Universal Serial Bus
Specification
CompaqIntel
Microsoft
NEC

Revision 1.1
September 23, 1998

Universal Serial Bus Specification Revision 1.1

Scope of this Revision
The 1.1 revision of the specification is intended for product design. Every attempt has been made to ensure a
consistent and implementable specification. Implementations should ensure compliance with this revision.

Revision History
RevisionIssue Date

Comments

0.7

November 11, 1994

Supersedes 0.6e.

0.8

December 30, 1994

Revisions to Chapters 3-8, 10, and 11. Added
appendixes.

0.9

April 13, 1995

Revisions to all the chapters.

0.99

August 25, 1995

Revisions to all the chapters.

1.0 FDR

November 13, 1995

Revisions to Chapters 1, 2, 5-11.

1.0

January 15, 1996

Edits to Chapters 5, 6, 7, 8, 9, 10, and 11 forconsistency.

1.1

September 23, 1998

Updates to all chapters to fix problems identified.

Universal Serial Bus Specification
Copyright © 1998, Compaq Computer Corporation,
Intel Corporation, Microsoft Corporation, NEC Corporation.
All rights reserved.
INTELLECTUAL PROPERTY DISCLAIMER
THIS SPECIFICATION IS PROVIDED “AS IS” WITH NO WARRANTIES WHATSOEVER INCLUDINGANY WARRANTY OF MERCHANTABILITY, FITNESS FOR ANY PARTICULAR PURPOSE, OR ANY
WARRANTY OTHERWISE ARISING OUT OF ANY PROPOSAL, SPECIFICATION, OR SAMPLE.
A LICENSE IS HEREBY GRANTED TO REPRODUCE AND DISTRIBUTE THIS SPECIFICATION FOR
INTERNAL USE ONLY. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OROTHERWISE, TO ANY OTHER INTELLECTUAL PROPERTY RIGHTS IS GRANTED OR INTENDED
HEREBY.
AUTHORS OF THIS SPECIFICATION DISCLAIM ALL LIABILITY, INCLUDING LIABILITY FORINFRINGEMENT OF PROPRIETARY RIGHTS, RELATING TO IMPLEMENTATION OF
INFORMATION IN THIS SPECIFICATION. AUTHORS OF THIS SPECIFICATION ALSO DO NOTWARRANT OR REPRESENT THAT SUCH IMPLEMENTATION(S) WILL NOT INFRINGE SUCHRIGHTS.

GeoPort and Apple Desktop Bus are trademarks of Apple Computer, Inc.
Windows and Windows NT are trademarks and Microsoft and Win32 are registered trademarks of Microsoft Corporation.
IBM, PS/2, and Micro Channel are registered trademarks of International Business Machines Corporation.
AT&T is a registered trademark of American Telephone and Telegraph Company.
Compaq is a registered trademark of Compaq Computer Corporation.
UNIX is a registered trademark of UNIX System Laboratories.
2
I C is a trademark of Phillips Semiconductors.
DEC is a trademark of Digital Equipment Corporation.
All other product names are trademarks, registered trademarks, or servicemarks of their respective owners.
Please send comments via electronic mail to techsup@usb.org
For industry information, refer to the USB Implementers Forum web page at http://www.usb.org

iiUniversal Serial Bus Specification Revision 1.1

Contents
CHAPTER 1 INTRODUCTION ................................................................................. 1
1.1

Motivation .................................................................................................................. .............................1

1.2

Objective of the Specification .............................................................................................. ..................1

1.3

Scope of the Document ....................................................................................................... ....................2

1.4

Document Organization....................................................................................................... ..................2

CHAPTER 2 TERMS AND ABBREVIATIONS .......................................................... 3
CHAPTER 3 BACKGROUND.................................................................................. 11
3.1

Goals for the Universal Serial Bus .......................................................................................... ............11

3.2

Taxonomy of Application Space............................................................................................... ...........12

3.3

Feature List ...........................................................................................................................................12

CHAPTER 4 ARCHITECTURAL OVERVIEW ........................................................ 15
4.1 USB System Description ......................................................................................................................15
4.1.1 Bus Topology ..................................................................................................................................16
4.2 Physical Interface .................................................................................................................................174.2.1 Electrical .........................................................................................................................................17
4.2.2 Mechanical ......................................................................................................................................17
4.3 Power .....................................................................................................................................................18
4.3.1 Power Distribution ..........................................................................................................................18
4.3.2 Power Management.........................................................................................................................18
4.4

Bus Protocol ..........................................................................................................................................18

4.5 Robustness.............................................................................................................................................194.5.1 Error Detection................................................................................................................................19
4.5.2 Error Handling.................................................................................................................................19
4.6 System Configuration...........................................................................................................................19
4.6.1 Attachment of USB Devices ...........................................................................................................19
4.6.2 Removal of USB Devices................................................................................................................20
4.6.3 Bus Enumeration .............................................................................................................................20
4.7 Data Flow Types ...................................................................................................................................20
4.7.1 Control Transfers.............................................................................................................................20iii

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4.7.2
4.7.3
4.7.4
4.7.5

Bulk Transfers.................................................................................................................................20
Interrupt Transfers...........................................................................................................................21
Isochronous Transfers .....................................................................................................................21
Allocating USB Bandwidth.............................................................................................................21

4.8 USB Devices ..........................................................................................................................................21
4.8.1 Device Characterizations.................................................................................................................21
4.8.2 Device Descriptions ........................................................................................................................22
4.9

USB Host: Hardware and Software...................................................................................................24

4.10 Architectural Extensions .....................................................................................................................24

CHAPTER 5 USB DATA FLOW MODEL................................................................ 25
5.1

Implementer Viewpoints......................................................................................................................25

5.2 Bus Topology ........................................................................................................................................27
5.2.1 USB Host ........................................................................................................................................27
5.2.2 USB Devices ...................................................................................................................................28
5.2.3 Physical Bus Topology....................................................................................................................29
5.2.4 Logical Bus Topology.....................................................................................................................30
5.2.5 Client Software-to-function Relationship........................................................................................30
5.3 USB Communication Flow ..................................................................................................................31
5.3.1 Device Endpoints ............................................................................................................................32
5.3.2 Pipes................................................................................................................................................33
5.4

Transfer Types......................................................................................................................................35

5.5 Control Transfers .................................................................................................................................36
5.5.1 Control Transfer Data Format .........................................................................................................36
5.5.2 Control Transfer Direction ..............................................................................................................37
5.5.3 Control Transfer Packet Size Constraints........................................................................................37
5.5.4 Control Transfer Bus Access Constraints........................................................................................38
5.5.5 Control Transfer Data Sequences....................................................................................................40
5.6 Isochronous Transfers .........................................................................................................................41
5.6.1 Isochronous Transfer Data Format..................................................................................................41
5.6.2 Isochronous Transfer Direction.......................................................................................................41
5.6.3 Isochronous Transfer Packet Size Constraints ................................................................................41
5.6.4 Isochronous Transfer Bus Access Constraints ................................................................................425.6.5 Isochronous Transfer Data Sequences.............................................................................................43
5.7 Interrupt Transfers ..............................................................................................................................43
5.7.1 Interrupt Transfer Data Format .......................................................................................................43
5.7.2 Interrupt Transfer Direction ............................................................................................................43
5.7.3 Interrupt Transfer Packet Size Constraints......................................................................................43
5.7.4 Interrupt Transfer Bus Access Constraints......................................................................................44
5.7.5 Interrupt Transfer Data Sequences ..................................................................................................46
5.8 Bulk Transfers ......................................................................................................................................46
5.8.1 Bulk Transfer Data Format .............................................................................................................47
5.8.2 Bulk Transfer Direction ..................................................................................................................47
5.8.3 Bulk Transfer Packet Size Constraints ............................................................................................47
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Universal Serial Bus Specification Revision 1.1

5.8.4
5.8.5

Bulk Transfer Bus Access Constraints ............................................................................................47
Bulk Transfer Data Sequences ........................................................................................................48

5.9 Bus Access for Transfers......................................................................................................................49
5.9.1 Transfer Management......................................................................................................................49
5.9.2 Transaction Tracking.......................................................................................................................52
5.9.3 Calculating Bus Transaction Times.................................................................................................54
5.9.4 Calculating Buffer Sizes in Functions and Software.......................................................................55
5.9.5 Bus Bandwidth Reclamation ...........................................................................................................55
5.10 Special Considerations for Isochronous Transfers............................................................................55
5.10.1 Example Non-USB Isochronous Application..................................................................................56
5.10.2 USB Clock Model ...........................................................................................................................59
5.10.3 Clock Synchronization ....................................................................................................................61
5.10.4 Isochronous Devices........................................................................................................................61
5.10.5 Data Prebuffering ............................................................................................................................69
5.10.6 SOF Tracking ..................................................................................................................................70
5.10.7 Error Handling.................................................................................................................................70
5.10.8 Buffering for Rate Matching ...........................................................................................................71

CHAPTER 6 MECHANICAL.................................................................................... 73
6.1

Architectural Overview........................................................................................................................73

6.2

Keyed Connector Protocol...................................................................................................................73

6.3

Cable......................................................................................................................................................74

6.4 Cable Assembly.....................................................................................................................................74
6.4.1 Detachable Cable Assemblies .........................................................................................................74
6.4.2 Full-speed Captive Cable Assemblies .............................................................................................76
6.4.3 Low-speed Captive Cable Assemblies ............................................................................................78
6.4.4 Prohibited Cable Assemblies...........................................................................................................80
6.5 Connector Mechanical Configuration and Material Requirements ................................................80
6.5.1 USB Icon Location..........................................................................................................................81
6.5.2 USB Connector Termination Data ..................................................................................................82
6.5.3 Series “A” and Series “B” Receptacles ...........................................................................................82
6.5.4 Series “A” and Series “B” Plugs .....................................................................................................86
6.6 Cable Mechanical Configuration and Material Requirements ........................................................90
6.6.1 Description ......................................................................................................................................90
6.6.2 Construction ....................................................................................................................................91
6.6.3 Electrical Characteristics .................................................................................................................93
6.6.4 Cable Environmental Characteristics ..............................................................................................93
6.6.5 Listing .............................................................................................................................................94
6.7 Electrical, Mechanical and Environmental Compliance Standards ................................................94
6.7.1 Applicable Documents ..................................................................................................................102
6.8

USB Grounding ..................................................................................................................................102

6.9

PCB Reference Drawings...................................................................................................................102v

Universal Serial Bus Specification Revision 1.1

CHAPTER 7 ELECTRICAL................................................................................... 107
7.1 Signaling................................................................................................................... ...........................107
7.1.1 USB Driver Characteristics ................................................................................................ ...........107
7.1.2 Data Signal Rise and Fall ..............................................................................................................110
7.1.3 Cable Skew....................................................................................................................................112
7.1.4 Receiver Characteristics................................................................................................................112
7.1.5 Device Speed Identification ..........................................................................................................113
7.1.6 Input Characteristics......................................................................................................................114
7.1.7 Signaling Levels............................................................................................................................115
7.1.8 Data Encoding/Decoding ..............................................................................................................123
7.1.9 Bit Stuffing....................................................................................................................................124
7.1.10 Sync Pattern ..................................................................................................................................126
7.1.11 Data Signaling Rate.......................................................................................................................126
7.1.12 Frame Interval and Frame Interval Adjustment ............................................................................126
7.1.13 Data Source Signaling...................................................................................................................127
7.1.14 Hub Signaling Timings .................................................................................................................128
7.1.15 Receiver Data Jitter .......................................................................................................................130
7.1.16 Cable Delay...................................................................................................................................132
7.1.17 Cable Attenuation..........................................................................................................................133
7.1.18 Bus Turn-around Time and Inter-packet Delay.............................................................................133
7.1.19 Maximum End-to-end Signal Delay..............................................................................................133
7.2 Power Distribution .............................................................................................................................134
7.2.1 Classes of Devices.........................................................................................................................134
7.2.2 Voltage Drop Budget ....................................................................................................................138
7.2.3 Power Control During Suspend/Resume.......................................................................................139
7.2.4 Dynamic Attach and Detach..........................................................................................................140
7.3 Physical Layer ....................................................................................................................................141
7.3.1 Regulatory Requirements..............................................................................................................142
7.3.2 Bus Timing/Electrical Characteristics...........................................................................................142
7.3.3 Timing Waveforms .......................................................................................................................151

CHAPTER 8 PROTOCOL LAYER ........................................................................ 155
8.1

Bit Ordering................................................................................................................ ........................155

8.2

SYNC Field .................................................................................................................. .......................155

8.3 Packet Field Formats ........................................................................................................ .................155
8.3.1 Packet Identifier Field ................................................................................................... ................155
8.3.2 Address Fields............................................................................................................ ...................156
8.3.3 Frame Number Field ........................................................................................................ .............157
8.3.4 Data Field ......................................................................................................................................157
8.3.5 Cyclic Redundancy Checks...........................................................................................................158
8.4 Packet Formats ...................................................................................................................................158
8.4.1 Token Packets ...............................................................................................................................159
8.4.2 Start-of-Frame Packets..................................................................................................................159
8.4.3 Data Packets ..................................................................................................................................160
8.4.4 Handshake Packets........................................................................................................................160
8.4.5 Handshake Responses ...................................................................................................................161vi

Universal Serial Bus Specification Revision 1.1

8.5 Transaction Formats ..........................................................................................................................162
8.5.1 Bulk Transactions..........................................................................................................................163
8.5.2 Control Transfers...........................................................................................................................164
8.5.3 Interrupt Transactions....................................................................................................................167
8.5.4 Isochronous Transactions ..............................................................................................................168
8.6 Data Toggle Synchronization and Retry ..........................................................................................168
8.6.1 Initialization via SETUP Token ....................................................................................................169
8.6.2 Successful Data Transactions ........................................................................................................169
8.6.3 Data Corrupted or Not Accepted...................................................................................................170
8.6.4 Corrupted ACK Handshake...........................................................................................................170
8.6.5 Low-speed Transactions................................................................................................................171
8.7 Error Detection and Recovery...........................................................................................................172
8.7.1 Packet Error Categories.................................................................................................................172
8.7.2 Bus Turn-around Timing...............................................................................................................172
8.7.3 False EOPs ....................................................................................................................................173
8.7.4 Babble and Loss of Activity Recovery..........................................................................................174

CHAPTER 9 USB DEVICE FRAMEWORK ........................................................... 175
9.1 USB Device States........................................................................................................... ....................175
9.1.1 Visible Device States..................................................................................................... ................175
9.1.2 Bus Enumeration ........................................................................................................... ................179
9.2 Generic USB Device Operations .......................................................................................................180
9.2.1 Dynamic Attachment and Removal...............................................................................................180
9.2.2 Address Assignment......................................................................................................................180
9.2.3 Configuration ................................................................................................................................180
9.2.4 Data Transfer.................................................................................................................................181
9.2.5 Power Management.......................................................................................................................181
9.2.6 Request Processing........................................................................................................................181
9.2.7 Request Error.................................................................................................................................182
9.3 USB Device Requests..........................................................................................................................183
9.3.1 bmRequestType.............................................................................................................................183
9.3.2 bRequest ........................................................................................................................................184
9.3.3 wValue ..........................................................................................................................................184
9.3.4 wIndex...........................................................................................................................................184
9.3.5 wLength.........................................................................................................................................184
9.4 Standard Device Requests..................................................................................................................185
9.4.1 Clear Feature .................................................................................................................................188
9.4.2 Get Configuration..........................................................................................................................189
9.4.3 Get Descriptor ...............................................................................................................................189
9.4.4 Get Interface..................................................................................................................................190
9.4.5 Get Status ......................................................................................................................................190
9.4.6 Set Address....................................................................................................................................192
9.4.7 Set Configuration ..........................................................................................................................193
9.4.8 Set Descriptor................................................................................................................................193
9.4.9 Set Feature.....................................................................................................................................1949.4.10 Set Interface...................................................................................................................................1959.4.11 Synch Frame..................................................................................................................................195
9.5

Descriptors ..........................................................................................................................................196vii

Universal Serial Bus Specification Revision 1.1

9.6 Standard USB Descriptor Definitions...............................................................................................196
9.6.1 Device ...........................................................................................................................................196
9.6.2 Configuration ................................................................................................................................199
9.6.3 Interface ........................................................................................................................................201
9.6.4 Endpoint ........................................................................................................................................2039.6.5 String.............................................................................................................................................204
9.7 Device Class Definitions .....................................................................................................................2059.7.1 Descriptors ....................................................................................................................................2059.7.2 Interface(s) and Endpoint Usage ...................................................................................................2059.7.3 Requests ........................................................................................................................................206

CHAPTER 10 USB HOST: HARDWARE AND SOFTWARE .............................. 207
10.1 Overview of the USB Host .................................................................................................................207
10.1.1 Overview.......................................................................................................................................207
10.1.2 Control Mechanisms .....................................................................................................................210
10.1.3 Data Flow......................................................................................................................................210
10.1.4 Collecting Status and Activity Statistics .......................................................................................211
10.1.5 Electrical Interface Considerations ...............................................................................................211
10.2 Host Controller Requirements ..........................................................................................................211
10.2.1 State Handling...............................................................................................................................212
10.2.2 Serializer/Deserializer ...................................................................................................................212
10.2.3 Frame Generation..........................................................................................................................212
10.2.4 Data Processing.............................................................................................................................213
10.2.5 Protocol Engine.............................................................................................................................213
10.2.6 Transmission Error Handling ........................................................................................................213
10.2.7 Remote Wakeup ............................................................................................................................21410.2.8 Root Hub .......................................................................................................................................21410.2.9 Host System Interface ...................................................................................................................21410.3 Overview of Software Mechanisms...................................................................................................21410.3.1 Device Configuration ....................................................................................................................215
10.3.2 Resource Management ..................................................................................................................217
10.3.3 Data Transfers ...............................................................................................................................217
10.3.4 Common Data Definitions ............................................................................................................218
10.4 Host Controller Driver.......................................................................................................................218
10.5 Universal Serial Bus Driver...............................................................................................................219
10.5.1 USBD Overview ...........................................................................................................................219
10.5.2 USBD Command Mechanism Requirements................................................................................221
10.5.3 USBD Pipe Mechanisms...............................................................................................................223
10.5.4 Managing the USB via the USBD Mechanisms............................................................................225
10.5.5 Passing USB Preboot Control to the Operating System................................................................227
10.6 Operating System Environment Guides...........................................................................................227

CHAPTER 11 HUB SPECIFICATION ................................................................... 229
11.1 Overview ................................................................................................................... ..........................229
11.1.1 Hub Architecture......................................................................................................... ..................230
11.1.2 Hub Connectivity ......................................................................................................... .................230

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11.2 Hub Frame Timer...............................................................................................................................232
11.2.1 Frame Timer Synchronization.......................................................................................................233
11.2.2 EOF1 and EOF2 Timing Points ....................................................................................................234
11.3 Host Behavior at End-of-Frame ........................................................................................................235
11.3.1 Latest Host Packet .........................................................................................................................235
11.3.2 Packet Nullification.......................................................................................................................235
11.3.3 Transaction Completion Prediction...............................................................................................236
11.4 Internal Port .......................................................................................................................................237
11.4.1 Inactive ..........................................................................................................................................237
11.4.2 Suspend Delay...............................................................................................................................237
11.4.3 Full Suspend (Fsus) .......................................................................................................................237
11.4.4 Generate Resume (GResume) .......................................................................................................238
11.5 Downstream Ports ..............................................................................................................................239
11.5.1 Downstream Port State Descriptions.............................................................................................240
11.5.2 Disconnect Detect Timer...............................................................................................................243
11.6 Upstream Port.....................................................................................................................................244
11.6.1 Receiver.........................................................................................................................................244
11.6.2 Transmitter ....................................................................................................................................246
11.7 Hub Repeater......................................................................................................................................249
11.7.1 Wait for Start of Packet from Upstream Port (WFSOPFU) ..........................................................250
11.7.2 Wait for End of Packet from Upstream Port (WFEOPFU) ...........................................................250
11.7.3 Wait for Start of Packet (WFSOP) ................................................................................................251
11.7.4 Wait for End of Packet (WFEOP) .................................................................................................25111.8 Bus State Evaluation ..........................................................................................................................251
11.8.1 Port Error.......................................................................................................................................251
11.8.2 Speed Detection.............................................................................................................................251
11.8.3 Collision ........................................................................................................................................252
11.8.4 Full- versus Low-speed Behavior..................................................................................................252
11.9 Suspend and Resume..........................................................................................................................253
11.10 Hub Reset Behavior............................................................................................................................254
11.10.1 Hub Receiving Reset on Upstream Port ........................................................................................254
11.11 Hub Port Power Control....................................................................................................................255
11.11.1 Multiple Gangs..............................................................................................................................255
11.12 Hub I/O Buffer Requirements...........................................................................................................25611.12.1 Pull-up and Pull-down Resistors ...................................................................................................25611.12.2 Edge Rate Control .........................................................................................................................25611.13 Hub Controller ...................................................................................................................................25611.13.1 Endpoint Organization ..................................................................................................................257
11.13.2 Hub Information Architecture and Operation................................................................................257
11.13.3 Port Change Information Processing.............................................................................................259
11.13.4 Hub and Port Status Change Bitmap .............................................................................................259
11.13.5 Over-current Reporting and Recovery ..........................................................................................260
11.14 Hub Configuration .............................................................................................................................261

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11.15 Descriptors ..........................................................................................................................................263
11.15.1 Standard Descriptors .....................................................................................................................263
11.15.2 Class-specific Descriptors .............................................................................................................264
11.16 Requests...............................................................................................................................................266
11.16.1 Standard Requests .........................................................................................................................266
11.16.2 Class-specific Requests .................................................................................................................267

INDEX.................................................................................................................... 281

xUniversal Serial Bus Specification Revision 1.1

FiguresFigure 3-1. Application Space Taxonomy ...........................................................................................................12
Figure 4-1. Bus Topology....................................................................................................................................16
Figure 4-2. USB Cable.........................................................................................................................................17
Figure 4-3. A Typical Hub...................................................................................................................................22
Figure 4-4. Hubs in a Desktop Computer Environment ......................................................................................23
Figure 5-1. Simple USB Host/Device View ........................................................................................................25
Figure 5-2. USB Implementation Areas ..............................................................................................................26
Figure 5-3. Host Composition .............................................................................................................................27
Figure 5-4. Physical Device Composition ...........................................................................................................28
Figure 5-5. USB Physical Bus Topology.............................................................................................................29
Figure 5-6. USB Logical Bus Topology ..............................................................................................................30
Figure 5-7. Client Software-to-function Relationships........................................................................................30
Figure 5-8. USB Host/Device Detailed View ......................................................................................................31
Figure 5-9. USB Communication Flow ...............................................................................................................32
Figure 5-10. USB Information Conversion From Client Software to Bus...........................................................50
Figure 5-11. Transfers for Communication Flows...............................................................................................52
Figure 5-12. Arrangement of IRPs to Transactions/Frames ................................................................................53
Figure 5-13. Non-USB Isochronous Example .....................................................................................................57
Figure 5-14. USB Isochronous Application.........................................................................................................60
Figure 5-15. Example Source/Sink Connectivity ................................................................................................66
Figure 5-16. Data Prebuffering ............................................................................................................................70
Figure 5-17. Packet and Buffer Size Formulas for Rate-Matched Isochronous Transfers...................................72
Figure 6-1. Keyed Connector Protocol ................................................................................................................73
Figure 6-2. USB Detachable Cable Assembly......................................................................................................75
Figure 6-3. USB Full-speed Hardwired Cable Assembly .....................................................................................77
Figure 6-4. USB Low-speed Hardwired Cable Assembly ....................................................................................79
Figure 6-5. USB Icon...........................................................................................................................................81
Figure 6-6. Typical USB Plug Orientation ...........................................................................................................81
Figure 6-7. USB Series "A" Receptacle Interface and Mating Drawing .............................................................83
Figure 6-8. USB Series "B" Recptacle Interface and Mating Drawing ...............................................................84
Figure 6-9. USB Series "A" Plug Interface Drawing...........................................................................................87
Figure 6-10. USB Series “B” Plug Interface Drawing.........................................................................................88
Figure 6-11. Typical Full-speed Cable Construction...........................................................................................90
Figure 6-12. Single Pin-Type Series "A" Receptacle ........................................................................................103xi

Universal Serial Bus Specification Revision 1.1

Figure 6-13. Dual Pin-Type Series "A" Receptacle...........................................................................................104
Figure 6-14. Single Pin-Type Series "B" Receptacle ........................................................................................105
Figure 7-1. Maximum Input Waveforms for USB Signaling ............................................................................107
Figure 7-2. Example Full-speed CMOS Driver Circuit.....................................................................................108
Figure 7-3. Full-speed Buffer V/I Characteristics .............................................................................................109
Figure 7-4. Full-speed Signal Waveforms.........................................................................................................110
Figure 7-5. Low-speed Driver Signal Waveforms.............................................................................................110
Figure 7-6. Data Signal Rise and Fall Time ......................................................................................................111
Figure 7-7. Full-speed Load ..............................................................................................................................111
Figure 7-8. Low-speed Port Loads ....................................................................................................................112
Figure 7-9. Differential Input Sensitivity Range ...............................................................................................113
Figure 7-10. Full-speed Device Cable and Resistor Connections......................................................................113
Figure 7-11. Low-speed Device Cable and Resistor Connections.....................................................................114
Figure 7-12. Placement of Optional Edge Rate Control Capacitors ..................................................................115
Figure 7-13. Upstream Full-speed Port Transceiver..........................................................................................117
Figure 7-14. Downstream Port Transceiver.......................................................................................................117
Figure 7-15. Disconnect Detection ....................................................................................................................118
Figure 7-16. Full-speed Device Connect Detection...........................................................................................118
Figure 7-17. Low-speed Device Connect Detection..........................................................................................119
Figure 7-18. Bus State Evaluation after reset (optional)....................................................................................119
Figure 7-19. Power-on and Connection Events Timing ....................................................................................120
Figure 7-20. Packet Voltage Levels...................................................................................................................121
Figure 7-21. NRZI Data Encoding ....................................................................................................................124
Figure 7-22. Bit Stuffing ...................................................................................................................................124
Figure 7-23. Illustration of Extra Bit Preceding EOP........................................................................................125
Figure 7-24. Flow Diagram for Bit Stuffing......................................................................................................125
Figure 7-25. Sync Pattern ..................................................................................................................................126
Figure 7-26. Data Jitter Taxonomy....................................................................................................................127
Figure 7-27. SE0 for EOP Width Timing ..........................................................................................................128
Figure 7-28. Hub Propagation Delay of Full-speed Differential Signals...........................................................129
Figure 7-29. Full-speed Cable Delay.................................................................................................................132
Figure 7-30. Low-speed Cable Delay ................................................................................................................132
Figure 7-31. Worst-case End to End Signal Delay Model.................................................................................134
Figure 7-32. Compound Bus-powered Hub.......................................................................................................135
Figure 7-33. Compound Self-powered Hub ......................................................................................................136
Figure 7-34. Low-power Bus-powered Function...............................................................................................137
Figure 7-35. High-power Bus-powered Function..............................................................................................137
Figure 7-36. Self-powered Function..................................................................................................................138xii

Universal Serial Bus Specification Revision 1.1

Figure 7-37. Worst-case Voltage Drop Topology (Steady State) ......................................................................138
Figure 7-38. Typical Suspend Current Averaging Profile .................................................................................139
Figure 7-39. Differential Data Jitter...................................................................................................................151
Figure 7-40. Differential-to-EOP Transition Skew and EOP Width .................................................................151
Figure 7-41. Receiver Jitter Tolerance...............................................................................................................151
Figure 7-42. Hub Differential Delay, Differential Jitter, and SOP Distortion ...................................................152
Figure 7-43. Hub EOP Delay and EOP Skew....................................................................................................153
Figure 8-1. PID Format......................................................................................................................................155
Figure 8-2. ADDR Field ....................................................................................................................................157
Figure 8-3. Endpoint Field.................................................................................................................................157
Figure 8-4. Data Field Format ...........................................................................................................................157
Figure 8-5. Token Format..................................................................................................................................159
Figure 8-6. SOF Packet......................................................................................................................................159
Figure 8-7. Data Packet Format.........................................................................................................................160
Figure 8-8. Handshake Packet ...........................................................................................................................160
Figure 8-9. Bulk Transaction Format.................................................................................................................163
Figure 8-10. Bulk Reads and Writes..................................................................................................................164
Figure 8-11. Control SETUP Transaction .........................................................................................................164
Figure 8-12. Control Read and Write Sequences...............................................................................................165
Figure 8-13. Interrupt Transaction Format ........................................................................................................167
Figure 8-14. Isochronous Transaction Format...................................................................................................168
Figure 8-15. SETUP Initialization .....................................................................................................................169
Figure 8-16. Consecutive Transactions..............................................................................................................169
Figure 8-17. NAKed Transaction with Retry.....................................................................................................170
Figure 8-18. Corrupted ACK Handshake with Retry ........................................................................................170
Figure 8-19. Low-speed Transaction .................................................................................................................171
Figure 8-20. Bus Turn-around Timer Usage......................................................................................................173
Figure 9-1. Device State Diagram .....................................................................................................................176
Figure 9-2. wIndex Format when Specifying an Endpoint ................................................................................184
Figure 9-3. wIndex Format when Specifying an Interface ................................................................................184
Figure 9-4. Information Returned by a GetStatus() Request to a Device ..........................................................191
Figure 9-5. Information Returned by a GetStatus() Request to a Interface .......................................................191
Figure 9-6. Information Returned by a GetStatus() Request to an Endpoint .....................................................192
Figure 10-1. Interlayer Communications Model................................................................................................207
Figure 10-2. Host Communications...................................................................................................................208
Figure 10-3. Frame Creation..............................................................................................................................212
Figure 10-4. Configuration Interactions ............................................................................................................215
Figure 10-5. Universal Serial Bus Driver Structure...........................................................................................219
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Universal Serial Bus Specification Revision 1.1

Figure 11-1. Hub Architecture...........................................................................................................................230
Figure 11-2. Hub Signaling Connectivity...........................................................................................................231
Figure 11-3. Resume Connectivity .....................................................................................................................232
Figure 11-4. EOF Timing Points .......................................................................................................................234
Figure 11-5. Internal Port State Machine...........................................................................................................237
Figure 11-6. Downstream Hub Port State Machine............................................................................................239
Figure 11-7. Upstream Port Receiver State Machine .........................................................................................244
Figure 11-8. Upstream Hub Port Transmitter State Machine .............................................................................246
Figure 11-9. Hub Repeater State Machine..........................................................................................................249
Figure 11-10. Example Remote-Wakeup Resume Signaling ............................................................................254
Figure 11-11. Example Hub Controller Organization .......................................................................................257
Figure 11-12. Relationship of Status, Status Change, and Control Information to Device States.....................258
Figure 11-13. Port Status Handling Method ......................................................................................................259
Figure 11-14. Hub and Port Status Change Bitmap...........................................................................................260
Figure 11-15. Example Hub and Port Change Bit Sampling .............................................................................260

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Universal Serial Bus Specification Revision 1.1

TablesTable 5-1. Full-speed Control Transfer Limits ....................................................................................................39
Table 5-2. Low-speed Control Transfer Limits ...................................................................................................40
Table 5-3. Isochronous Transaction Limits .........................................................................................................42
Table 5-4. Full-speed Interrupt Transaction Limits .............................................................................................45
Table 5-5. Low-speed Interrupt Transaction Limits ............................................................................................46
Table 5-6. Bulk Transaction Limits .....................................................................................................................48
Table 5-7. Synchronization Characteristics .........................................................................................................62
Table 5-8. Connection Requirements ..................................................................................................................68
Table 6-1. USB Connector Termination Assignment ..........................................................................................82
Table 6-2. Power Pair ..........................................................................................................................................91
Table 6-3. Signal Pair ..........................................................................................................................................91
Table 6-4. Drain Wire Signal Pair .......................................................................................................................92
Table 6-5. Nominal Cable Diameter....................................................................................................................93
Table 6-6. Conductor Resistance .........................................................................................................................93
Table 6-7. USB Electrical, Mechnaical and Environmental Compliance Standards ...........................................94
Table 7-1. Signaling Levels ...............................................................................................................................116
Table 7-2. Full-speed Jitter Budget....................................................................................................................131
Table 7-3. Low-speed Jitter Budget...................................................................................................................131
Table 7-4. Signal Attenuation............................................................................................................................133
Table 7-5. DC Electrical Characteristics............................................................................................................142
Table 7-6. Full-speed Source Electrical Characteristics ....................................................................................144
Table 7-7. Low-speed Source Electrical Characteristics ...................................................................................145
Table 7-8. Hub/Repeater Electrical Characteristics ...........................................................................................146
Table 7-9. Cable Characteristics (Note 14)........................................................................................................147
Table 7-10. Hub Event Timings ........................................................................................................................148
Table 7-11. Device Event Timings ....................................................................................................................149
Table 8-1. PID Types.........................................................................................................................................156
Table 8-2. Function Responses to IN Transactions ...........................................................................................161
Table 8-3. Host Responses to IN Transactions ..................................................................................................162
Table 8-4. Function Responses to OUT Transactions in Order of Precedence..................................................162
Table 8-5. Status Stage Responses.....................................................................................................................166
Table 8-6. Packet Error Types ...........................................................................................................................172
Table 9-1. Visible Device States........................................................................................................................177
Table 9-2. Format of Setup Data .......................................................................................................................183

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Universal Serial Bus Specification Revision 1.1

Table 9-3. Standard Device Requests ................................................................................................................186
Table 9-4. Standard Request Codes...................................................................................................................187
Table 9-5. Descriptor Types ..............................................................................................................................187
Table 9-6. Standard Feature Selectors ...............................................................................................................188
Table 9-7. Standard Device Descriptor..............................................................................................................197
Table 9-8. Standard Configuration Descriptor ..................................................................................................199
Table 9-9. Standard Interface Descriptor...........................................................................................................202
Table 9-10. Standard Endpoint Descriptor ........................................................................................................203
Table 9-11. Codes Representing Languages Supported by the Device .............................................................205
Table 9-12. UNICODE String Descriptor .........................................................................................................205
Table 11-1. Hub and Host EOF Timing Points..................................................................................................235
Table 11-2. Internal Port Signal/Event Definitions ...........................................................................................237
Table 11-3. Downstream Hub Port Signal/Event Definitions............................................................................240
Table 11-4. Upstream Hub Port Receiver Signal/Event Definitions .................................................................245
Table 11-5. Upstream Hub Port Transmit Signal/Event Definitions .................................................................247
Table 11-6. Hub Repeater Signal/Event Definitions .........................................................................................250
Table 11-7. Hub Power Operating Mode Summary ..........................................................................................261
Table 11-8. Hub Descriptor ...............................................................................................................................264
Table 11-9. Hub Responses to Standard Device Requests ................................................................................266
Table 11-10. Hub Class Requests ......................................................................................................................267
Table 11-11. Hub Class Request Codes.............................................................................................................268
Table 11-12. Hub Class Feature Selectors.........................................................................................................268
Table 11-13. Hub Status Field, wHubStatus......................................................................................................272
Table 11-14. Hub Change Field, wHubChange.................................................................................................272
Table 11-15. Port Status Field, wPortStatus......................................................................................................274
Table 11-16. Port Change Field, wPortChange.................................................................................................277

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Chapter 1
Introduction
1.1 Motivation
The motivation for the Universal Serial Bus (USB) comes from three interrelated considerations:Connection of the PC to the telephone
It is well understood that the merge of computing and communication will be the basis for the next
generation of productivity applications. The movement of machine-oriented and human-oriented datatypes from one location or environment to another depends on ubiquitous and cheap connectivity.
Unfortunately, the computing and communication industries have evolved independently. The USB
provides a ubiquitous link that can be used across a wide range of PC-to-telephone interconnects.

Ease-of-use
The lack of flexibility in reconfiguring the PC has been acknowledged as the Achilles’ heel to its
further deployment. The combination of user-friendly graphical interfaces and the hardware andsoftware mechanisms associated with new-generation bus architectures have made computers lessconfrontational and easier to reconfigure. However, from the end user’s point of view, the PC’s I/Ointerfaces, such as serial/parallel ports, keyboard/mouse/joystick interfaces, etc., do not have theattributes of plug-and-play.

Port expansion
The addition of external peripherals continues to be constrained by port availability. The lack of a bidirectional, low-cost, low-to-mid speed peripheral bus has held back the creative proliferation of
peripherals such as telephone/fax/modem adapters, answering machines, scanners, PDA’s, keyboards,
mice, etc. Existing interconnects are optimized for one or two point products. As each new function
or capability is added to the PC, a new interface has been defined to address this need.
The USB is the answer to connectivity for the PC architecture. It is a fast, bi-directional, isochronous,
low-cost, dynamically attachable serial interface that is consistent with the requirements of the PC
platform of today and tomorrow.

1.2 Objective of the Specification
This document defines an industry-standard USB. The specification describes the bus attributes, theprotocol definition, types of transactions, bus management, and the programming interface required todesign and build systems and peripherals that are compliant with this standard.
The goal is to enable such devices from different vendors to interoperate in an open architecture. The
specification is intended as an enhancement to the PC architecture, spanning portable, business desktop,
and home environments. It is intended that the specification allow system OEMs and peripheral developers
adequate room for product versatility and market differentiation without the burden of carrying obsolete
interfaces or losing compatibility.

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Universal Serial Bus Specification Revision 1.1

1.3 Scope of the Document


Target audience
The specification is primarily targeted to peripheral developers and system OEMs, but provides
valuable information for platform operating system/ BIOS/ device driver, adapter IHVs/ISVs, andplatform/adapter controller vendors.



BenefitThis version of the USB Specification can be used for planning new products, engineering an early
prototype, and preliminary software development. All final products are required to be compliant with
the USB Specification 1.1.

1.4 Document Organization
Chapters 1 through 5 provide an overview for all readers, while Chapters 6 through 11 contain detailed
technical information defining the USB.


Peripheral implementers should particularly read Chapters 5 through 11.

USB Host Controller implementers should particularly read Chapters 5 through 8, 10, and 11.

USB device driver implementers should particularly read Chapters 5, 9, and 10.

This document is complemented and referenced by the Universal Serial Bus Device Class Specifications.
Device class specifications exist for a wide variety of devices. Please contact the USB Implementers
Forum for further details.
Readers are also requested to contact operating system vendors for operating system bindings specific to
the USB.

2

Universal Serial Bus Specification Revision 1.1

Chapter 2
Terms and Abbreviations
This chapter lists and defines terms and abbreviations used throughout this specification.

ACK

Handshake packet indicating a positive acknowledgment.

Active Device

A device that is powered and is not in the Suspend state.

Asynchronous Data

Data transferred at irregular intervals with relaxed latency requirements.

Asynchronous RA

The incoming data rate, Fsi, and the outgoing data rate, Fso, of the RA process
are independent (i.e., there is no shared master clock). See also Rate
Adaptation.

Asynchronous SRC

The incoming sample rate, Fsi, and outgoing sample rate, Fso, of the SRC
process are independent (i.e., there is no shared master clock). See also
Sample Rate Conversion.

Audio Device

A device that sources or sinks sampled analog data.

AWG#

The measurement of a wire’s cross section, as defined by the American Wire
Gauge standard.

BabbleUnexpected bus activity that persists beyond a specified point in a frame.

Bandwidth

The amount of data transmitted per unit of time, typically bits per second (b/s)
or bytes per second (B/s).

Big Endian

A method of storing data that places the most significant byte of multiple-bytevalues at a lower storage addresses. For example, a 16-bit integer stored in big
endian format places the least significant byte at the higher address and themost significant byte at the lower address. See also Little Endian.

Bit

A unit of information used by digital computers. Represents the smallest
piece of addressable memory within a computer. A bit expresses the choice
between two possibilities and is typically represented by a logical one (1) or
zero (0).

Bit Stuffing

Insertion of a “0” bit into a data stream to cause an electrical transition on thedata wires, allowing a PLL to remain locked.

b/s

Transmission rate expressed in bits per second.

B/s

Transmission rate expressed in bytes per second.

BufferStorage used to compensate for a difference in data rates or time of occurrenceof events, when transmitting data from one device to another.

Bulk Transfer

One of the four USB transfer types. Bulk transfers are non-periodic, large
bursty communication typically used for a transfer that can use any available
bandwidth and can also be delayed until bandwidth is available. See also
Transfer Type.

Bus Enumeration

Detecting and identifying USB devices.

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Universal Serial Bus Specification Revision 1.1

Byte

A data element that is eight bits in size.

CapabilitiesThose attributes of a USB device that are administrated by the host.

Characteristics

Those qualities of a USB device that are unchangeable; for example, thedevice class is a device characteristic.

ClientSoftware resident on the host that interacts with the USB System Software toarrange data transfer between a function and the host. The client is often thedata provider and consumer for transferred data.

Configuring
Software

Software resident on the host software that is responsible for configuring a
USB device. This may be a system configurator or software specific to thedevice.

Control Endpoint

A pair of device endpoints with the same endpoint number that are used by acontrol pipe. Control endpoints transfer data in both directions and therefore
use both endpoint directions of a device address and endpoint number
combination. Thus, each control endpoint consumes two endpoint addresses.

Control Pipe

Same as a message pipe.

Control Transfer

One of the four USB transfer types. Control transfers supportconfiguration/command/status type communications between client andfunction. See also Transfer Type.

CRC

See Cyclic Redundancy Check.

CTI

Computer Telephony Integration.

Cyclic Redundancy
Check (CRC)

A check performed on data to see if an error has occurred in transmitting,reading, or writing the data. The result of a CRC is typically stored ortransmitted with the checked data. The stored or transmitted result iscompared to a CRC calculated for the data to determine if an error has
occurred.

Default Address

An address defined by the USB Specification and used by a USB device when
it is first powered or reset. The default address is 00H.

Default Pipe

The message pipe created by the USB System Software to pass control andstatus information between the host and a USB device’s endpoint zero.

DeviceA logical or physical entity that performs a function. The actual entity
described depends on the context of the reference. At the lowest level, device
may refer to a single hardware component, as in a memory device. At ahigher level, it may refer to a collection of hardware components that performa particular function, such as a USB interface device. At an even higher level,
device may refer to the function performed by an entity attached to the USB;for example, a data/FAX modem device. Devices may be physical, electrical,
addressable, and logical.
When used as a non-specific reference, a USB device is either a hub or afunction.

Device Address

4

A seven-bit value representing the address of a device on the USB. The
device address is the default address (00H) when the USB device is firstpowered or the device is reset. Devices are assigned a unique device addressby the USB System Software.

Universal Serial Bus Specification Revision 1.1

Device Endpoint

A uniquely addressable portion of a USB device that is the source or sink of
information in a communication flow between the host and device. See also
Endpoint Address.

Device Resources

Resources provided by UB devices, such as buffer space and endpoints. See
also Host Resources and Universal Serial Bus Resources.

Device Software

Software that is responsible for using a USB device. This software may or
may not also be responsible for configuring the device for use.

Downstream

The direction of data flow from the host or away from the host. A
downstream port is the port on a hub electrically farthest from the host thatgenerates downstream data traffic from the hub. Downstream ports receive
upstream data traffic.

DriverWhen referring to hardware, an I/O pad that drives an external load. Whenreferring to software, a program responsible for interfacing to a hardware
device; that is, a device driver.

DWORD

Double word. A data element that is two words (i.e., four bytes or 32 bits) insize.

Dynamic Insertion
and Removal

The ability to attach and remove devices while the host is in operation.

2

E PROMSee Electrically Erasable Programmable Read Only Memory.

EEPROMSee Electrically Erasable Programmable Read Only Memory.

Electrically
Erasable
Programmable
Read Only Memory(EEPROM)

Non-volatile rewritable memory storage technology.

End User

The user of a host.

Endpoint

See Device Endpoint.

Endpoint Address

The combination of an endpoint number and an endpoint direction on a USB
device. Each endpoint address supports data transfer in one direction.

Endpoint Direction

The direction of data transfer on the USB. The direction can be either IN or
OUT. IN refers to transfers to the host; OUT refers to transfers from the host.

Endpoint Number

A four-bit value between 0H and FH, inclusive, associated with an endpoint
on a USB device.

EOF

End-of-Frame.

EOP

End-of-Packet.

External PortSee Port.

False EOP

A spurious, usually noise-induced event that is interpreted by a packet receiver
as an EOP.

Frame

The time from the start of one SOF token to the start of the subsequent SOF
token; consists of a series of transactions.

5Universal Serial Bus Specification Revision 1.1

6

Frame Pattern

A sequence of frames that exhibit a repeating pattern in the number of samples
transmitted per frame. For a 44.1kHz audio transfer, the frame pattern could
be nine frames containing 44 samples followed by one frame containing 45
samples.

Fs

See Sample Rate.

Full-duplex

Computer data transmission occurring in both directions simultaneously.

FunctionA USB device that provides a capability to the host, such as an ISDN
connection, a digital microphone, or speakers.

Handshake Packet

A packet that acknowledges or rejects a specific condition. For examples, see
ACK and NAK.

Host

The host computer system where the USB Host Controller is installed. This
includes the host hardware platform (CPU, bus, etc.) and the operating system
in use.

Host Controller

The host’s USB interface.

Host Controller
Driver (HCD)The USB software layer that abstracts the Host Controller hardware. The Host
Controller Driver provides an SPI for interaction with a Host Controller. The
Host Controller Driver hides the specifics of the Host Controller hardware
implementation.

Host Resources

Resources provided by the host, such as buffer space and interrupts. See also
Device Resources and Universal Serial Bus Resources.

Hub

A USB device that provides additional connections to the USB.

Hub Tier

The level of connect within a USB network topology, given as the number ofhubs through which the data has to flow.

Interrupt Request(IRQ)

A hardware signal that allows a device to request attention from a host. The
host typically invokes an interrupt service routine to handle the condition thatcaused the request.

Interrupt Transfer

One of the four USB transfer types. Interrupt transfer characteristics are smalldata, non-periodic, low-frequency, and bounded-latency. Interrupt transfers
are typically used to handle service needs. See also Transfer Type.

I/O Request Packet

An identifiable request by a software client to move data between itself (on thehost) and an endpoint of a device in an appropriate direction.

IRP

See I/O Request Packet.

IRQ

See Interrupt Request.

Isochronous Data

A stream of data whose timing is implied by its delivery rate.

Isochronous Device

An entity with isochronous endpoints, as defined in the USB Specification,that sources or sinks sampled analog streams or synchronous data streams.

Isochronous Sink
Endpoint

An endpoint that is capable of consuming an isochronous data stream that issent by the host.

Isochronous Source
Endpoint

An endpoint that is capable of producing an isochronous data stream andsending it to the host.

IsochronousTransfer

One of the four USB transfer types. Isochronous transfers are used whenworking with isochronous data. Isochronous transfers provide periodic,
continuous communication between host and device. See also Transfer Type.

Universal Serial Bus Specification Revision 1.1

JitterA tendency toward lack of synchronization caused by mechanical or electricalchanges. More specifically, the phase shift of digital pulses over atransmission medium.

kb/sTransmission rate expressed in kilobits per second.

kB/sTransmission rate expressed in kilobytes per second.

Little Endian

Method of storing data that places the least significant byte of multiple-bytevalues at lower storage addresses. For example, a 16-bit integer stored in little
endian format places the least significant byte at the lower address and themost significant byte at the next address. See also Big Endian.

LOA

Loss of bus activity characterized by an SOP without a corresponding EOP.

LSb

Least significant bit.

LSB

Least significant byte.

Mb/sTransmission rate expressed in megabits per second.

MB/s

Transmission rate expressed in megabytes per second.

Message Pipe

A bi-directional pipe that transfers data using a request/data/status paradigm.
The data has an imposed structure that allows requests to be reliably identified
and communicated.

MSb

Most significant bit.

MSB

Most significant byte.

NAK

Handshake packet indicating a negative acknowledgment.

Non Return to ZeroInvert (NRZI)A method of encoding serial data in which ones and zeroes are represented by
opposite and alternating high and low voltages where there is no return to zero(reference) voltage between encoded bits. Eliminates the need for clock
pulses.

NRZI

See Non Return to Zero Invert.

ObjectHost software or data structure representing a USB entity.

PacketA bundle of data organized in a group for transmission. Packets typicallycontain three elements: control information (e.g., source, destination, andlength), the data to be transferred, and error detection and correction bits.

Packet Buffer

The logical buffer used by a USB device for sending or receiving a single
packet. This determines the maximum packet size the device can send or
receive.

Packet ID (PID)

A field in a USB packet that indicates the type of packet, and by inference, theformat of the packet and the type of error detection applied to the packet.

Phase

A token, data, or handshake packet; a transaction has three phases.

Phase Locked Loop
(PLL)

A circuit that acts as a phase detector to keep an oscillator in phase with anincoming frequency.

Physical Device

A device that has a physical implementation; e.g., speakers, microphones, andCD players.

PID

See Packet ID.

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Universal Serial Bus Specification Revision 1.1

8

Pipe

A logical abstraction representing the association between an endpoint on a
device and software on the host. A pipe has several attributes; for example, apipe may transfer data as streams (stream pipe) or messages (message pipe).
See also Stream Pipe and Message Pipe.

PLL

See Phase Locked Loop.

Polling

Asking multiple devices, one at a time, if they have any data to transmit.

POR

See Power On Reset.

Port

Point of access to or from a system or circuit. For the USB, the point where aUSB device is attached.

Power On Reset(POR)

Restoring a storage device, register, or memory to a predetermined state whenpower is applied.

Programmable
Data Rate

Either a fixed data rate (single-frequency endpoints), a limited number of data
rates (32kHz, 44.1kHz, 48kHz, …), or a continuously programmable data rate.
The exact programming capabilities of an endpoint must be reported in theappropriate class-specific endpoint descriptors.

ProtocolA specific set of rules, procedures, or conventions relating to format andtiming of data transmission between two devices.

RA

See Rate Adaptation.

Rate Adaptation

The process by which an incoming data stream, sampled at Fsi, is converted toan outgoing data stream, sampled at Fso,with a certain loss of quality,
determined by the rate adaptation algorithm. Error control mechanisms are
required for the process. Fsi and Fso can be different and asynchronous. Fsi isthe input data rate of the RA; Fso is the output data rate of the RA.

RequestA request made to a USB device contained within the data portion of a SETUP
packet.

Retire

The action of completing service for a transfer and notifying the appropriate
software client of the completion.

Root HubA USB hub directly attached to the Host Controller. This hub is attached tothe host (tier 0).

Root PortThe downstream port on a Root Hub.

SampleThe smallest unit of data on which an endpoint operates; a property of an
endpoint.

Sample Rate (Fs)

The number of samples per second, expressed in Hertz (Hz).

Sample Rate
Conversion (SRC)

A dedicated implementation of the RA process for use on sampled analog data
streams. The error control mechanism is replaced by interpolating techniques.

Service

A procedure provided by a System Programming Interface (SPI).

Service Interval

The period between consecutive requests to a USB endpoint to send or receive
data.

Service Jitter

The deviation of service delivery from its scheduled delivery time.

Service Rate

The number of services to a given endpoint per unit time.

SOF

See Start-of-Frame.

SOP

Start-of-Packet.

Universal Serial Bus Specification Revision 1.1

SPI

See System Programming Interface.

SRC

See Sample Rate Conversion.

Stage

One part of the sequence composing a control transfer; stages include the
Setup stage, the Data stage, and the Status stage.

Start-of-Frame(SOF)

The first transaction in each frame. An SOF allows endpoints to identify thestart of the frame and synchronize internal endpoint clocks to the host.

Stream Pipe

A pipe that transfers data as a stream of samples with no defined USB
structure.

Synchronization
Type

A classification that characterizes an isochronous endpoint’s capability toconnect to other isochronous endpoints.

Synchronous RA

The incoming data rate, Fsi, and the outgoing data rate, Fso, of the RA process
are derived from the same master clock. There is a fixed relation between Fsi
and Fso.

Synchronous SRC

The incoming sample rate, Fsi, and outgoing sample rate, Fso, of the SRC
process are derived from the same master clock. There is a fixed relation
between Fsi and Fso.

SystemProgramming
Interface (SPI)

A defined interface to services provided by system software.

TDM

See Time Division Multiplexing.

Termination

Passive components attached at the end of cables to prevent signals from beingreflected or echoed.

Time Division
Multiplexing(TDM)

A method of transmitting multiple signals (data, voice, and/or video)
simultaneously over one communications medium by interleaving a piece ofeach signal one after another.

Timeout

The detection of a lack of bus activity for some predetermined interval.

Token Packet

A type of packet that identifies what transaction is to be performed on the bus.

Transaction

The delivery of service to an endpoint; consists of a token packet, optional
data packet, and optional handshake packet. Specific packets areallowed/required based on the transaction type.

Transfer

One or more bus transactions to move information between a software clientand its function.

Transfer Type

Determines the characteristics of the data flow between a software client andits function. Four transfer types are defined: control, interrupt, bulk, andisochronous.

Turn-around TimeThe time a device needs to wait to begin transmitting a packet after a packet
has been received to prevent collisions on the USB. This time is based on thelength and propagation delay characteristics of the cable and the location of
the transmitting device in relation to other devices on the USB.

USBD

See Universal Serial Bus Driver.

Universal Serial
Bus Driver (USBD)

The host resident software entity responsible for providing common services
to clients that are manipulating one or more functions on one or more Host
Controllers.

9Universal Serial Bus Specification Revision 1.1

10

Universal Serial
Bus Resources

Resources provided by the USB, such as bandwidth and power. See also
Device Resources and Host Resources

Upstream

The direction of data flow towards the host. An upstream port is the port on a
device electrically closest to the host that generates upstream data traffic fromthe hub. Upstream ports receive downstream data traffic.

Virtual Device

A device that is represented by a software interface layer. An example of avirtual device is a hard disk with its associated device driver and client
software that makes it able to reproduce an audio .WAV file.

Word

A data element that is two bytes (16 bits) in size.

Universal Serial Bus Specification Revision 1.1

Chapter 3
Background
This chapter presents a brief description of the background of the Universal Serial Bus (USB), including
design goals, features of the bus, and existing technologies.

3.1

Goals for the Universal Serial Bus
The USB is specified to be an industry-standard extension to the PC architecture with a focus on Computer
Telephony Integration (CTI), consumer, and productivity applications. The following criteria were appliedin defining the architecture for the USB:

Ease-of-use for PC peripheral expansion



Low-cost solution that supports transfer rates up to 12Mb/sFull support for real-time data for voice, audio, and compressed videoProtocol flexibility for mixed-mode isochronous data transfers and asynchronous messaging

Integration in commodity device technologyComprehension of various PC configurations and form factors

Provision of a standard interface capable of quick diffusion into productEnablement of new classes of devices that augment the PC’s capability.

11

Universal Serial Bus Specification Revision 1.1

3.2

Taxonomy of Application Space
Figure 3-1 describes a taxonomy for the range of data traffic workloads that can be serviced over a USB.
As can be seen, a 12Mb/s bus comprehends the mid-speed and low-speed data ranges. Typically, midspeed data types are isochronous, while low-speed data comes from interactive devices. The USB being
proposed is primarily a desktop bus but can be readily applied to the mobile environment. The softwarearchitecture allows for future extension of the USB by providing support for multiple USB Host
Controllers.

PERFORMANCEAPPLICATIONSATTRIBUTESLOW-SPEED

Keyboard, Mouse
StylusGame Peripherals
Virtual Reality Peripherals
Monitor Configuration

Lower Cost
Hot Plug-unplug
Ease-of-use
Multiple Peripherals• Interactive Devices• 10 – 100kb/s

MEDIUM-SPEED• Phone, Audio,
Compressed Video
500Kb/S - 10Mb/s

ISDN
PBX
POTS
Audio

Low Cost
Ease-of-use
Guaranteed Latency
Guaranteed Bandwidth
Dynamic Attach-Detach
Multiple devices

HIGH-SPEED• Video, Disk

Video
Disk

• 25 - 500Mb/s

High Bandwidth
Guaranteed Latency
Ease-of-use

Figure 3-1. Application Space Taxonomy

3.3

Feature List
The USB Specification provides a selection of attributes that can achieve multiple price/performance
integration pointsi and can enable functions that allow differentiation at the system and component level.
Features are categorized by the following benefits:
Easy to use for end user

12Single model for cabling and connectors

Electrical details isolated from end user (e.g., bus terminations)

Self-identifying peripherals, automatic mapping of function to driver, and configuration



Dynamically attachable and reconfigurable peripherals

Universal Serial Bus Specification Revision 1.1

Wide range of workloads and applications
Suitable for device bandwidths ranging from a few kb/s to several Mb/s

Supports isochronous as well as asynchronous transfer types over the same set of wires



Supports concurrent operation of many devices (multiple connections)Supports up to 127 physical devices



Supports transfer of multiple data and message streams between the host and devicesAllows compound devices (i.e., peripherals composed of many functions)

Lower protocol overhead, resulting in high bus utilization

Isochronous bandwidthGuaranteed bandwidth and low latencies appropriate for telephony, audio, etc.



Isochronous workload may use entire bus bandwidth

FlexibilitySupports a wide range of packet sizes, which allows a range of device buffering options

Allows a wide range of device data rates by accommodating packet buffer size and latencies



Flow control for buffer handling is built into the protocol

RobustnessError handling/fault recovery mechanism is built into the protocolDynamic insertion and removal of devices is identified in user-perceived real-timeSupports identification of faulty devices

Synergy with PC industryProtocol is simple to implement and integrateConsistent with the PC plug-and-play architectureLeverages existing operating system interfaces

Low-cost implementation


Low-cost subchannel at 1.5Mb/s



Optimized for integration in peripheral and host hardwareSuitable for development of low-cost peripherals

Low-cost cables and connectors

Uses commodity technologies

Upgrade pathArchitecture upgradeable to support multiple USB Host Controllers in a system

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Universal Serial Bus Specification Revision 1.1

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Universal Serial Bus Specification Revision 1.1

Chapter 4
Architectural Overview
This chapter presents an overview of the Universal Serial Bus (USB) architecture and key concepts. The
USB is a cable bus that supports data exchange between a host computer and a wide range ofsimultaneously accessible peripherals. The attached peripherals share USB bandwidth through a hostscheduled, token-based protocol. The bus allows peripherals to be attached, configured, used, anddetached while the host and other peripherals are in operation.
Later chapters describe the various components of the USB in greater detail.

4.1 USB System Description
A USB system is described by three definitional areas:USB interconnectUSB devicesUSB host.

The USB interconnect is the manner in which USB devices are connected to and communicate with thehost. This includes the following:

Bus Topology: Connection model between USB devices and the host.

Inter-layer Relationships: In terms of a capability stack, the USB tasks that are performed at eachlayer in the system.



Data Flow Models: The manner in which data moves in the system over the USB between producers
and consumers.

USB Schedule: The USB provides a shared interconnect. Access to the interconnect is scheduled inorder to support isochronous data transfers and to eliminate arbitration overhead.

USB devices and the USB host are described in detail in subsequent sections.

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Universal Serial USB Specification Revision 1.1

4.1.1 Bus Topology
The USB connects USB devices with the USB host. The USB physical interconnect is a tiered star
topology. A hub is at the center of each star. Each wire segment is a point-to-point connection between
the host and a hub or function, or a hub connected to another hub or function. Figure 4-1 illustrates the
topology of the USB.

Host
RootHub

Host (Root Tier)
Tier 1

Hub 1

Tier 2
Node
Hub 2

Node

Tier 3
Hub 3

Hub 4

Node

Node

Tier 4
Node

Node

Node

Figure 4-1. Bus Topology

4.1.1.1 USB Host
There is only one host in any USB system. The USB interface to the host computer system is referred to as
the Host Controller. The Host Controller may be implemented in a combination of hardware, firmware, orsoftware. A root hub is integrated within the host system to provide one or more attachment points.
Additional information concerning the host may be found in Section 4.9 and in Chapter 10.

4.1.1.2 USB Devices
USB devices are one of the following:16



Hubs, which provide additional attachment points to the USB

Functions, which provide capabilities to the system, such as an ISDN connection, a digital joystick, or
speakers.

Universal Serial Bus Specification Revision 1.1

USB devices present a standard USB interface in terms of the following:

Their comprehension of the USB protocol



Their response to standard USB operations, such as configuration and resetTheir standard capability descriptive information.

Additional information concerning USB devices may be found in Section 4.8 and in Chapter 9.

4.2 Physical Interface
The physical interface of the USB is described in the electrical (Chapter 7) and mechanical (Chapter 6)
specifications for the bus.

4.2.1 Electrical
The USB transfers signal and power over a four-wire cable, shown in Figure 4-2. The signaling occurs
over two wires on each point-to-point segment.
There are two data rates:


The USB full-speed signaling bit rate is 12Mb/s.



A limited capability low-speed signaling mode is also defined at 1.5Mb/s.

The low-speed mode requires less EMI protection. Both modes can be supported in the same USB bus byautomatic dynamic mode switching between transfers. The low-speed mode is defined to support a limited
number of low-bandwidth devices, such as mice, because more general use would degrade bus utilization.
The clock is transmitted, encoded along with the differential data. The clock encoding scheme is NRZI
with bit stuffing to ensure adequate transitions. A SYNC field precedes each packet to allow thereceiver(s) to synchronize their bit recovery clocks.

VBUS
D+
DGND

...
...

VBUS
D+
DGND

Figure 4-2. USB Cable
The cable also carries VBUS and GND wires on each segment to deliver power to devices. VBUS is
nominally +5V at the source. The USB allows cable segments of variable lengths, up to several meters, bychoosing the appropriate conductor gauge to match the specified IR drop and other attributes such asdevice power budget and cable flexibility. In order to provide guaranteed input voltage levels and proper
termination impedance, biased terminations are used at each end of the cable. The terminations also permit
the detection of attach and detach at each port and differentiate between full-speed and low-speed devices.

4.2.2 Mechanical
The mechanical specifications for cables and connectors are provided in Chapter 6. All devices have an
upstream connection. Upstream and downstream connectors are not mechanically interchangeable, thus
eliminating illegal loopback connections at hubs. The cable has four conductors: a twisted signal pair ofstandard gauge and a power pair in a range of permitted gauges. The connector is four-position, withshielded housing, specified robustness, and ease of attach-detach characteristics.

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Universal Serial USB Specification Revision 1.1

4.3 Power
The specification covers two aspects of power:Power distribution over the USB deals with the issues of how USB devices consume power provided
by the host over the USB.

Power management deals with how the USB System Software and devices fit into the host-based
power management system.

4.3.1 Power Distribution
Each USB segment provides a limited amount of power over the cable. The host supplies power for use by
USB devices that are directly connected. In addition, any USB device may have its own power supply.
USB devices that rely totally on power from the cable are called bus-powered devices. In contrast, thosethat have an alternate source of power are called self-powered devices. A hub also supplies power for its
connected USB devices. The architecture permits bus-powered hubs within certain constraints of topologythat are discussed later in Chapter 11. In Figure 4-4 (see Section 4.8.2.1), the keyboard, pen, and mouse
can all be bus-powered devices.

4.3.2 Power Management
A USB host may have a power management system that is independent of the USB. The USB System
Software interacts with the host’s power management system to handle system power events such assuspend or resume. Additionally, USB devices typically implement additional power management featuresthat allow them to be power managed by system software.
The power distribution and power management features of the USB allow it to be designed into powersensitive systems such as battery-based notebook computers.

4.4 Bus Protocol
The USB is a polled bus. The Host Controller initiates all data transfers.
All bus transactions involve the transmission of up to three packets. Each transaction begins when the Host
Controller, on a scheduled basis, sends a USB packet describing the type and direction of transaction, theUSB device address, and endpoint number. This packet is referred to as the “token packet.” The USB
device that is addressed selects itself by decoding the appropriate address fields. In a given transaction,
data is transferred either from the host to a device or from a device to the host. The direction of datatransfer is specified in the token packet. The source of the transaction then sends a data packet or indicatesit has no data to transfer. The destination, in general, responds with a handshake packet indicating whether
the transfer was successful.
The USB data transfer model between a source or destination on the host and an endpoint on a device isreferred to as a pipe. There are two types of pipes: stream and message. Stream data has no USB-definedstructure, while message data does. Additionally, pipes have associations of data bandwidth, transfer
service type, and endpoint characteristics like directionality and buffer sizes. Most pipes come into
existence when a USB device is configured. One message pipe, the Default Control Pipe, always exists
once a device is powered, in order to provide access to the device’s configuration, status, and control
information.
The transaction schedule allows flow control for some stream pipes. At the hardware level, this prevents
buffers from underrun or overrun situations by using a NAK handshake to throttle the data rate. When
NAKed, a transaction is retried when bus time is available. The flow control mechanism permits theconstruction of flexible schedules that accommodate concurrent servicing of a heterogeneous mix of stream
pipes. Thus, multiple stream pipes can be serviced at different intervals and with packets of different sizes.

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Universal Serial Bus Specification Revision 1.1

4.5 Robustness
There are several attributes of the USB that contribute to its robustness:

Signal integrity using differential drivers, receivers, and shielding

CRC protection over control and data fields



Detection of attach and detach and system-level configuration of resources

Self-recovery in protocol, using timeouts for lost or corrupted packets

Flow control for streaming data to ensure isochrony and hardware buffer management

Data and control pipe constructs for ensuring independence from adverse interactions between
functions.

4.5.1 Error Detection
The core bit error rate of the USB medium is expected to be close to that of a backplane and any glitches
will very likely be transient in nature. To provide protection against such transients, each packet includes
error protection fields. When data integrity is required, such as with lossless data devices, an error
recovery procedure may be invoked in hardware or software.
The protocol includes separate CRCs for control and data fields of each packet. A failed CRC isconsidered to indicate a corrupted packet. The CRC gives 100% coverage on single- and double-bit errors.

4.5.2 Error Handling
The protocol allows for error handling in hardware or software. Hardware error handling includes
reporting and retry of failed transfers. A USB Host Controller will try a transmission that encounters errors
up to three times before informing the client software of the failure. The client software can recover in an
implementation-specific way.

4.6 System Configuration
The USB supports USB devices attaching to and detaching from the USB at any time. Consequently,
system software must accommodate dynamic changes in the physical bus topology.

4.6.1 Attachment of USB Devices
All USB devices attach to the USB through ports on specialized USB devices known as hubs. Hubs havestatus indicators that indicate the attachment or removal of a USB device on one of its ports. The hostqueries the hub to retrieve these indicators In the case of an attachment, the host enables the port andaddresses the USB device through the device’s control pipe at the default address.
The host assigns a unique USB address the to the device and then determines if the newly attached USB
device is a hub or a function The host establishes its end of the control pipe for the USB device using theassigned USB address and endpoint number zero.
If the attached USB device is a hub and USB devices are attached to its ports, then the above procedure isfollowed for each of the attached USB devices.
If the attached USB device is a function, then attachment notifications will be handled by host software thatis appropriate for the function.

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Universal Serial USB Specification Revision 1.1

4.6.2 Removal of USB Devices
When a USB device has been removed from one of a hub’s ports, the hub disables the port and provides anindication of device removal to the host The removal indication is then handled by appropriate USB
System Software. If the removed USB device is a hub, the USB System Software must handle the removalof both the hub and of all of the USB devices that were previously attached to the system through the hub.

4.6.3 Bus Enumeration
Bus enumeration is the activity that identifies and assigns unique addresses to devices attached to a bus.
Because the USB allows USB devices to attach to or detach from the USB at any time, bus enumeration isan on-going activity for the USB System Software. Additionally, bus enumeration for the USB also
includes the detection and processing of removals.

4.7 Data Flow Types
The USB supports functional data and control exchange between the USB host and a USB device as a setof either uni-directional or bi-directional pipes. USB data transfers take place between host software and aparticular endpoint on a USB device. Such associations between the host software and a USB device
endpoint are called pipes. In general, data movement though one pipe is independent from the data flow in
any other pipe. A given USB device may have many pipes. As an example, a given USB device could
have an endpoint that supports a pipe for transporting data to the USB device and another endpoint that
supports a pipe for transporting data from the USB device.
The USB architecture comprehends four basic types of data transfers:
Control Transfers: Used to configure a device at attach time and can be used for other device-specificpurposes, including control of other pipes on the device.

Bulk Data Transfers: Generated or consumed in relatively large and bursty quantities and have wide
dynamic latitude in transmission constraints.

Interrupt Data Transfers: Used for characters or coordinates with human-perceptible echo or feedback
response characteristics.

Isochronous Data Transfers: Occupy a prenegotiated amount of USB bandwidth with a prenegotiateddelivery latency. (Also called streaming real time transfers).

A pipe supports only one of the types of transfers described above for any given device configuration. The
USB data flow model is described in more detail in Chapter 5.

4.7.1 Control Transfers
Control data is used by the USB System Software to configure devices when they are first attached. Other
driver software can choose to use control transfers in implementation-specific ways. Data delivery islossless.

4.7.2 Bulk Transfers
Bulk data typically consists of larger amounts of data, such as that used for printers or scanners. Bulk data
is sequential. Reliable exchange of data is ensured at the hardware level by using error detection in
hardware and invoking a limited number of retries in hardware. Also, the bandwidth taken up by bulk data
can vary, depending on other bus activities.

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Universal Serial Bus Specification Revision 1.1

4.7.3 Interrupt Transfers
A small, limited-latency transfer to or from a device is referred to as interrupt data. Such data may bepresented for transfer by a device at any time and is delivered by the USB at a rate no slower than is
specified by the device.
Interrupt data typically consists of event notification, characters, or coordinates that are organized as one ormore bytes. An example of interrupt data is the coordinates from a pointing device. Although an explicit
timing rate is not required, interactive data may have response time bounds that the USB must support.

4.7.4 Isochronous Transfers
Isochronous data is continuous and real-time in creation, delivery, and consumption. Timing-related
information is implied by the steady rate at which isochronous data is received and transferred.
Isochronous data must be delivered at the rate received to maintain its timing. In addition to delivery rate,isochronous data may also be sensitive to delivery delays. For isochronous pipes, the bandwidth required
is typically based upon the sampling characteristics of the associated function. The latency required isrelated to the buffering available at each endpoint.
A typical example of isochronous data is voice. If the delivery rate of these data streams is not maintained,
drop-outs in the data stream will occur due to buffer or frame underruns or overruns. Even if data isdelivered at the appropriate rate by USB hardware, delivery delays introduced by software may degrade
applications requiring real-time turn-around, such as telephony-based audio conferencing.
The timely delivery of isochronous data is ensured at the expense of potential transient losses in the data
stream. In other words, any error in electrical transmission is not corrected by hardware mechanisms suchas retries. In practice, the core bit error rate of the USB is expected to be small enough not to be an issue.
USB isochronous data streams are allocated a dedicated portion of USB bandwidth to ensure that data canbe delivered at the desired rate. The USB is also designed for minimal delay of isochronous data transfers.

4.7.5 Allocating USB Bandwidth
USB bandwidth is allocated among pipes. The USB allocates bandwidth for some pipes when a pipe isestablished. USB devices are required to provide some buffering of data. It is assumed that USB devices
requiring more bandwidth are capable of providing larger buffers. The goal for the USB architecture is toensure that buffering-induced hardware delay is bounded to within a few milliseconds.
The USB’s bandwidth capacity can be allocated among many different data streams. This allows a wide
range of devices to be attached to the USB. For example, telephony devices ranging from 1B+D all theway up to T1 capacity can be accommodated. Further, different device bit rates, with a wide dynamic
range, can be concurrently supported.
The USB Specification defines the rules for how each transfer type is allowed access to the bus.

4.8 USB Devices
USB devices are divided into device classes such as hub, locator, or text device. The hub device classindicates a specially designated USB device that provides additional USB attachment points (refer to
Chapter 11). USB devices are required to carry information for self-identification and genericconfiguration. They are also required at all times to display behavior consistent with defined USB device
states.

4.8.1 Device Characterizations
All USB devices are accessed by a USB address that is assigned when the device is attached andenumerated. Each USB device additionally supports one or more pipes through which the host may
communicate with the device. All USB devices must support a specially designated pipe at endpoint zero21

Universal Serial USB Specification Revision 1.1

to which the USB device’s USB control pipe will be attached. All USB devices support a common
accesses mechanism for accessing information through this control pipe.
Associated with the control pipe at endpoint zero is the information required to completely describe theUSB device. This information falls into the following categories:Standard: This is information whose definition is common to all USB devices and includes items suchas vendor identification, device class, and power management. Device, configuration, interface, andendpoint descriptions carry configuration-related information about the device. Detailed information
about these descriptors can be found in Chapter 9.

Class: The definition of this information varies, depending on the device class of the USB device.



USB Vendor: The vendor of the USB device is free to put any information desired here. The format,
however, is not determined by this specification.

Additionally, each USB device carries USB control and status information.

4.8.2 Device Descriptions
Two major divisions of device classes exist: hubs and functions. Only hubs have the ability to provide
additional USB attachment points. Functions provide additional capabilities to the host.

4.8.2.1 Hubs
Hubs are a key element in the plug-and-play architecture of the USB. Figure 4-3 shows a typical hub.
Hubs serve to simplify USB connectivity from the user’s perspective and provide robustness at low costand complexity.
Hubs are wiring concentrators and enable the multiple attachment characteristics of the USB. Attachment
points are referred to as ports. Each hub converts a single attachment point into multiple attachment points.
The architecture supports concatenation of multiple hubs.
The upstream port of a hub connects the hub towards the host. Each of the downstream ports of a hub
allows connection to another hub or function. Hubs can detect attach and detach at each downstream port
and enable the distribution of power to downstream devices. Each downstream port can be individuallyenabled and attached to either full- or low-speed devices. The hub isolates low-speed ports from full-speed
signaling.
A hub consists of two portions: the Hub Controller and the Hub Repeater. The Hub Repeater is a
protocol-controlled switch between the upstream port and downstream ports. It also has hardware support
for reset and suspend/resume signaling. The Host Controller provides the interface registers to allow
communication to/from the host. Hub-specific status and control commands permit the host to configure a
hub and to monitor and control its ports.

Port
#1

Upstream
Port

Port
#2

HUB Port
#4

Port
#7
Figure 4-3. A Typical Hub
22

Port
#3

Port
#6

Port
#5

Universal Serial Bus Specification Revision 1.1

Figure 4-4 illustrates how hubs provide connectivity in a typical computer environment.

USB

TYPICAL USB ARCHITECTURALCONFIGURATION

Hub/Function

Hub/Function

Host/Hub

KBD

Monitor

PC

Pen

Mouse

Speaker

Mic

Phone

Hub

Function

Function

Function

Function

Function

Hub

Figure 4-4. Hubs in a Desktop Computer Environment

4.8.2.2 Functions
A function is a USB device that is able to transmit or receive data or control information over the bus. A
function is typically implemented as a separate peripheral device with a cable that plugs into a port on ahub. However, a physical package may implement multiple functions and an embedded hub with a single
USB cable. This is known as a compound device. A compound device appears to the host as a hub with
one or more non-removable USB devices.
Each function contains configuration information that describes its capabilities and resource requirements.
Before a function can be used, it must be configured by the host. This configuration includes allocating
USB bandwidth and selecting function-specific configuration options.
Examples of functions include the following:

A locator device such as a mouse, tablet, or light pen



An input device such as a keyboard

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Universal Serial USB Specification Revision 1.1



An output device such as a printer

A telephony adapter such as ISDN.

4.9 USB Host: Hardware and Software
The USB host interacts with USB devices through the Host Controller. The host is responsible for thefollowing:

Detecting the attachment and removal of USB devices

Managing control flow between the host and USB devices

Managing data flow between the host and USB devices

Collecting status and activity statistics



Providing power to attached USB devices.

The USB System Software on the host manages interactions between USB devices and host-based device
software. There are five areas of interactions between the USB System Software and device software:

Device enumeration and configurationIsochronous data transfers

Asynchronous data transfersPower management

Device and bus management information.

Whenever possible, the USB System Software uses existing host system interfaces to manage the above
interactions.

4.10 Architectural Extensions
The USB architecture comprehends extensibility at the interface between the Host Controller Driver andUSB Driver. Implementations with multiple Host Controllers, and associated Host Controller Drivers, arepossible.

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Universal Serial Bus Specification Revision 1.1

Chapter 5
USB Data Flow Model
This chapter presents information about how data is moved across the USB. The information in thischapter affects all implementers. The information presented is at a level above the signaling and protocol
definitions of the system. Consult Chapter 7 and Chapter 8 for more details about their respective parts of
the USB system. This chapter provides framework information that is further expanded in Chapters 9through 11. All implementers should read this chapter so they understand the key concepts of the USB.

5.1 Implementer Viewpoints
The USB provides communication services between a host and attached USB devices. However, thesimple view an end user sees of attaching one or more USB devices to a host, as in Figure 5-1, is in fact alittle more complicated to implement than is indicated by the figure. Different views of the system are
required to explain specific USB requirements from the perspective of different implementers. Several
important concepts and features must be supported to provide the end user with the reliable operation
demanded from today’s personal computers. The USB is presented in a layered fashion to ease explanation
and allow implementers of particular USB products to focus on the details related to their product.

USB Host

USB Device

Figure 5-1. Simple USB Host/Device View
Figure 5-2 shows a deeper overview of the USB, identifying the different layers of the system that will be
described in more detail in the remainder of the specification. In particular, there are four focus
implementation areas:USB Physical Device: A piece of hardware on the end of a USB cable that performs some useful end
user function.



Client Software: Software that executes on the host, corresponding to a USB device. This client
software is typically supplied with the operating system or provided along with the USB device.



USB System Software: Software that supports the USB in a particular operating system. The USB
System Software is typically supplied with the operating system, independently of particular USB
devices or client software.



USB Host Controller (Host Side Bus Interface): The hardware and software that allows USB devices
to be attached to a host.

There are shared rights and responsibilities between the four USB system components. The remainder ofthis specification describes the details required to support robust, reliable communication flows between afunction and its client.

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Universal Serial Bus Specification Revision 1.1

HostInterconnect

Physical Device

Client SW

Function

USB System
SW

USB Logical
DeviceUSB Host
Controller

USB BusInterface

Function Layer

USB Device
Layer

USB BusInterface Layer

Actual communications flow
Logical communications flow
Implementation Focus Area

Figure 5-2. USB Implementation Areas
As shown in Figure 5-2, the simple connection of a host to a device requires interaction between a numberof layers and entities. The USB Bus Interface layer provides physical/signaling/packet connectivity
between the host and a device. The USB Device Layer is the view the USB System Software has for
performing generic USB operations with a device. The Function Layer provides additional capabilities tothe host via an appropriate matched client software layer. The USB Device and Function layers each havea view of logical communication within their layer that actually uses the USB Bus Interface Layer toaccomplish data transfer.
The physical view of USB communication as described in Chapters 6, 7, and 8 is related to the logicalcommunication view presented in Chapters 9 and 10. This chapter describes those key concepts that affect
USB implementers and should be read by all before proceeding to the remainder of the specification to findthose details most relevant to their product.
To describe and manage USB communication, the following concepts are important:26



Bus Topology: Section 5.2 presents the primary physical and logical components of the USB and how
they interrelate.

Communication Flow Models: Sections 5.3 through 5.8 describe how communication flows between
the host and devices through the USB and defines the four USB transfer types.



Bus Access Management: Section 5.9 describes how bus access is managed within the host to support
a broad range of communication flows by USB devices.



Special Consideration for Isochronous Transfers: Section 5.10 presents features of the USB specific todevices requiring isochronous data transfers. Device implementers for non-isochronous devices do notneed to read Section 5.10.

Universal Serial Bus Specification Revision 1.1

5.2 Bus Topology
There are four main parts to USB topology:

Host and Devices: The primary components of a USB system.

Physical Topology: How USB elements are connected.

Logical Topology: The roles and responsibilities of the various USB elements and how the USB
appears from the perspective of the host and a device.

Client Software-to-function Relationships: How client software and its related function interfaces on a
USB device view each other.

5.2.1 USB Host
The host’s logical composition is shown in Figure 5-3, and includes the following:

USB Host Controller



Aggregate USB System Software (USB Driver, Host Controller Driver, and host software)

Client.
Host

Client SW

USB System SW

USB Host
Controller
Actual communications flow
Logical communications flow

Figure 5-3. Host Composition
The USB host occupies a unique position as the coordinating entity for the USB. In addition to its special
physical position, the host has specific responsibilities with regard to the USB and its attached devices.
The host controls all access to the USB. A USB device gains access to the bus only by being granted
access by the host. The host is also responsible for monitoring the topology of the USB.
For a complete discussion of the host and its duties, refer to Chapter 10.

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Universal Serial Bus Specification Revision 1.1

5.2.2 USB Devices
A USB physical device’s logical composition is shown in Figure 5-4, and includes the following:USB bus interfaceUSB logical deviceFunction.

USB physical devices provide additional functionality to the host. The types of functionality provided by
USB devices vary widely. However, all USB logical devices present the same basic interface to the host.
This allows the host to manage the USB-relevant aspects of different USB devices in the same manner.
To assist the host in identifying and configuring USB devices, each device carries and reports
configuration-related information. Some of the information reported is common among all logical devices.
Other information is specific to the functionality provided by the device. The detailed format of thisinformation varies, depending on the device class of the device.
For a complete discussion of USB devices, refer to Chapter 9.
Physical Device

Function

USB Logical
DeviceUSB BusInterface
Actual communications flow
Logical communications flow

Figure 5-4. Physical Device Composition28

Universal Serial Bus Specification Revision 1.1

5.2.3 Physical Bus Topology
Devices on the USB are physically connected to the host via a tiered star topology, as illustrated in
Figure 5-5. USB attachment points are provided by a special class of USB device known as a hub. The
additional attachment points provided by a hub are called ports. A host includes an embedded hub called
the root hub. The host provides one or more attachment points via the root hub. USB devices that provide
additional functionality to the host are known as functions. To prevent circular attachments, a tiered
ordering is imposed on the star topology of the USB. This results in the tree-like configuration illustrated
in Figure 5-5.

Host
DeviceDeviceRoot HubCompound Device

Hub

DeviceHub

DeviceDeviceDeviceDeviceFigure 5-5. USB Physical Bus Topology
Multiple functions may be packaged together in what appears to be a single physical device. For example,
a keyboard and a trackball might be combined in a single package. Inside the package, the individualfunctions are permanently attached to a hub and it is the internal hub that is connected to the USB. Whenmultiple functions are combined with a hub in a single package, they are referred to as a compound device.
From the host’s perspective, a compound device is the same as a separate hub with multiple functions
attached. Figure 5-5 also illustrates a compound device.

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Universal Serial Bus Specification Revision 1.1

5.2.4 Logical Bus Topology
While devices physically attach to the USB in a tiered, star topology, the host communicates with each
logical device as if it were directly connected to the root port. This creates the logical view illustrated in
Figure 5-6 that corresponds to the physical topology shown in Figure 5-5. Hubs are logical devices also,
but are not shown in Figure 5-6 to simplify the picture. Even though most host/logical device activities use
this logical perspective, the host maintains an awareness of the physical topology to support processing theremoval of hubs. When a hub is removed, all of the devices attached to the hub must be removed from thehost’s view of the logical topology. A more complete discussion of hubs can be found in Chapter 11.
Host

Logical
DeviceLogical
DeviceLogical
DeviceLogical
DeviceLogical
DeviceLogical
DeviceLogical
DeviceFigure 5-6. USB Logical Bus Topology

5.2.5 Client Software-to-function Relationship
Even though the physical and logical topology of the USB reflects the shared nature of the bus, client
software (CSw) manipulating a USB function interface is presented with the view that it deals only with its
interface(s) of interest. Client software for USB functions must use USB software programming interfaces
to manipulate their functions as opposed to directly manipulating their functions via memory or I/O
accesses as with other buses (e.g., PCI, EISA, PCMCIA, etc.). During operation, client software should be
independent of other devices that may be connected to the USB. This allows the designer of the device andclient software to focus on the hardware/software interaction design details. Figure 5-7 illustrates a device
designer’s perspective of the relationships of client software and USB functions with respect to the USB
logical topology of Figure 5-6.

ClientSoftware

Func

CSw
CSw

CSw
CSw

CSw

CSw
Func

Func

Func
Func

Func
Func

Figure 5-7. Client Software-to-function Relationships

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Universal Serial Bus Specification Revision 1.1

5.3 USB Communication Flow
The USB provides a communication service between software on the host and its USB function. Functions
can have different communication flow requirements for different client-to-function interactions. The USB
provides better overall bus utilization by allowing the separation of the different communication flows to a
USB function. Each communication flow makes use of some bus access to accomplish communication
between client and function. Each communication flow is terminated at an endpoint on a device. Device
endpoints are used to identify aspects of each communication flow.
Figure 5-8 shows a more detailed view of Figure 5-2. The complete definition of the actual
communication flows of Figure 5-2 supports the logical device and function layer communication flows.
These actual communication flows cross several interface boundaries. Chapters 6 through 8 describe themechanical, electrical, and protocol interface definitions of the USB “wire.” Chapter 9 describes the USB
device programming interface that allows a USB device to be manipulated from the host side of the wire.
Chapter 10 describes two host side software interfaces:

Host Controller Driver (HCD): The software interface between the USB Host Controller and USB
System Software. This interface allows a range of Host Controller implementations without requiring
all host software to be dependent on any particular implementation. One USB Driver can support
different Host Controllers without requiring specific knowledge of a Host Controller implementation.
A Host Controller implementer provides an HCD implementation that supports the Host Controller.



USB Driver (USBD): The interface between the USB System Software and the client software. This
interface provides clients with convenient functions for manipulating USB devices.
Host

Interconnect

Physical Device

Function

Client SW

a collection ofinterfaces

Interface x

manages an interface

Pipe Bundle
to an interface

Buffers

Interfacespecific

No USB
FormatNo USB
FormatEndpoint
Zero

USB System SW
manages devices

Default Pipe

USB Logical
Devicea collection of
endpoints

USB Device

to Endpoint Zero

(Chapter 9)
Transfers

USB
FramedData

USB BusInterface

USB Host(Chapter 10)

Data Per
Endpoint

USB
FramedData

USB BusInterface

Host
Controller USB Framed
Data
SIE
Transactions

SIE

USB Wire

Pipe: represents connection abstraction
between two horizontal entities

Mechanical,
Data transport mechanism

Electrical,
Protocol

USB-relevant format of transported data

(Chapter 6, 7, 8)

Figure 5-8. USB Host/Device Detailed View

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Universal Serial Bus Specification Revision 1.1

A USB logical device appears to the USB system as a collection of endpoints. Endpoints are grouped into
endpoint sets that implement an interface. Interfaces are views to the function. The USB System Software
manages the device using the Default Control Pipe. Client software manages an interface using pipe
bundles (associated with an endpoint set). Client software requests that data be moved across the USB
between a buffer on the host and an endpoint on the USB device. The Host Controller (or USB device,depending on transfer direction) packetizes the data to move it over the USB. The Host Controller also
coordinates when bus access is used to move the packet of data over the USB.
Figure 5-9 illustrates how communication flows are carried over pipes between endpoints and host side
memory buffers. The following sections describe endpoints, pipes, and communication flows in moredetail.
Host

ClientSoftware
Buffers
Communication
Flows

Pipes

Endpoints

USB Logical Device
Interface
Figure 5-9. USB Communication Flow

Software on the host communicates with a logical device via a set of communication flows. The set ofcommunication flows are selected by the device software/hardware designer(s) to efficiently match thecommunication requirements of the device to the transfer characteristics provided by the USB.

5.3.1 Device Endpoints
An endpoint is a uniquely identifiable portion of a USB device that is the terminus of a communication
flow between the host and device. Each USB logical device is composed of a collection of independent
endpoints. Each logical device has a unique address assigned by the system at device attachment time.
Each endpoint on a device is given at design time a unique device-determined identifier called the endpoint
number. Each endpoint has a device-determined direction of data flow. The combination of the device
address, endpoint number, and direction allows each endpoint to be uniquely referenced. Each endpoint isa simplex connection that supports data flow in one direction: either input (from device to host) or output(from host to device).
An endpoint has characteristics that determine the type of transfer service required between the endpoint
and the client software. Endpoints describe themselves by:32Their bus access frequency/latency requirements



Their bandwidth requirements



Their endpoint numberThe error handling behavior requirements

Maximum packet size that the endpoint is capable of sending or receiving

Universal Serial Bus Specification Revision 1.1



The transfer type for the endpoint (refer to Section 5.4 for details)

The direction data is transferred between the endpoint and the host.

Endpoints other than those with endpoint number zero are in an unknown state before being configured
and may not be accessed by the host before being configured.

5.3.1.1 Endpoint Zero Requirements
All USB devices are required to implement a default control method that uses both the input and output
endpoints with endpoint number zero. The USB System Software uses this default control method toinitialize and generically manipulate the logical device (e.g., to configure the logical device) as the Default
Control Pipe (see Section 5.3.2). The Default Control Pipe provides access to the device’s configuration
information and allows generic USB status and control access. The Default Control Pipe supports control
transfers as defined in Section 5.5. The endpoints with endpoint number zero are always accessible once a
device is attached, powered, and has received a bus reset.

5.3.1.2 Non-endpoint Zero Requirements
Functions can have additional endpoints as required for their implementation. Low-speed functions arelimited to two optional endpoints beyond the two required to implement the Default Control Pipe. Fullspeed devices can have additional endpoints only limited by the protocol definition (i.e., a maximum of 15
additional input endpoints and 15 additional output endpoints).
Endpoints other than those for the Default Control Pipe cannot be used until the device is configured as anormal part of the device configuration process (refer to Chapter 9).

5.3.2 Pipes
A USB pipe is an association between an endpoint on a device and software on the host. Pipes represent
the ability to move data between software on the host via a memory buffer and an endpoint on a device.
There are two different, mutually exclusive, pipe communication modes:Stream: Data moving through a pipe has no USB-defined structure

Message: Data moving through a pipe has some USB-defined structure.

The USB does not interpret the content of data it delivers through a pipe. Even though a message pipe
requires that data be structured according to USB definitions, the content of the data is not interpreted by
the USB.
Additionally, pipes have the following associated with them:

A claim on USB bus access and bandwidth usage.



A transfer type.



The associated endpoint’s characteristics, such as directionality and maximum data payload sizes. The
data payload is the data that is carried in the data field of a data packet within a bus transaction (asdefined in Chapter 8).

The pipe that consists of the two endpoints with endpoint number zero is called the Default Control Pipe.
This pipe is always available once a device is powered and has received a bus reset. Other pipes come into
existence when a USB device is configured. The Default Control Pipe is used by the USB System
Software to determine device identification and configuration requirements, and to configure the device.
The Default Control Pipe can also be used by device-specific software after the device is configured. The
USB System Software retains “ownership” of the Default Control Pipe and mediates use of the pipe byother client software.

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Universal Serial Bus Specification Revision 1.1

A software client normally requests data transfers via I/O Request Packets (IRPs) to a pipe and then either
waits or is notified when they are completed. Details about IRPs are defined in an operating systemspecific manner. This specification uses the term to simply refer to an identifiable request by a software
client to move data between itself (on the host) and an endpoint of a device in an appropriate direction. A
software client can cause a pipe to return all outstanding IRPs if it desires. The software client is notified
that an IRP has completed when the bus transactions associated with it have completed either successfullyor due to errors.
If there are no IRPs pending or in progress for a pipe, the pipe is idle and the Host Controller will take noaction with regard to the pipe; i.e., the endpoint for such a pipe will not see any bus transactions directed toit. The only time bus activity is present for a pipe is when IRPs are pending for that pipe.
If a non-isochronous pipe encounters a condition that causes it to send a STALL to the host (refer to
Chapter 8) or three bus errors are encountered on any packet of an IRP, the IRP is aborted/retired, alloutstanding IRPs are also retired, and no further IRPs are accepted until the software client recovers fromthe condition (in an implementation-dependent way) and acknowledges the halt or error condition via a
USBD call. An appropriate status informs the software client of the specific IRP result for error versus halt
(refer to Chapter 10). Isochronous pipe behavior is described in Section 5.6.
An IRP may require multiple data payloads to move the client data over the bus. The data payloads forsuch a multiple data payload IRP are expected to be of the maximum packet size until the last data payloadthat contains the remainder of the overall IRP. See the description of each transfer type for more details.
For such an IRP, short packets (i.e., less than maximum-sized data payloads) on input that do notcompletely fill an IRP data buffer can have one of two possible meanings, depending upon the expectationsof a client:A client can expect a variable-sized amount of data in an IRP. In this case, a short packet that does notfill an IRP data buffer can be used simply as an in-band delimiter to indicate “end of unit of data.”
The IRP should be retired without error and the Host Controller should advance to the next IRP.



A client can expect a specific-sized amount of data. In this case, a short packet that does not fill an
IRP data buffer is an indication of an error. The IRP should be retired, the pipe should be stalled, andany pending IRPs associated with the pipe should also be retired.

Because the Host Controller must behave differently in the two cases and cannot know on its own which
way to behave for a given IRP, it is possible to indicate per IRP which behavior the client desires.
An endpoint can inform the host that it is busy by responding with NAK. NAKs are not used as a retirecondition for returning an IRP to a software client. Any number of NAKs can be encountered during theprocessing of a given IRP. A NAK response to a transaction does not constitute an error and is not counted
as one of the three errors described above.

5.3.2.1 Stream Pipes
Stream pipes deliver data in the data packet portion of bus transactions with no USB-required structure on
the data content. Data flows in at one end of a stream pipe and out the other end in the same order. Stream
pipes are always uni-directional in their communication flow.
Data flowing through a stream pipe is expected to interact with what the USB believes is a single client.
The USB System Software is not required to provide synchronization between multiple clients that may beusing the same stream pipe. Data presented to a stream pipe is moved through the pipe in sequential order:
first-in, first-out.
A stream pipe to a device is bound to a single device endpoint number in the appropriate direction (i.e.,corresponding to an IN or OUT token as defined by the protocol layer). The device endpoint number forthe opposite direction can be used for some other stream pipe to the device.
Stream pipes support bulk, isochronous, and interrupt transfer types, which are explained in later sections.

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Universal Serial Bus Specification Revision 1.1

5.3.2.2 Message Pipes
Message pipes interact with the endpoint in a different manner than stream pipes. First, a request is sent to
the USB device from the host. This request is followed by data transfer(s) in the appropriate direction.
Finally, a Status stage follows at some later time. In order to accommodate the request/data/status
paradigm, message pipes impose a structure on the communication flow that allows commands to bereliably identified and communicated. Message pipes allow communication flow in both directions,although the communication flow may be predominately one-way. The Default Control Pipe is always amessage pipe.
The USB System Software ensures that multiple requests are not sent to a message pipe concurrently. A
device is required to service only a single message request at a time per message pipe. Multiple software
clients on the host can make requests via the Default Control Pipe, but they are sent to the device in a firstin, first-out order. A device can control the flow of information during the Data and Status stages based on
its ability to respond to the host transactions (refer to Chapter 8 for more details).
A message pipe will not normally be sent the next message from the host until the current message’s
processing at the device has been completed. However, there are error conditions whereby a message
transfer can be aborted by the host and the message pipe can be sent a new message transfer prematurely(from the device’s perspective). From the perspective of the software manipulating a message pipe, anerror on some part of an IRP retires the current IRP and all queued IRPs. The software client thatrequested the IRP is notified of the IRP completion with an appropriate error indication.
A message pipe to a device requires a single device endpoint number in both directions (IN and OUT
tokens). The USB does not allow a message pipe to be associated with different endpoint numbers for eachdirection.
Message pipes support the control transfer type, which is explained in Section 5.5.

5.4 Transfer Types
The USB transports data through a pipe between a memory buffer associated with a software client on thehost and an endpoint on the USB device. Data transported by message pipes is carried in a USB-definedstructure, but the USB allows device-specific structured data to be transported within the USB-defined
message data payload. The USB also defines that data moved over the bus is packetized for any pipe
(stream or message), but ultimately the formatting and interpretation of the data transported in the data
payload of a bus transaction is the responsibility of the client software and function using the pipe.
However, the USB provides different transfer types that are optimized to more closely match the service
requirements of the client software and function using the pipe. An IRP uses one or more bus transactions
to move information between a software client and its function.
Each transfer type determines various characteristics of the communication flow including the following:

Data format imposed by the USBDirection of communication flowPacket size constraints

Bus access constraints

Latency constraints



Required data sequences

Error handling.

The designers of a USB device choose the capabilities for the device’s endpoints. When a pipe isestablished for an endpoint, most of the pipe’s transfer characteristics are determined and remain fixed forthe lifetime of the pipe. Transfer characteristics that can be modified are described for each transfer type.

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Universal Serial Bus Specification Revision 1.1

The USB defines four transfer types:
Control Transfers: Bursty, non-periodic, host software-initiated request/response communication,
typically used for command/status operations.



Isochronous Transfers: Periodic, continuous communication between host and device, typically used
for time-relevant information. This transfer type also preserves the concept of time encapsulated in thedata. This does not imply, however, that the delivery needs of such data is always time-critical.

Interrupt Transfers: Small-data, low-frequency, bounded-latency communication.

Bulk Transfers: Non-periodic, large-packet bursty communication, typically used for data that can use
any available bandwidth and can also be delayed until bandwidth is available.

Each transfer type is described in detail in the following four major sections. The data for any IRP is
carried by the data field of the data packet as described in Section 8.4.3. Chapter 8 also describes details of
the protocol that are affected by use of each particular transfer type.

5.5 Control Transfers
Control transfers allow access to different parts of a device. Control transfers are intended to supportconfiguration/command/status type communication flows between client software and its function. A
control transfer is composed of a Setup bus transaction moving request information from host to function,
zero or more Data transactions sending data in the direction indicated by the Setup transaction, and a Statustransaction returning status information from function to host. The Status transaction returns “success”
when the endpoint has successfully completed processing the requested operation. Section 8.5.2 describes
the details of what packets, bus transactions, and transaction sequences are used to accomplish a control
transfer. Chapter 9 describes the details of the defined USB command codes.
Each USB device is required to implement the Default Control Pipe as a message pipe. This pipe is usedby the USB System Software. The Default Control Pipe provides access to the USB device’s
configuration, status, and control information. A function can, but is not required to, provide endpoints for
additional control pipes for its own implementation needs.
The USB device framework (refer to Chapter 9) defines standard, device class, or vendor-specific requests
that can be used to manipulate a device’s state. Descriptors are also defined that can be used to containdifferent information on the device. Control transfers provide the transport mechanism to access device
descriptors and make requests of a device to manipulate its behavior.
Control transfers are carried only through message pipes. Consequently, data flows using control transfers
must adhere to USB data structure definitions as described in Section 5.5.1.
The USB system will make a “best effort” to support delivery of control transfers between the host anddevices. A function and its client software cannot request specific bus access frequency or bandwidth forcontrol transfers. The USB System Software may restrict the bus access and bandwidth that a device may
desire for control transfers. These restrictions are defined in Section 5.5.3 and Section 5.5.4.

5.5.1 Control Transfer Data Format
The Setup packet has a USB-defined structure that accommodates the minimum set of commands required
to enable communication between the host and a device. The structure definition allows vendor-specificextensions for device specific commands. The Data transactions following Setup have a USB-definedstructure except when carrying vendor-specific information. The Status transaction also has a USB-definedstructure. Specific control transfer Setup/Data definitions are described in Section 8.5.2 and Chapter 9.

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Universal Serial Bus Specification Revision 1.1

5.5.2 Control Transfer Direction
Control transfers are supported via bi-directional communication flow over message pipes. As aconsequence, when a control pipe is configured, it uses both the input and output endpoint with thespecified endpoint number.

5.5.3 Control Transfer Packet Size Constraints
An endpoint for control transfers specifies the maximum data payload size that the endpoint can accept
from or transmit to the bus. The USB defines the allowable maximum control data payload sizes for fullspeed devices to be either 8, 16, 32, or 64 bytes. Low-speed devices are limited to only an eight-byte
maximum data payload size. This maximum applies to the data payloads of the Data packets following a
Setup; i.e., the size specified is for the data field of the packet as defined in Chapter 8, not including otherinformation that is required by the protocol. A Setup packet is always eight bytes. A control pipe(including the Default Control Pipe) always uses its wMaxPacketSize value for data payloads.
An endpoint reports in its configuration information the value for its maximum data payload size. The
USB does not require that data payloads transmitted be exactly the maximum size; i.e., if a data payload isless than the maximum, it does not need to be padded to the maximum size.
All Host Controllers are required to have support for 8-, 16-, 32-, and 64-byte maximum data payload sizes
for full-speed control endpoints and only eight-byte maximum data payload sizes for low-speed control
endpoints. No Host Controller is required to support larger or smaller maximum data payload sizes.
In order to determine the maximum packet size for the Default Control Pipe, the USB System Software
reads the device descriptor. The host will read the first eight bytes of the device descriptor. The device
always responds with at least these initial bytes in a single packet. After the host reads the initial part ofthe device descriptor, it is guaranteed to have read this default pipe’s wMaxPacketSize field (byte 7 of thedevice descriptor). It will then allow the correct size for all subsequent transactions. For all other control
endpoints, the maximum data payload size is known after configuration so that the USB System Software
can ensure that no data payload will be sent to the endpoint that is larger than the supported size. The hostwill always use a maximum data payload size of at least eight bytes.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
wMaxPacketSize (refer to Chapter 9). When a control transfer involves more data than can fit in one data
payload of the currently established maximum size, all data payloads are required to be maximum-sized
except for the last data payload, which will contain the remaining data.
The Data stage of a control transfer from an endpoint to the host is complete when the endpoint does one of
the following:

Has transferred exactly the amount of data specified during the Setup stage

Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet.

When a Data stage is complete, the Host Controller advances to the Status stage instead of continuing onwith another data transaction. If the Host Controller does not advance to the Status stage when the Data
stage is complete, the endpoint halts the pipe as was outlined in Section 5.3.2. If a larger-than-expected
data payload is received from the endpoint, the IRP for the control transfer will be aborted/retired.
The Data stage of a control transfer from the host to an endpoint is complete when all of the data has beentransferred. If the endpoint receives a larger-than-expected data payload from the host, it halts the pipe.

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Universal Serial Bus Specification Revision 1.1

5.5.4 Control Transfer Bus Access Constraints
Control transfers can be used by full-speed and low-speed USB devices.
An endpoint has no way to indicate a desired bus access frequency for a control pipe. The USB balances
the bus access requirements of all control pipes and the specific IRPs that are pending to provide “besteffort” delivery of data between client software and functions.
The USB requires that part of each frame be reserved to be available for use by control transfers as follows:

If the control transfers that are attempted (in an implementation-dependent fashion) consume less than
10% of the frame time, the remaining time can be used to support bulk transfers (refer to Section 5.8).



A control transfer that has been attempted and needs to be retried can be retried in the current or afuture frame; i.e., it is not required to be retried in the same frame.



If there are more control transfers than reserved time, but there is additional frame time that is not
being used for isochronous or interrupt transfers, a Host Controller may move additional control
transfers as they are available.



If there are too many pending control transfers for the available frame time, control transfers are
selected to be moved over the bus as appropriate.



If there are control transfers pending for multiple endpoints, control transfers for the different
endpoints are selected according to a fair access policy that is Host Controller implementationdependent.



A transaction of a control transfer that is frequently being retried should not be expected to consume
an unfair share of the bus time.

These requirements allow control transfers between host and devices to be regularly moved over the bus
with “best effort.”
The rate of control transfers to a particular endpoint can be varied by the USB System Software at its
discretion. An endpoint and its client software cannot assume a specific rate of service for control
transfers. A control endpoint may see zero or more transfers in a single frame. Bus time made available toa software client and its endpoint can be changed as other devices are inserted into and removed from thesystem or also as control transfers are requested for other device endpoints.
The bus frequency and frame timing limit the maximum number of successful control transfers within aframe for any USB system to less than 29 full-speed eight-byte data payloads or less than four low-speed
eight-byte data payloads. Table 5-1 lists information about different-sized full-speed control transfers andthe maximum number of transfers possible in a frame. This table was generated assuming that there is one
Data stage transaction and that the Data stage has a zero-length status phase. The table illustrates thepossible power of two data payloads less than or equal to the allowable maximum data payload sizes. The
table does not include the overhead associated with bit stuffing.

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Universal Serial Bus Specification Revision 1.1

Table 5-1. Full-speed Control Transfer Limits

Max

Protocol Overhead (45 bytes)(9 SYNC bytes, 9 PID bytes, 6 Endpoint + CRC bytes, 6
CRC bytes, 8 Setup data bytes, and a 7-byte interpacket
delay (EOP, etc.))

Data
PayloadFrame
BandwidthperTransfer

Max
Transfers

Bytes
Remaining

Max Bandwidth(bytes/second)

Bytes/Frame
Useful Data

1

32000

3%

32

23

32

2

62000

3%

31

43

62

4

120000

3%

30

30

120

8

224000

4%

28

16

224

16

384000

4%

24

36

384

32

608000

5%

19

37

608

64

832000

7%

13

83

832

1500000

1500

The 10% frame reservation for non-periodic transfers means that in a system with bus time fully allocated,
all full-speed control transfers in the system contend for a nominal three control transfers per frame.
Because the USB system uses control transfers for configuration purposes in addition to whatever othercontrol transfers other client software may be requesting, a given software client and its function should not
expect to be able to make use of this full bandwidth for its own control purposes. Host Controllers are also
free to determine how the individual bus transactions for specific control transfers are moved over the bus
within and across frames. An endpoint could see all bus transactions for a control transfer within the same
frame or spread across several noncontiguous frames. A Host Controller, for various implementation
reasons, may not be able to provide the theoretical maximum number of control transfers per frame.
Both full-speed and low-speed control transfers contend for the same available frame time. Low-speed
control transfers simply take longer to transfer. Table 5-2 lists information about different-sized low-speed
packets and the maximum number of packets possible in a frame. The table does not include the overhead
associated with bit stuffing. For both speeds, because a control transfer is composed of several packets, thepackets can be spread over several frames to spread the bus time required across several frames.

39Universal Serial Bus Specification Revision 1.1

Table 5-2. Low-speed Control Transfer Limits
Protocol Overhead (46 bytes)

Max

Data
PayloadMax Bandwidth(Approximate)

Frame
Bandwidth
per Transfer

Max
Transfers

Bytes
Remaining

1

3000

25%

3

46

3

2

6000

26%

3

43

6

4

12000

27%

3

37

12

8

24000

29%

3

25

24

187500

Bytes/Frame
Useful Data

187

5.5.5 Control Transfer Data Sequences
Control transfers require that a Setup bus transaction be sent from the host to a device to describe the typeof control access that the device should perform. The Setup transaction is followed by zero or more
control Data transactions that carry the specific information for the requested access. Finally, a Statustransaction completes the control transfer and allows the endpoint to return the status of the control transfer
to the client software. After the Status transaction for a control transfer is completed, the host can advanceto the next control transfer for the endpoint. As described in Section 5.5.4, each control transaction and thenext control transfer will be moved over the bus at some Host Controller implementation-defined time.
The endpoint can be busy for a device-specific time during the Data and Status transactions of the control
transfer. During these times when the endpoint indicates it is busy (refer to Chapter 8 and Chapter 9 fordetails), the host will retry the transaction at a later time.
If a Setup transaction is received by an endpoint before a previously initiated control transfer is completed,the device must abort the current transfer/operation and handle the new control Setup transaction. A Setup
transaction should not normally be sent before the completion of a previous control transfer. However, if atransfer is aborted, for example, due to errors on the bus, the host can send the next Setup transaction
prematurely from the endpoint’s perspective.
After a halt condition is encountered or an error is detected by the host, a control endpoint is allowed torecover by accepting the next Setup PID; i.e., recovery actions via some other pipe are not required forcontrol endpoints. For the Default Control Pipe, a device reset will ultimately be required to clear the halt
or error condition if the next Setup PID is not accepted.
The USB provides robust error detection and recovery/retransmission for errors that occur during control
transfers. Transmitters and receivers can remain synchronized with regard to where they are in a control
transfer and recover with minimum effort. Retransmission of Data and Status packets can be detected by areceiver via data retry indicators in the packet. A transmitter can reliably determine that its corresponding
receiver has successfully accepted a transmitted packet by information returned in a handshake to thepacket. The protocol allows for distinguishing a retransmitted packet from its original packet except for acontrol Setup packet. Setup packets may be retransmitted due to a transmission error; however, Setup
packets cannot indicate that a packet is an original or a retried transmission.

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Universal Serial Bus Specification Revision 1.1

5.6 Isochronous Transfers
In non-USB environments, isochronous transfers have the general implication of constant-rate, errortolerant transfers. In the USB environment, requesting an isochronous transfer type provides the requesterwith the following:

Guaranteed access to USB bandwidth with bounded latencyGuaranteed constant data rate through the pipe as long as data is provided to the pipeIn the case of a delivery failure due to error, no retrying of the attempt to deliver the data.

While the USB isochronous transfer type is designed to support isochronous sources and destinations, it is
not required that software using this transfer type actually be isochronous in order to use the transfer type.
Section 5.10 presents more detail on special considerations for handling isochronous data on the USB.

5.6.1 Isochronous Transfer Data Format
The USB imposes no data content structure on communication flows for isochronous pipes.

5.6.2 Isochronous Transfer Direction
An isochronous pipe is a stream pipe and is, therefore, always uni-directional. An endpoint description
identifies whether a given isochronous pipe’s communication flow is into or out of the host. If a device
requires bi-directional isochronous communication flow, two isochronous pipes must be used, one in eachdirection.

5.6.3 Isochronous Transfer Packet Size Constraints
An endpoint in a given configuration for an isochronous pipe specifies the maximum size data payload thatit can transmit or receive. The USB System Software uses this information during configuration to ensurethat there is sufficient bus time to accommodate this maximum data payload in each frame. If there issufficient bus time for the maximum data payload, the configuration is established; if not, the configuration
is not established. The USB System Software does not adjust the maximum data payload size for anisochronous pipe as is the case for a control pipe. An isochronous pipe can simply either be supported or
not supported in a given USB system configuration.
The USB limits the maximum data payload size to 1,023 bytes for each isochronous pipe. Table 5-3 lists
information about different-sized isochronous transactions and the maximum number of transactions
possible in a frame. The table does not include the overhead associated with bit stuffing.

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Universal Serial Bus Specification Revision 1.1

Table 5-3. Isochronous Transaction Limits

Max

Protocol Overhead (9 bytes)(2 SYNC bytes, 2 PID bytes, 2 Endpoint + CRC bytes, 2 CRC
bytes, and a 1-byte interpacket delay)

Data
PayloadFrame
Bandwidth
per Transfer

Max
Transfers

Bytes
RemainingMax
Bandwidth

Bytes/Frame
Useful Data

1

150000

1%

150

0

150

2

272000

1%

136

4

272

4

460000

1%

115

5

460

8

704000

1%

88

4

704

16

960000

2%

60

0

960

32

1152000

3%

36

24

1152

64

1280000

5%

20

40

1280

128

1280000

9%

10

130

1280

256

1280000

18%

5

175

1280

512

1024000

35%

2

458

1024

1023

1023000

69%

1

468

1023

1500000

1500

Any given transaction for a isochronous pipe need not be exactly the maximum size specified for theendpoint. The size of a data payload is determined by the transmitter (client software or function) and canvary as required from transaction to transaction. The USB ensures that whatever size is presented to the
Host Controller is delivered on the bus. The actual size of a data payload is determined by the datatransmitter and may be less than the prenegotiated maximum size. Bus errors can change the actual packet
size seen by the receiver. However, these errors can be detected by either CRC on the data or byknowledge the receiver has about the expected size for any transaction.

5.6.4 Isochronous Transfer Bus Access Constraints
Isochronous transfers can be used only by full-speed devices.
The USB requires that no more than 90% of any frame be allocated for periodic (isochronous andinterrupt) transfers.
An endpoint for an isochronous pipe does not include information about bus access frequency. All
isochronous pipes normally move exactly one data packet each frame (i.e., every 1ms). Errors on the bus
or delays in operating system scheduling of client software can result in no packet being transferred for aframe. An error indication should be returned as status to the client software in such a case. A device canalso detect this situation by tracking SOF tokens and noticing two SOF tokens without an intervening data
packet for an isochronous endpoint.
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Universal Serial Bus Specification Revision 1.1

The bus frequency and frame timing limit the maximum number of successful isochronous transactions
within a frame for any USB system to less than 151 full-speed one-byte data payloads. A Host Controller,
for various implementation reasons, may not be able to provide the theoretical maximum number ofisochronous transactions per frame.

5.6.5 Isochronous Transfer Data Sequences
Isochronous transfers do not support data retransmission in response to errors on the bus. A receiver candetermine that a transmission error occurred. The low-level USB protocol does not allow handshakes to be
returned to the transmitter of an isochronous pipe. Normally, handshakes would be returned to tell thetransmitter whether a packet was successfully received or not. For isochronous transfers, timeliness ismore important than correctness/retransmission, and given the low error rates expected on the bus, theprotocol is optimized by assuming transfers normally succeed. Isochronous receivers can determine
whether they missed data during a frame. Also, a receiver can determine how much data was lost. Section
5.10 describes these USB mechanisms in more detail.
An endpoint for isochronous transfers never halts because there is no handshake to report a halt condition.
Errors are reported as status associated with the IRP for an isochronous transfer, but the isochronous pipe is
not halted in an error case. If an error is detected, the host continues to process the data associated with thenext frame of the transfer. Limited error detection is possible because the protocol for isochronoustransactions does not allow per-transaction handshakes.

5.7 Interrupt Transfers
The interrupt transfer type is designed to support those devices that need to send or receive small amountsof data infrequently, but with bounded service periods. Requesting a pipe with an interrupt transfer typeprovides the requester with the following:

Guaranteed maximum service period for the pipeRetry of transfer attempts at the next period, in the case of occasional delivery failure due to error on
the bus.

5.7.1 Interrupt Transfer Data Format
The USB imposes no data content structure on communication flows for interrupt pipes.

5.7.2 Interrupt Transfer Direction
An interrupt pipe is a stream pipe and is therefore always uni-directional. An endpoint description
identifies whether a given interrupt pipe’s communication flow is into or out of the host.

5.7.3 Interrupt Transfer Packet Size Constraints
An endpoint for an interrupt pipe specifies the maximum size data payload that it will transmit or receive.
The maximum allowable interrupt data payload size is 64 bytes or less for full-speed. Low-speed devices
are limited to eight bytes or less maximum data payload size. This maximum applies to the data payloadsof the data packets; i.e., the size specified is for the data field of the packet as defined in Chapter 8, notincluding other protocol-required information. The USB does not require that data packets be exactly the
maximum size; i.e., if a data packet is less than the maximum, it does not need to be padded to the
maximum size.
All Host Controllers are required to have support for up to 64-byte maximum data payload sizes for fullspeed interrupt endpoints and eight bytes or less maximum data payload sizes for low-speed interrupt
endpoints. No Host Controller is required to support larger maximum data payload sizes.

43Universal Serial Bus Specification Revision 1.1

The USB System Software determines the maximum data payload size that will be used for a interrupt pipeduring device configuration. This size remains constant for the lifetime of a device’s configuration. The
USB System Software uses the maximum data payload size determined during configuration to ensure that
there is sufficient bus time to accommodate this maximum data payload in its assigned period. If there issufficient bus time, the pipe is established; if not, the pipe is not established. The USB System Software
does not adjust the bus time made available to an interrupt pipe as is the case for a control pipe. Aninterrupt pipe can simply either be supported or not supported in a given USB system configuration.
However, the actual size of a data payload is still determined by the data transmitter and may be less thanthe maximum size.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
wMaxPacketSize value. A device can move data via an interrupt pipe that is larger than wMaxPacketSize.
A software client can accept this data via an IRP for the interrupt transfer that requires multiple bus
transactions without requiring an IRP-complete notification per transaction. This can be achieved by
specifying a buffer that can hold the desired data size. The size of the buffer is a multiple of
wMaxPacketSize with some remainder. The endpoint must transfer each transaction except the last as
wMaxPacketSize and the last transaction is the remainder. The multiple data transactions are moved overthe bus at the period established for the pipe.
When an interrupt transfer involves more data than can fit in one data payload of the currently establishedmaximum size, all data payloads are required to be maximum-sized except for the last data payload, which
will contain the remaining data. An interrupt transfer is complete when the endpoint does one of thefollowing:

Has transferred exactly the amount of data expectedTransfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet.

When an interrupt transfer is complete, the Host Controller retires the current IRP and advances to the next
IRP. If a data payload is received that is larger than expected, the interrupt IRP will be aborted/retired andthe pipe will stall future IRPs until the condition is corrected and acknowledged.

5.7.4 Interrupt Transfer Bus Access Constraints
Interrupt transfers can be used by full-speed and low-speed devices.
The USB requires that no more than 90% of any frame be allocated for periodic (isochronous andinterrupt) transfers.
The bus frequency and frame timing limit the maximum number of successful interrupt transactions withina frame for any USB system to less than 108 full-speed one-byte data payloads or 14 low-speed one-byte
data payloads. A Host Controller, for various implementation reasons, may not be able to provide theabove maximum number of interrupt transactions per frame.
Table 5-4 lists information about different sized full-speed interrupt transactions and the maximum numberof transactions possible in a frame. Table 5-5 lists similar information for low-speed interrupt transactions.
The tables do not include the overhead associated with bit stuffing.

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Universal Serial Bus Specification Revision 1.1

Table 5-4. Full-speed Interrupt Transaction Limits

Max

Protocol Overhead (13 bytes)(3 SYNC bytes, 3 PID bytes, 2 Endpoint + CRC bytes, 2
CRC bytes, and a 3-byte interpacket delay)

Data
PayloadFrame
Bandwidth
per Transfer

Max
Transfers

Bytes
RemainingMax
Bandwidth

Bytes/Frame
Useful Data

1

107000

1%

107

2

107

2

200000

1%

100

0

200

4

352000

1%

88

4

352

8

568000

1%

71

9

568

16

816000

2%

51

21

816

32

1056000

3%

33

15

1056

64

1216000

5%

19

37

1216

1500000

1500

An endpoint for an interrupt pipe specifies its desired bus access period. A full-speed endpoint can specify
a desired period from 1ms to 255ms. Low-speed endpoints are limited to specifying only 10ms to 255ms.
The USB System Software will use this information during configuration to determine a period that can besustained. The period provided by the system may be shorter than that desired by the device up to theshortest period defined by the USB (1ms). The client software and device can depend only on the fact thatthe host will ensure that the time duration between two transaction attempts with the endpoint will be nolonger than the desired period. Note that errors on the bus can prevent an interrupt transaction from being
successfully delivered over the bus and consequently exceed the desired period. Also, the endpoint is only
polled when the software client has an IRP for an interrupt transfer pending. If the bus time for performing
an interrupt transfer arrives and there is no IRP pending, the endpoint will not be given an opportunity totransfer data at that time. Once an IRP is available, its data will be transferred at the next allocated period.

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Universal Serial Bus Specification Revision 1.1

Table 5-5. Low-speed Interrupt Transaction Limits
Protocol Overhead (13 bytes)

Data
PayloadMax Bandwidth(Approximate)

Frame
Bandwidth
per Transfer

Max
Transfers

Bytes
Remaining

1

13000

7%

13

5

13

2

24000

8%

12

7

24

4

44000

9%

11

0

44

8

64000

11%

8

19

64

Max

187500

Bytes/Frame
Useful Data

187

Interrupt transfers are moved over the USB by accessing an interrupt endpoint every period. For input
interrupt endpoints, the host has no way to determine whether an endpoint will source an interrupt without
accessing the endpoint and requesting an interrupt transfer. If the endpoint has no interrupt data to transmit
when accessed by the host, it responds with NAK. An endpoint should only provide interrupt data when it
has an interrupt pending to avoid having a software client erroneously notified of IRP complete. A zerolength data payload is a valid transfer and may be useful for some implementations.

5.7.5 Interrupt Transfer Data Sequences
Interrupt transactions may use either alternating data toggle bits, such that the bits are toggled only uponsuccessful transfer completion, or a continuously toggling of data toggle bits. The host in any case must
assume that the device is obeying full handshake/retry rules as defined in Chapter 8. A device may choose
to always toggle DATA0/DATA1 PIDs so that it can ignore handshakes from the host. However, in thiscase, the client software can miss some data packets when an error occurs, because the Host Controllerinterprets the next packet as a retry of a missed packet.
If a halt condition is detected on an interrupt pipe due to transmission errors or a STALL handshake being
returned from the endpoint, all pending IRPs are retired. Removal of the halt condition is achieved via
software intervention through a separate control pipe. This recovery will reset the data toggle bit to
DATA0 for the endpoint on both the host and the device. Interrupt transactions are retried due to errors
detected on the bus that affect a given transfer.

5.8 Bulk Transfers
The bulk transfer type is designed to support devices that need to communicate relatively large amounts ofdata at highly variable times where the transfer can use any available bandwidth. Requesting a pipe with abulk transfer type provides the requester with the following:

Access to the USB on a bandwidth-available basis

Retry of transfers, in the case of occasional delivery failure due to errors on the bus

Guaranteed delivery of data, but no guarantee of bandwidth or latency.

Bulk transfers occur only on a bandwidth-available basis. For a USB with large amounts of freebandwidth, bulk transfers may happen relatively quickly; for a USB with little bandwidth available, bulk
transfers may trickle out over a relatively long period of time.
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Universal Serial Bus Specification Revision 1.1

5.8.1 Bulk Transfer Data Format
The USB imposes no data content structure on communication flows for bulk pipes.

5.8.2 Bulk Transfer Direction
A bulk pipe is a stream pipe and, therefore, always has communication flowing either into or out of thehost for a given pipe. If a device requires bi-directional bulk communication flow, two bulk pipes must be
used, one in each direction.

5.8.3 Bulk Transfer Packet Size Constraints
An endpoint for bulk transfers specifies the maximum data payload size that the endpoint can accept fromor transmit to the bus. The USB defines the allowable maximum bulk data payload sizes to be only 8, 16,
32, or 64 bytes. This maximum applies to the data payloads of the data packets; i.e.; the size specified isfor the data field of the packet as defined in Chapter 8, not including other protocol-required information.
A bulk endpoint is designed to support a maximum data payload size. A bulk endpoint reports in its
configuration information the value for its maximum data payload size. The USB does not require thatdata payloads transmitted be exactly the maximum size; i.e., if a data payload is less than the maximum, it
does not need to be padded to the maximum size.
All Host Controllers are required to have support for 8-, 16-, 32-, and 64-byte maximum packet sizes forbulk endpoints. No Host Controller is required to support larger or smaller maximum packet sizes.
During configuration, the USB System Software reads the endpoint’s maximum data payload size andensures that no data payload will be sent to the endpoint that is larger than the supported size.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
reported wMaxPacketSize value. When a bulk IRP involves more data than can fit in one maximum-sizeddata payload, all data payloads are required to be maximum size except for the last data payload, which
will contain the remaining data. A bulk transfer is complete when the endpoint does one of the following:

Has transferred exactly the amount of data expectedTransfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet.

When a bulk transfer is complete, the Host Controller retires the current IRP and advances to the next IRP.
If a data payload is received that is larger than expected, all pending bulk IRPs for that endpoint will beaborted/retired.

5.8.4 Bulk Transfer Bus Access Constraints
Bulk transfers can be used only by full-speed devices.
An endpoint has no way to indicate a desired bus access frequency for a bulk pipe. The USB balances thebus access requirements of all bulk pipes and the specific IRPs that are pending to provide “good effort”
delivery of data between client software and functions. Moving control transfers over the bus has priority
over moving bulk transfers.
There is no time guaranteed to be available for bulk transfers as there is for control transfers. Bulk
transfers are moved over the bus only on a bandwidth-available basis. If there is bus time that is not being
used for other purposes, bulk transfers will be moved over the bus. If there are bulk transfers pending for
multiple endpoints, bulk transfers for the different endpoints are selected according to a fair access policythat is Host Controller implementation-dependent.
All bulk transfers pending in a system contend for the same available bus time. Because of this, the bus
time made available for bulk transfers to a particular endpoint can be varied by the USB System Softwareat its discretion. An endpoint and its client software cannot assume a specific rate of service for bulk

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Universal Serial Bus Specification Revision 1.1

transfers. Bus time made available to a software client and its endpoint can be changed as other devices areinserted into and removed from the system or also as bulk transfers are requested for other device
endpoints. Client software cannot assume ordering between bulk and control transfers; i.e., in somesituations, bulk transfers can be delivered ahead of control transfers.
The bus frequency and frame timing limit the maximum number of successful bulk transactions within aframe for any USB system to less than 72 eight-byte data payloads. Table 5-6 lists information about
different-sized bulk transactions and the maximum number of transactions possible in a frame. The table
does not include the overhead associated with bit stuffing.
Table 5-6. Bulk Transaction Limits

Max

Protocol Overhead (13 bytes)(3 SYNC bytes, 3 PID bytes, 2 Endpoint + CRC bytes, 2 CRC
bytes, and a 3-byte interpacket delay)

Data
PayloadFrame
Bandwidth
per Transfer

Max
Transfers

Bytes
Remaining

Max Bandwidth(bytes/second)

Bytes/Frame
Useful Data

1

107000

1%

107

2

107

2

200000

1%

100

0

200

4

352000

1%

88

4

352

8

568000

1%

71

9

568

16

816000

2%

51

21

816

32

1056000

3%

33

15

1056

64

1216000

5%

19

37

1216

1500000

1500

Host Controllers are free to determine how the individual bus transactions for specific bulk transfers are
moved over the bus within and across frames. An endpoint could see all bus transactions for a bulk
transfer within the same frame or spread across several frames. A Host Controller, for various
implementation reasons, may not be able to provide the above maximum number of transactions per frame.

5.8.5 Bulk Transfer Data Sequences
Bulk transactions use data toggle bits that are toggled only upon successful transaction completion topreserve synchronization between transmitter and receiver when transactions are retried due to errors.
Bulk transactions are initialized to DATA0 when the endpoint is configured by an appropriate control
transfer. The host will also start the first bulk transaction with DATA0. If a halt condition is detected on
an bulk pipe due to transmission errors or a STALL handshake being returned from the endpoint, allpending IRPs are retired. Removal of the halt condition is achieved via software intervention through aseparate control pipe. This recovery will reset the data toggle bit to DATA0 for the endpoint on both thehost and the device.
Bulk transactions are retried due to errors detected on the bus that affect a given transaction.

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Universal Serial Bus Specification Revision 1.1

5.9 Bus Access for Transfers
Accomplishing any data transfer between the host and a USB device requires some use of the USB
bandwidth. Supporting a wide variety of isochronous and asynchronous devices requires that eachdevice’s transfer requirements are accommodated. The process of assigning bus bandwidth to devices iscalled transfer management. There are several entities on the host that coordinate the information flowing
over the USB: client software, the USB Driver (USBD), and the Host Controller Driver (HCD).
Implementers of these entities need to know the key concepts related to bus access:

Transfer Management: The entities and the objects that support communication flow over the USB.

Transaction Tracking: The USB mechanisms that are used to track transactions as they move throughthe USB system.

Bus Time: The time it takes to move a packet of information over the bus.



Device/Software Buffer Size: The space required to support a bus transaction.



Bus Bandwidth Reclamation: Conditions where bandwidth that was allocated to other transfers but
was not used and can now be possibly reused by control and bulk transfers.

The previous sections focused on how client software relates to a function and what the logical flows areover a pipe between the two entities. This section focuses on the different parts of the host and how theymust interact to support moving data over the USB. This information may also be of interest to device
implementers so they understand aspects of what the host is doing when a client requests a transfer andhow that transfer is presented to the device.

5.9.1 Transfer Management
Transfer management involves several entities that operate on different objects in order to movetransactions over the bus:

Client Software: Consumes/generates function-specific data to/from a function endpoint via calls andcallbacks requesting IRPs with the USBD interface.



USB Driver (USBD): Converts data in client IRPs to/from device endpoint via calls/callbacks with theappropriate HCD. A single client IRP may involve one or more transfers.



Host Controller Driver (HCD): Converts IRPs to/from transactions (as required by a Host Controller
implementation) and organizes them for manipulation by the Host Controller. Interactions between
the HCD and its hardware is implementation-dependent and is outside the scope of the USB
Specification.



Host Controller: Takes transactions and generates bus activity via packets to move function-specificdata across the bus for each transaction.

Figure 5-10 shows how the entities are organized as information flows between client software and theUSB. The objects of primary interest to each entity are shown at the interfaces between entities.

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Universal Serial Bus Specification Revision 1.1

Client Software

Data

USBD
Interface
IRPs

USBD

HCD
Interface

HCD

Transfers

Transaction
Transaction List

Transactions

Transaction

HW/SW
Interface

Host Controller

Packets
USB

Figure 5-10. USB Information Conversion From Client Software to Bus

5.9.1.1 Client Software
Client software determines what transfers need to be made with a function. It uses appropriate operating
system-specific interfaces to request IRPs. Client software is aware only of the set of pipes (i.e., theinterface) it needs to manipulate its function. The client is aware of and adheres to all bus access andbandwidth constraints as described previously for each transfer type. The requests made by the client
software are presented via the USBD interface.
Some clients may manipulate USB functions via other device class interfaces defined by the operating
system and may themselves not make direct USBD calls. However, there is always some lowest level
client that makes USBD calls to pass IRPs to the USBD. All IRPs presented are required to adhere to theprenegotiated bandwidth constraints set when the pipe was established. If a function is moved from a nonUSB environment to the USB, the driver that would have directly manipulated the function hardware via
memory or I/O accesses is the lowest client software in the USB environment that now interacts with the
USBD to manipulate the driver’s USB function.

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Universal Serial Bus Specification Revision 1.1

After client software has requested a transfer of its function and the request has been serviced, the client
software receives notification of the completion status of the IRP. If the transfer involved function-to-hostdata transfer, the client software can access the data in the data buffer associated with the completed IRP.
The USBD interface is defined in Chapter 10.

5.9.1.2 USB Driver
The Universal Serial Bus Driver (USBD) is involved in mediating bus access at two general times:

While a device is attached to the bus during configuration



During normal transfers.

When a device is attached and configured, the USBD is involved to ensure that the desired device
configuration can be accommodated on the bus. The USBD receives configuration requests from theconfiguring software that describe the desired device configuration: endpoint(s), transfer type(s), transfer
period(s), data size(s), etc. The USBD either accepts or rejects a configuration request based on bandwidthavailability and the ability to accommodate that request type on the bus. If it accepts the request, theUSBD creates a pipe for the requester of the desired type and with appropriate constraints as defined forthe transfer type. Bandwidth allocation for periodic endpoints does not have to be made when the device isconfigured and, once made, an bandwidth allocation can be released without changing the device
configuration.
The configuration aspects of the USBD are typically operating system-specific and heavily leverage theconfiguration features of the operating system to avoid defining additional (redundant) interfaces.
Once a device is configured, the software client can request IRPs to move data between it and its function
endpoints.

5.9.1.3 Host Controller Driver
The Host Controller Driver (HCD) is responsible for tracking the IRPs in progress and ensuring that USB
bandwidth and frame time maximums are never exceeded. When IRPs are made for a pipe, the HCD adds
them to the transaction list. When an IRP is complete, the HCD notifies the requesting software client of
the completion status for the IRP. If the IRP involved data transfer from the function to the software client,the data was placed in the client-indicated data buffer.
IRPs are defined in an operating system-dependent manner.

5.9.1.4 Transaction List
The transaction list is a Host Controller implementation-dependent description of the current outstanding
set of bus transactions that need to be run on the bus. Only the HCD and its Host Controller have access tothe specific representation. Each description contains transaction descriptions in which parameters, such asdata size in bytes, the device address and endpoint number, and the memory area to which data is to be sentor received, are identified.
A transaction list and the interface between the HCD and its Host Controller is typically represented in an
implementation-dependent fashion and is not defined explicitly as part of the USB Specification.

5.9.1.5 Host Controller
The Host Controller has access to the transaction list and translates it into bus activity. In addition, theHost Controller provides a reporting mechanism whereby the status of a transaction (done, pending, halted,etc.) can be obtained. The Host Controller converts transactions into appropriate implementationdependent activities that result in USB packets moving over the bus topology rooted in the root hub.

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Universal Serial Bus Specification Revision 1.1

The Host Controller ensures that the bus access rules defined by the protocol are obeyed, such as
inter-packet timings, timeouts, babble, etc. The HCD interface provides a way for the Host Controller toparticipate in deciding whether a new pipe is allowed access to the bus. This is done because Host
Controller implementations can have restrictions/constraints on the minimum inter-transaction times they
may support for combinations of bus transactions.
The interface between the transaction list and the Host Controller is hidden within an HCD and Host
Controller implementation.

5.9.2 Transaction Tracking
A USB function sees data flowing across the bus in packets as described in Chapter 8. The Host Controller
uses some implementation-dependent representation to track what packets to transfer to/from what
endpoints at what time or in what order. Most client software does not want to deal with packetizedcommunication flows because this involves a degree of complexity and interconnect dependency that limits
the implementation. The USB System Software (USBD and HCD) provides support for matching data
movement requirements of a client to packets on the bus. The Host Controller hardware and software uses
IRPs to track information about one or more transactions that combine to deliver a transfer of information
between the client software and the function. Figure 5-11 summarizes how transactions are organized into
IRPs for the four transfer types. Detailed protocol information for each transfer type can be found in
Chapter 8. More information about client software views of IRPs can be found in Chapter 10 and in theoperating system specific-information for a particular operating system.
Data Flow Types
IRP
Transaction

Transaction

Transaction

Control Transfer
IRP
Setup
Transaction

Data
Transaction

StatusTransaction

Additional
Control Transfers

Interrupt Transfer

All transfers arecomposed of one or moretransactions. An IRP
corresponds to one ormore transfers.

A control transfer is an OUT
Setup transaction followedby multiple IN or OUT Datatransactions followed by one“opposite of data direction”
Status transaction.

An interrupt transfer is one
or more IN / OUT Datatransactions.

IRP
Transaction

Transaction

Isochronous Transfer
An isochronous transfer
is one or more IN / OUT
Data transactions.

IRP
Transaction

Transaction

Transaction

Bulk Transfer

A bulk transfer is one
or more IN / OUT Datatransactions.

IRP
Transaction

Transaction

Transaction

Figure 5-11. Transfers for Communication Flows52

Universal Serial Bus Specification Revision 1.1

Even though IRPs track the bus transactions that need to occur to move a specific data flow over the USB,
Host Controllers are free to choose how the particular bus transactions are moved over the bus subject tothe USB-defined constraints (e.g., exactly one transaction per frame for isochronous transfers). In any
case, an endpoint will see transactions in the order they appear within an IRP unless errors occur. For
example, Figure 5-12 shows two IRPs, one each for two pipes where each IRP contains three transactions.
For any transfer type, a Host Controller is free to move the first transaction of the first IRP followed by the
first transaction of the second IRP somewhere in Frame 1, while moving the second transaction of each
IRP in opposite order somewhere in Frame 2. If these are isochronous transfer types, that is the only
degree of freedom a Host Controller has. If these are control or bulk transfers, a Host Controller could
further move more or less transactions from either IRP within either frame. Functions cannot depend on
seeing transactions within an IRP back-to-back within a frame nor should they depend on not seeing
transactions back-to-back within a frame.

Pipe

Pipe

IRP 1

IRP 2
Transaction
1-0

Transaction
1-1

Frame 1
Token Data,
Handshake(1-0)

Transaction
1-2

Transaction
2-0

Transaction
2-1

Transaction
2-2

Frame 2
Token, Data,
Handshake(2-0)

Token, Data,
Handshake(2-1)

Token, Data,
Handshake(1-1)

Figure 5-12. Arrangement of IRPs to Transactions/Frames

53Universal Serial Bus Specification Revision 1.1

5.9.3 Calculating Bus Transaction Times
When the USB System Software allows a new pipe to be created for the bus, it must calculate how muchbus time is required for a given transaction. That bus time is based on the maximum packet size
information reported for an endpoint, the protocol overhead for the specific transaction type request, theoverhead due to signaling imposed bit stuffing, inter-packet timings required by the protocol,inter-transaction timings, etc. These calculations are required to ensure that the time available in a frame is
not exceeded. The equations used to determine transaction bus time are:
KEY:
Data_bc

The byte count of data payload

Host_Delay

The time required for the host to prepare for orrecover from the transmission; Host Controller
implementation-specific

Floor()The integer portion of argument

Hub_LS_Setup

The time provided by the Host Controller for hubs toenable low-speed ports; measured as the delay from theend of the PRE PID to the start of the low-speed SYNC;
minimum of four full-speed bit times

BitStuffTime

Function that calculates theoretical additional timerequired due to bit stuffing in signaling; worst caseis (1.1667*8*Data_bc)

Full-speed (Input)
Non-Isochronous Transfer (Handshake Included)= 9107 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Isochronous Transfer (No Handshake)= 7268 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Full-speed (Output)
Non-Isochronous Transfer (Handshake Included)= 9107 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Isochronous Transfer (No Handshake)= 6265 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Low-speed (Input)= 64060 + (2 * Hub_LS_Setup) +(676.67 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Low-speed (Output)= 64107 + (2 * Hub_LS_Setup) +(667.0 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay

The bus times in the above equations are in nanoseconds and take into account propagation delays due tothe distance the device is from the host. These are typical equations that can be used to calculate bus time;
however, different implementations may choose to use coarser approximations of these times.
The actual bus time taken for a given transaction will almost always be less than that calculated because bit
stuffing overhead is data-dependent. Worst case bit stuffing is calculated as 1.1667 (7/6) times the raw
time (i.e., the BitStuffTime function multiplies the Data_bc by 8*1.1667 in the equations). This means that
there will almost always be time unused on the bus (subject to data pattern specifics) after all regularly

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Universal Serial Bus Specification Revision 1.1

scheduled transactions have completed. The bus time made available due to less bit stuffing can be reused
as discussed in Section 5.9.5.
The Host_Delay term in the equations is Host Controller- and system-dependent and allows for additionaltime a Host Controller may require due to delays in gaining access to memory or other implementation
dependencies. This term is incorporated into an implementation of these equations by using the transfer
management functions provided by the HCD interface. These equations are typically implemented by a
combination of USBD and HCD software working in cooperation. The results of these calculations areused to determine whether a transfer or pipe creation can be supported in a given USB configuration.

5.9.4 Calculating Buffer Sizes in Functions and Software
Client software and functions both need to provide buffer space for pending data transactions awaiting theirturn on the bus. For non-isochronous pipes, this buffer space needs to be just large enough to hold the next
data packet. If more than one transaction request is pending for a given endpoint, the buffering for eachtransaction must be supplied. Methods to calculate the precise absolute minimum buffering a function may
require because of specific interactions defined between its client software and the function are outside thescope of the USB Specification.
The Host Controller is expected to be able to support an unlimited number of transactions pending for thebus subject to available system memory for buffer and descriptor space, etc. Host Controllers are allowed
to limit how many frames into the future they allow a transaction to be requested.
For isochronous pipes, Section 5.10.4 describes details affecting host side and device side buffering
requirements. In general, buffers need to be provided to hold approximately twice the amount of data thatcan be transferred in 1ms.

5.9.5 Bus Bandwidth Reclamation
The USB bandwidth and bus access are granted based on a calculation of worst case bus transmission timeand required latencies. However, due to the constraints placed on different transfer types and the fact thatthe bit stuffing bus time contribution is calculated as a constant but is data-dependent, there will frequently
be bus time remaining in each frame time versus what the frame transmission time was calculated to be. In
order to support the most efficient use of the bus bandwidth, control and bulk transfers are candidates to be
moved over the bus as bus time becomes available. Exactly how a Host Controller supports this is
implementation-dependent. A Host Controller can take into account the transfer types of pending IRPs andimplementation-specific knowledge of remaining frame time to reuse reclaimed bandwidth.

5.10 Special Considerations for Isochronous Transfers
Support for isochronous data movement between the host and a device is one of the system capabilities
supported by the USB. Delivering isochronous data reliably over the USB requires careful attention todetail. The responsibility for reliable delivery is shared by several USB entities:

The device/function

The busThe Host Controller

One or more software agents.

Because time is a key part of an isochronous transfer, it is important for USB designers to understand how
time is dealt with within the USB by these different entities.
All isochronous devices must report their capabilities in the form of device-specific descriptors. The
capabilities should also be provided in a form that the potential customer can use to decide whether the55

Universal Serial Bus Specification Revision 1.1

device offers a solution to his problem(s). The specific capabilities of a device can justify pricedifferences.
In any communication system, the transmitter and receiver must be synchronized enough to deliver datarobustly. In an asynchronous communication system, data can be delivered robustly by allowing thetransmitter to detect that the receiver has not received a data item correctly and simply retrying
transmission of the data.
In an isochronous communication system, the transmitter and receiver must remain time- and datasynchronized to deliver data robustly. The USB does not support transmission retry of isochronous data sothat minimal bandwidth can be allocated to isochronous transfers and time synchronization is not lost due
to a retry delay. However, it is critical that a USB isochronous transmitter/receiver pair still remain
synchronized both in normal data transmission cases and in cases where errors occur on the bus.
In many systems that deal with isochronous data, a single global clock is used to which all entities in thesystem synchronize. An example of such a system is the PSTN (Public Switched Telephone Network).
Given that a broad variety of devices with different natural frequencies may be attached to the USB, nosingle clock can provide all the features required to satisfy the synchronization requirements of all devices
and software while still supporting the cost targets of mass-market PC products. The USB defines a clock
model that allows a broad range of devices to coexist on the bus and have reasonable cost implementations.
This section presents options or features that can be used by isochronous endpoints to minimize behavior
differences between a non-USB implemented function and a USB version of the function. An example isincluded to illustrate the similarities and differences between the non-USB and USB versions of a function.
The remainder of the section presents the following key concepts:


USB Clock Model: What clocks are present in a USB system that have impact on isochronous datatransfers



USB Frame Clock-to-function Clock Synchronization Options: How the USB frame clock can relate
to a function clock

SOF Tracking: Responsibilities and opportunities of isochronous endpoints with respect to the SOF
token and USB frames

Data Prebuffering: Requirements for accumulating data before generation, transmission, andconsumption

Error Handling: Isochronous-specific details for error handling

Buffering for Rate Matching: Equations that can be used to calculate buffer space required forisochronous endpoints.

5.10.1 Example Non-USB Isochronous Application
The example used is a reasonably generalized example. Other simpler or more complex cases are possible
and the relevant USB features identified can be used or not as appropriate.
The example consists of an 8kHz mono microphone connected through a mixer driver that sends the input
data stream to 44kHz stereo speakers. The mixer expects the data to be received and transmitted at somesample rate and encoding. A rate matcher driver on input and output converts the sample rate andencoding from the natural rate and encoding of the device to the rate and encoding expected by the mixer.
Figure 5-13 illustrates this example.

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Universal Serial Bus Specification Revision 1.1

Each DD has
independent
service rate

Mixer Device
DriverRate
Matcher

Rate
Matcher

1 speaker DD
service period(n sample)
slop buffer

Speaker
Device Driver

20ms service
periodMaster Clock20ms service
periodMicrophone
Device Driver

2x160 Byte Buffer(2 Services,
160 samples per service)Transfer
Complete
Interrupt

Transfer
Complete
Interrupt

2x3528 Byte Buffer(2 Services,
882 samples per service)

DMA
controller

SoftwareHardwareTraditional Bus(e.g. PCI, ISA, ...)
Single sample
transfers
1 sample at a time

Mono
Microphone

8MHz Bus Clock1 sample at a time

CD Stereo
Speakers

2x1 Byte Buffer(2 Samples)

8kHz Sample Clock(1 byte/sample)

2x4 Byte Buffer(2 Samples)

44.1KHz Sample Clock(4 bytes/sample)

Figure 5-13. Non-USB Isochronous Example

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Universal Serial Bus Specification Revision 1.1

A master clock (which can be provided by software driven from the real time clock) in the PC is used toawaken the mixer to ask the input source for input data and to provide output data to the output sink. In
this example, assume it awakens every 20ms. The microphone and speakers each have their own sample
clocks that are unsynchronized with respect to each other or the master mixer clock. The microphone
produces data at its natural rate (one-byte samples, 8,000 times a second) and the speakers consume data at
their natural rate (four-byte samples, 44,100 times a second). The three clocks in the system can drift andjitter with respect to each other. Each rate matcher may also be running at a different natural rate thaneither the mixer driver, the input source/driver, or output sink/driver.
The rate matchers also monitor the long-term data rate of their device compared to the master mixer clock
and interpolate an additional sample or merge two samples to adjust the data rate of their device to the data
rate of the mixer. This adjustment may be required every couple of seconds, but typically occurs
infrequently. The rate matchers provide some additional buffering to carry through a rate match.
Note: Some other application might not be able to tolerate sample adjustment and would need some othermeans of accommodating master clock-to-device clock drift or else would require some means ofsynchronizing the clocks to ensure that no drift could occur.
The mixer always expects to receive exactly a service period of data (20ms service period) from its input
device and produce exactly a service period of data for its output device. The mixer can be delayed up toless than a service period if data or space is not available from its input/output device. The mixer assumes
that such delays do not accumulate.
The input and output devices and their drivers expect to be able to put/get data in response to a hardwareinterrupt from the DMA controller when their transducer has processed one service period of data. They
expect to get/put exactly one service period of data. The input device produces 160 bytes (ten samples)
every service period of 20ms. The output device consumes 3,528 bytes (882 samples) every 20ms service
period. The DMA controller can move a single sample between the device and the host buffer at a rate
much faster than the sample rate of either device.
The input and output device drivers provide two service periods of system buffering. One buffer is always
being processed by the DMA controller. The other buffer is guaranteed to be ready before the current
buffer is exhausted. When the current buffer is emptied, the hardware interrupt awakens the device driverand it calls the rate matcher to give it the buffer. The device driver requests a new IRP with the buffer
before the current buffer is exhausted.
The devices can provide two samples of data buffering to ensure that they always have a sample to process
for the next sample period while the system is reacting to the previous/next sample.
The service periods of the drivers are chosen to survive interrupt latency variabilities that may be present in
the operating system environment. Different operating system environments will require different service
periods for reliable operation. The service periods are also selected to place a minimum interrupt load on
the system, because there may be other software in the system that requires processing time.

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Universal Serial Bus Specification Revision 1.1

5.10.2 USB Clock Model
Time is present in the USB system via clocks. In fact, there are multiple clocks in a USB system that must
be understood:Sample Clock: This clock determines the natural data rate of samples moving between client softwareon the host and the function. This clock does not need to be different between non-USB and USB
implementations.



Bus Clock: This clock runs at a 1.000ms period (1kHz frequency) and is indicated by the rate of SOF
packets on the bus. This clock is somewhat equivalent to the 8MHz clock in the non-USB example.
In the USB case, the bus clock is often a lower-frequency clock than the sample clock, whereas the bus
clock is almost always a higher-frequency clock than the sample clock in a non-USB case.

Service Clock: This clock is determined by the rate at which client software runs to service IRPs that
may have accumulated between executions. This clock also can be the same in the USB and non-USB
cases.

In most existing operating systems, it is not possible to support a broad range of isochronous
communication flows if each device driver must be interrupted for each sample for fast sample rates.
Therefore, multiple samples, if not multiple packets, will be processed by client software and then given to
the Host Controller to sequence over the bus according to the prenegotiated bus access requirements.
Figure 5-14 presents an example for a reasonable USB clock environment equivalent to the non-USB
example in Figure 5-13.

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Universal Serial Bus Specification Revision 1.1

Each DD has
independent
service rate

Mixer Device
DriverRate
Matcher

1 speaker DD
service period(n sample)
slop buffer

Rate
Matcher
Master Clock20ms service
period1 sampleslop buffer

Microphone
Device Driver

Transfer
Complete
Interrupt
QueueBuffer

2x161 Byte Buffer(2 Services,159-161 samples per
service,
20 packets/service)Transfer
Complete
Interrupt
USB SW

Host
Controller

1KHz Bus Clock
7-9 Byte Packets
7-9 samples per packet

Speaker
Device Driver

20ms service
periodQueueBuffer

2x3532 Byte Buffer(2 Services,
881-883 samples per
service
20 packets/service)

1x3 Byte Buffer(1 Services,
1 feedback per service
1 packets/service)

172-184 Byte Packets
43-46 samples per packet

3 byte packets
Feedback Info
Hub

Mono
Microphone

8+9 Byte Buffer(2 Packets)

8kHz Sample Clock(1 byte/sample)

CD Stereo
Speakers(44+45+1+1)x4
Byte Buffer(2 Packets)

44.1KHz Sample Clock(4 bytes/sample)

Figure 5-14. USB Isochronous Application

60

1x3 Byte
Buffer(1 Packets)

SoftwareHardwareUniversal Serial Bus Specification Revision 1.1

Figure 5-14 shows a typical round trip path of information from a microphone as an input device to a
speaker as an output device. The clocks, packets, and buffering involved are also shown. Figure 5-14 will
be explored in more detail in the following sections.
The focus of this example is to identify the differences introduced by the USB compared to the previous
non-USB example. The differences are in the areas of buffering, synchronization given the existence of a
USB bus clock, and delay. The client software above the device drivers can be unaffected in most cases.

5.10.3 Clock Synchronization
In order for isochronous data to be manipulated reliably, the three clocks identified above must be
synchronized in some fashion. If the clocks are not synchronized, several clock-to-clock attributes can bepresent that can be undesirable:


Clock Drift: Two clocks that are nominally running at the same rate can, in fact, have implementationdifferences that result in one clock running faster or slower than the other over long periods of time. If
uncorrected, this variation of one clock compared to the other can lead to having too much or too little
data when data is expected to always be present at the time required.



Clock Jitter: A clock may vary its frequency over time due to changes in temperature, etc. This may
also alter when data is actually delivered compared to when it is expected to be delivered.



Clock-to-clock Phase Differences: If two clocks are not phase locked, different amounts of data may
be available at different points in time as the beat frequency of the clocks cycle out over time. Thiscan lead to quantization/sampling related artifacts.

The bus clock provides a central clock with which USB hardware devices and software can synchronize to
one degree or another. However, the software will, in general, not be able to phase- or frequency-lockprecisely to the bus clock given the current support for “real time-like” operating system schedulingsupport in most PC operating systems. Software running in the host can, however, know that data moved
over the USB is packetized. For isochronous transfer types, a single packet of data is moved exactly once
per frame and the frame clock is reasonably precise. Providing the software with this information allows it
to adjust the amount of data it processes to the actual frame time that has passed.

5.10.4 Isochronous Devices
The USB includes a framework for isochronous devices that defines synchronization types, howisochronous endpoints provide data rate feedback, and how they can be connected together. Isochronous
devices include sampled analog devices (for example, audio and telephony devices) and synchronous data
devices. Synchronization type classifies an endpoint according to its capability to synchronize its data rate
to the data rate of the endpoint to which it is connected. Feedback is provided by indicating accurately
what the required data rate is, relative to the SOF frequency. The ability to make connections depends on
the quality of connection that is required, the endpoint synchronization type, and the capabilities of the hostapplication that is making the connection. Additional device class-specific information may be required,depending on the application.
Note: the term “data” is used very generally, and may refer to data that represents sampled analog
information (like audio), or it may be more abstract information. “Data rate” refers to the rate at whichanalog information is sampled, or the rate at which data is clocked.

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Universal Serial Bus Specification Revision 1.1

The following information is required in order to determine how to connect isochronous endpoints:
Synchronization type:

Asynchronous: Unsynchronized, although sinks provide data rate feedback

Synchronous: Synchronized to the USB’s SOF

Adaptive: Synchronized using feedback or feedforward data rate informationAvailable data rates



Available data formats.

Synchronization type and data rate information are needed to determine if an exact data rate match exists
between source and sink, or if an acceptable conversion process exists that would allow the source to beconnected to the sink. It is the responsibility of the application to determine whether the connection can besupported within available processing resources and other constraints (like delay). Specific USB deviceclasses define how to describe synchronization type and data rate information.
Data format matching and conversion is also required for a connection, but it is not a unique requirement
for isochronous connections. Details about format conversion can be found in other documents related to
specific formats.

5.10.4.1 Synchronization TypeThree distinct synchronization types are defined. Table 5-7 presents an overview of endpoint
synchronization characteristics for both source and sink endpoints. The types are presented in order ofincreasing capability.
Table 5-7. Synchronization Characteristics

Asynchronous

Synchronous

Adaptive

SourceSink

Free running Fs

Free running Fs

Provides implicit feedforward (data stream)

Provides explicit feedback (interrupt pipe)

Fs locked to SOF

Fs locked to SOF

Uses implicit feedback (SOF)

Uses implicit feedback (SOF)

Fs locked to sinkFs locked to data flow

Uses explicit feedback (control pipe)

Uses implicit feedforward (data stream)

5.10.4.1.1 Asynchronous
Asynchronous endpoints cannot synchronize to SOF or any other clock in the USB domain. They sourceor sink an isochronous data stream at either a fixed data rate (single-frequency endpoints), a limited
number of data rates (32kHz, 44.1kHz, 48kHz, …), or a continuously programmable data rate. If the data
rate is programmable, it is set during initialization of the isochronous endpoint. Asynchronous devices
must report their programming capabilities in the class-specific endpoint descriptor as described in theirdevice class specification. The data rate is locked to a clock external to the USB or to a free-runninginternal clock. These devices place the burden of data rate matching elsewhere in the USB environment.
Asynchronous source endpoints carry their data rate information implicitly in the number of samples they
produce per frame. Asynchronous sink endpoints must provide explicit feedback information to an
adaptive driver (refer to Section 5.10.4.2).
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Universal Serial Bus Specification Revision 1.1

An example of an asynchronous source is a CD-audio player that provides its data based on an internal
clock or resonator. Another example is a Digital Audio Broadcast (DAB) receiver or a Digital Satellite
Receiver (DSR). Here too, the sample rate is fixed at the broadcasting side and is beyond USB control.
Asynchronous sink endpoints could be low-cost speakers, running off of their internal sample clock.
Another case arises when there are two or more devices present on the USB that need to have mastershipcontrol over SOF generation in order to operate as synchronous devices. This could happen if there were
two telephony devices, each locked to a different external clock. One telephony device could be digitallyconnected to a Private Branch Exchange (PBX) that is not synchronized to the ISDN. The other device
could be connected directly to the ISDN. Each device will source or sink data to/from the network side atan externally driven rate. Because only one of the devices can take mastership over the SOF, the other willsink or source data at a rate that is asynchronous to the SOF. This example indicates that every device
capable of SOF mastership may be forced to operate as an asynchronous device.

5.10.4.1.2 Synchronous
Synchronous endpoints can have their clock system (their notion of time) controlled externally throughSOF synchronization. These endpoints must be doing one of the following:

Slaving their sample clock to the 1ms SOF tick (by means of a programmable PLL).

Controlling the rate of USB SOF generation so that their data rate becomes automatically locked to
SOF. In case these endpoints are not granted SOF mastership, they must degenerate to theasynchronous mode of operation (refer to the asynchronous example).

Synchronous endpoints may source or sink isochronous data streams at either a fixed data rate (singlefrequency endpoints), a limited number of data rates (32kHz, 44.1kHz, 48kHz, …), or a continuously
programmable data rate. If programmable, the operating data rate is set during initialization of theisochronous endpoint. The number of samples or data units generated in a series of USB frames isdeterministic and periodic. Synchronous devices must report their programming capabilities in the classspecific endpoint descriptor as described in their device class specification.
An example of a synchronous source is a digital microphone that synthesizes its sample clock from SOF
and produces a fixed number of audio samples every USB frame. Another possibility is a 64kb/s bitstream from an ISDN “modem.” If the USB SOF generation is locked to the PSTN clock (perhaps throughthe same ISDN device), the data generation will also be locked to SOF and the endpoint will produce astable 64kb/s data stream, referenced to the SOF time notion.

5.10.4.1.3 Adaptive
Adaptive endpoints are the most capable endpoints possible. They are able to source or sink data at any
rate within their operating range. Adaptive source endpoints produce data at a rate that is controlled by thedata sink. The sink provides feedback (refer to Section 5.10.4.2) to the source, which allows the source toknow the desired data rate of the sink. Adaptive endpoints can communicate with all types of sink
endpoints. For adaptive sink endpoints, the data rate information is embedded in the data stream. The
average number of samples received during a certain averaging time determines the instantaneous data rate.
If this number changes during operation, the data rate is adjusted accordingly.
The data rate operating range may center around one rate (e.g., 8kHz), select between several
programmable or auto-detecting data rates (32kHz, 44.1kHz, 48kHz, …), or may be within one or moreranges (e.g., 5kHz to 12kHz or 44kHz to 49kHz). Adaptive devices must report their programming
capabilities in the class-specific endpoint descriptor as described in their device class specification
An example of an adaptive source is a CD player that contains a fully adaptive sample rate converter (SRC)
so that the output sample frequency no longer needs to be 44.1kHz but can be anything within theoperating range of the SRC. Adaptive sinks include such endpoints as high-end digital speakers, headsets,etc.
63Universal Serial Bus Specification Revision 1.1

5.10.4.2 Feedback
An asynchronous sink provides feedback to an adaptive source by indicating accurately what its desired
data rate (Ff) is, relative to the USB SOF frequency. The required data rate is accurate to better than one
sample per second (1Hz) in order to allow a high-quality source rate to be created and to tolerate delays
and errors in the feedback loop.
The Ff value consists of a fractional part, in order to get the required resolution with 1kHz frames, and an
integer part, which gives the minimum number of samples per frame. Ten bits are required to resolve one
sample within a 1kHz frame frequency (1000 / 2^10 = 0.98). This is a ten-bit fraction, represented inunsigned fixed binary point 0.10 format. The integer part needs ten bits (2^10 = 1024) to encode up to
1,023 one-byte samples per frame. The ten-bit integer is represented in unsigned fixed binary point 10.0
format. The combined Ff value can be coded in unsigned fixed binary point 10.10 format, which fits into
three bytes (24 bits). Because the maximum integer value is fixed to 1,023, the 10.10 number will be leftjustified in the 24 bits, so that it has a 10.14 format. Only the first ten bits behind the binary point are
required. The lower four bits may be optionally used to extend the precision of Ff , otherwise, they shall bereported as zero. The bit and byte ordering follows the definitions of other multi-byte fields contained in
Chapter 8.
Each frame, the adaptive source adds Ff to any remaining fractional sample count from the previous frame,
sources the number of samples in the integer part of the sum, and retains the fractional sample count for the
next frame. The source can look at the behavior of Ff over many frames to determine an even moreaccurate rate, if it needs to.
The sink can determine Ff by counting cycles of a clock with a frequency of Fs * 2^P for a period of2^(10-P) frames, where P is an integer. P is practically bound to be in the range [0,10] because there is no
point in using a clock slower than Fs, and no point in trying to update more than once a frame. The
counter is read into Ff and reset every 2^(10-P) frames. As long as no clock cycles are skipped, the countwill be accurate over the long term. An endpoint needs to implement only the number of counter bits thatit requires for its maximum Ff.
A digital telephony endpoint, for example, will usually derive its 8kHz Fs by dividing down the 64kHz
clock (P=3) which it uses to serialize the data stream. The 64kHz clock phase can also give an additional
one bit of accuracy, effectively giving P=4. This would give Ff updates every 2^(10-4) = 64 frames. A 13bit counter would be required to obtain Ff, with three bits for eight samples per frame, and ten bits for thefractional part. The 13 bits would provide a 3.10 field within the 10.14 Ff value, with the remaining bits
set to zero.
The choice of P is endpoint-specific. Use the following guidelines when choosing P:P should be in the range [1,9].

Larger values of P are preferred, because they reduce the size of the frame counter and increase therate at which Ff is updated. More frequent updates result in a tighter control of the source data rate,
which reduces the buffer space required to handle Ff changes.



P should be less than ten so that Ff is averaged across at least two frames in order to reduce SOF jitter
effects.



P should not be zero in order to keep the deviation in the number of samples sourced to less than 1 inthe event of a lost Ff value.

Isochronous transfers are used to read Ff from the feedback register. The desired reporting rate for the
feedback should be 2^(10-P) frames. Ff will be reported at most once per update period. There is nothing
to be gained by reporting the same Ff value more than once per update period. The endpoint may choose
to report Ff only if the updated value has changed from the previous Ff value.
It is possible that the source will deliver one too many or one too few samples over a long period, due toerrors or accumulated inaccuracies in measuring Ff. The sink must have sufficient buffer capability toaccommodate this. When the sink recognizes this condition, it should adjust the reported Ff value to64

Universal Serial Bus Specification Revision 1.1

correct it. This may also be necessary to compensate for relative clock drifts. The implementation of thiscorrection process is endpoint-specific and is not specified.
An adaptive source may obtain the sink data rate information from an adaptive sink that is locked to thesame clock as the sink, as would be the case for a two-way speech connection. In this case, the feedback
pipe is not needed.

5.10.4.3 Connectivity
In order to fully describe the source-to-sink connectivity process, an interconnect model is presented. The
model indicates the different components involved and how they interact to establish the connection.
The model provides for multi-source/multi-sink situations. Figure 5-15 illustrates a typical situation(highly condensed and incomplete). A physical device is connected to the host application software
through different hardware and software layers as described in the USB Specification. At the clientinterface level, a virtual device is presented to the application. From the application standpoint, only
virtual devices exist. It is up to the device driver and client software to decide what the exact relation is
between physical and virtual device.

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Universal Serial Bus Specification Revision 1.1

Host Environment
CD-ROM

DeviceDriverClientIsoc. Pipe

DeviceDriverClientIsoc. Pipe

DeviceDriverClientPhysical Sources

SourceSourceVirtual Sources

Application

USB Environment

Virtual Sinks
Sink

Sink

Isoc. Pipe

DeviceDriverClientIsoc. Pipe

DeviceDriverClientDeviceDriverClientPhysical Sinks

Hard Disk
Figure 5-15. Example Source/Sink Connectivity
Device manufacturers (or operating system vendors) must provide the necessary device driver software andclient interface software to convert their device from the physical implementation to a USB-compliant
software implementation (the virtual device). As stated before, depending on the capabilities built into thissoftware, the virtual device can exhibit different synchronization behavior from the physical device.
However, the synchronization classification applies equally to both physical and virtual devices. All
physical devices belong to one of the three possible synchronization types. Therefore, the capabilities thathave to be built into the device driver and/or client software are the same as the capabilities of a physicaldevice. The word “application” must be replaced by “device driver/client software.” In the case of aphysical source to virtual source connection, “virtual source device” must be replaced by “physical source
device” and “virtual sink device” must be replaced by “virtual source device.” In the case of a virtual sinkto physical sink connection, “virtual source device” must be replaced by “virtual sink device” and “virtual
sink device” must be replaced by “physical sink device.”

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Universal Serial Bus Specification Revision 1.1

Placing the rate adaptation (RA) functionality into the device driver/client software layer has the distinct
advantage of isolating all applications, relieving the device from the specifics and problems associated with
rate adaptation. Applications that would otherwise be multi-rate degenerate to simpler mono-rate systems.
Note: the model is not limited to only USB devices. For example, a CD-ROM drive containing 44.1kHz
audio can appear as either an asynchronous, synchronous, or adaptive source. Asynchronous operationmeans that the CD-ROM fills its buffer at the rate that it reads data from the disk, and the driver empties
the buffer according to its USB service interval. Synchronous operation means that the driver uses theUSB service interval (e.g., 10ms) and nominal sample rate of the data (44.1kHz) to determine to put out
441 samples every USB service interval. Adaptive operation would build in a sample rate converter tomatch the CD-ROM output rate to different sink sampling rates.
Using this reference model, it is possible to define what operations are necessary to establish connections
between various sources and sinks. Furthermore, the model indicates at what level these operations mustor can take place. First there is the stage where physical devices are mapped onto virtual devices and viceversa. This is accomplished by the driver and/or client software. Depending on the capabilities included inthis software, a physical device can be transformed into a virtual device of an entirely different
synchronization type. The second stage is the application that uses the virtual devices. Placing rate
matching capabilities at the driver/client level of the software stack relieves applications communicatingwith virtual devices from the burden of performing rate matching for every device that is attached to them.
Once the virtual device characteristics are decided, the actual device characteristics are not any moreinteresting than the actual physical device characteristics of another driver.
As an example, consider a mixer application that connects at the source side to different sources, eachrunning at their own frequencies and clocks. Before mixing can take place, all streams must be converted
to a common frequency and locked to a common clock reference. This action can be performed in thephysical-to-virtual mapping layer or it can be handled by the application itself for each source device
independently. Similar actions must be performed at the sink side. If the application sends the mixed data
stream out to different sink devices, it can either do the rate matching for each device itself or it can rely on
the driver/client software to do that, if possible.
Table 5-8 indicates at the intersections what actions the application must perform to connect a source
endpoint to a sink endpoint.

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Universal Serial Bus Specification Revision 1.1

Table 5-8. Connection Requirements
Source Endpoint
Sink Endpoint

Asynchronous

Synchronous

Adaptive

Asynchronous

Async Source/Sink RA
See Note 1.

Async SOF/Sink RA
See Note 2.

Data + Feedback
FeedthroughSee Note 3.

Synchronous

Async Source/SOF RA
See Note 4.

Sync RASee Note 5.

Data Feedthrough +Application Feedback
See Note 6.

Adaptive

Data FeedthroughSee Note 7.

Data FeedthroughSee Note 8.

Data FeedthroughSee Note 9.

Notes:
1. Asynchronous RA in the application. Fsi is determined by the source, using the feedforward information
embedded in the data stream. Fso is determined by the sink, based on feedback information from thesink. If nominally Fsi = Fso, the process degenerates to a feedthrough connection if slips/stuffs due tolack of synchronization are tolerable. Such slips/stuffs will cause audible degradation in audio
applications.
2. Asynchronous RA in the application. Fsi is determined by the source but locked to SOF. Fso isdetermined by the sink, based on feedback information from the sink. If nominally Fsi = Fso, theprocess degenerates to a feedthrough connection if slips/stuffs due to lack of synchronization aretolerable. Such slips/stuffs will cause audible degradation in audio applications.
3. If Fso falls within the locking range of the adaptive source, a feedthrough connection can be established.
Fsi = Fso and both are determined by the asynchronous sink, based on feedback information from thesink. If Fso falls outside the locking range of the adaptive source, the adaptive source is switched tosynchronous mode and Note 2 applies.
4. Asynchronous RA in the application. Fsi is determined by the source. Fso is determined by the sinkand locked to SOF. If nominally Fsi = Fso, the process degenerates to a feedthrough connection if
slips/stuffs due to lack of synchronization are tolerable. Such slips/stuffs will cause audible degradation
in audio applications.
5. Synchronous RA in the application. Fsi is determined by the source and locked to SOF. Fso isdetermined by the sink and locked to SOF. If Fsi = Fso, the process degenerates to a loss-free
feedthrough connection.
6. The application will provide feedback to synchronize the source to SOF. The adaptive source appears
to be a synchronous endpoint and Note 5 applies.
7. If Fsi falls within the locking range of the adaptive sink, a feedthrough connection can be established.
Fsi = Fso and both are determined by and locked to the source.
If Fsi falls outside the locking range of the adaptive sink, synchronous RA is done in the host to provide
an Fso that is within the locking range of the adaptive sink.
8. If Fsi falls within the locking range of the adaptive sink, a feedthrough connection can be established.
Fso = Fsi and both are determined by the source and locked to SOF.
If Fsi falls outside the locking range of the adaptive sink, synchronous RA is done in the host to provide
an Fso that is within the locking range of the adaptive sink.
9. The application will use feedback control to set Fso of the adaptive source when the connection is set
up. The adaptive source operates as an asynchronous source in the absence of ongoing feedback
information and Note 7 applies.

68Universal Serial Bus Specification Revision 1.1

In cases where RA is needed but not available, the rate adaptation process could be mimicked by sample
dropping/stuffing. The connection could then still be made, possibly with a warning about poor quality;
otherwise, the connection cannot be made.

5.10.4.3.1 Audio Connectivity
When the above is applied to audio data streams, the RA process is replaced by sample rate conversion,
which is a specialized form of rate adaptation. Instead of error control, some form of sample interpolation
is used to match incoming and outgoing sample rates. Depending on the interpolation techniques used, theaudio quality (distortion, signal to noise ratio, etc.) of the conversion can vary significantly. In general,higher quality requires more processing power.

5.10.4.3.2 Synchronous Data Connectivity
For the synchronous data case, RA is used. Occasional slips/stuffs may be acceptable to many applications
that implement some form of error control. Error control includes error detection and discard, error
detection and retransmit, or forward error correction. The rate of slips/stuffs will depend on the clock
mismatch between the source and sink, and may be the dominant error source of the channel. If the error
control is sufficient, then the connection can still be made.

5.10.5 Data Prebuffering
The USB requires that devices prebuffer data before processing/transmission to allow the host more
flexibility in managing when each pipe’s transaction is moved over the bus from frame to frame.
For transfers from function to host, the endpoint must accumulate samples during frame X until it receives
the SOF token for frame X+1. It “latches” the data from frame X into its packet buffer and is now ready tosend the packet containing those samples during frame X+1. When it will send that data during the frame
is determined solely by the Host Controller and can vary from frame to frame.
For transfers from host to function, the endpoint will accept a packet from the host sometime during frame
Y. When it receives the SOF for frame Y+1, it can then start processing the data received in frame Y.
This approach allows an endpoint to use the SOF token as a stable clock with very little jitter and/or drift
when the Host Controller moves the packet over the bus. This approach also allows the Host Controller tovary within a frame precisely when the packet is actually moved over the bus. This prebuffering
introduces some additional delay between when a sample is available at an endpoint and when it moves
over the bus compared to an environment where the bus access is at exactly the same time offset from SOF
from frame to frame.
Figure 5-16 shows the time sequence for a function-to-host transfer (IN process). Data D0 is accumulated
during frame Fi at time Ti, and transmitted to the host during frame Fi+1. Similarly, for a host-to-function
transfer (OUT process), data D0 is received by the endpoint during frame Fi+1 and processed during frame
Fi+2.

69

Universal Serial Bus Specification Revision 1.1

Time:

Ti

T i+1

T i+2

T i+3

...

Tm

T m+1

...

Frame:

Fi

F i+1

F i+2

F i+3

...

Fm

F m+1

...

D0

D1

D2

...

D0

D1

...

D0

D1

...

D0

...

Data on Bus:OUT Process:
IN Process

D0

D1

...

D0

...

Figure 5-16. Data Prebuffering

5.10.6 SOF Tracking
Functions supporting isochronous pipes must receive and comprehend the SOF token to support
prebuffering as previously described. Given that SOFs can be corrupted, a device must be prepared torecover from a corrupted SOF. These requirements limit isochronous transfers to full-speed devices only,
because low-speed devices do not see SOFs on the bus. Also, because SOF packets can be damaged intransmission, devices that support isochronous transfers need to be able to synthesize the existence of an
SOF that they may not see due to a bus error.
Isochronous transfers require the appropriate data to be transmitted in the corresponding frame. The USB
requires that when an isochronous transfer is presented to the Host Controller, it identifies the frame
number for the first frame. The Host Controller must not transmit the first transaction before the indicatedframe number. Each subsequent transaction in the IRP must be transmitted in succeeding frames. If there
are no transactions pending for the current frame, then the Host Controller must not transmit anything for
an isochronous pipe. If the indicated frame number has passed, the Host Controller must skip (i.e., nottransmit) all transactions until the one corresponding to the current frame is reached.

5.10.7 Error Handling
Isochronous transfers provide no data packet retries (i.e., no handshakes are returned to a transmitter by areceiver) so that timeliness of data delivery is not perturbed. However, it is still important for the agents
responsible for data transport to know when an error occurs and how the error affects the communication
flow. In particular, for a sequence of data packets (A, B, C, D), the USB allows sufficient information suchthat a missing packet (A, _, C, D) can be detected and will not unknowingly be turned into an incorrect
data or time sequence (A, C, D or A, _, B, C, D). The protocol provides four mechanisms that support this:exactly one packet per frame, SOF, CRC, and bus transaction timeout.

70Isochronous transfers require exactly one data transaction every frame for normal operation. The USB
does not dictate what data is transmitted in each frame. The data transmitter/source determines
specifically what data to provide. This regular data-per-frame provides a framework that isfundamental to detecting missing data errors. Any phase of a transaction can be damaged duringtransmission on the bus. Chapter 8 describes how each error case affects the protocol.

Because every frame is preceded by an SOF and a receiver can see SOFs on the bus, a receiver candetermine that its expected transaction did not occur between two SOFs. Additionally, because even
an SOF can be damaged, a device must be able to reconstruct the existence of a missed SOF as
described in Section 5.10.6.

Universal Serial Bus Specification Revision 1.1

A data packet may be corrupted on the bus; therefore, CRC protection allows a receiver to determinethat the data packet it received was corrupted.

The protocol defines the details that allow a receiver to determine via bus transaction timeout that it is
not going to receive its data packet after it has successfully seen its token packet.

Once a receiver has determined that a data packet was not received, it may need to know the size of thedata that was missed in order to recover from the error with regard to its functional behavior. If thecommunication flow is always the same data size per frame, then the size is always a known constant.
However, in some cases the data size can vary from frame to frame. In this case, the receiver andtransmitter have an implementation-dependent mechanism to determine the size of the lost packet.
In summary, whether a transaction is actually moved successfully over the bus or not, the transmitter andreceiver always advance their data/buffer streams one transaction per frame to keep data-per-timesynchronization. The detailed mechanisms described above allow detection, tracking, and reporting of
damaged transactions so that a function or its client software can react to the damage in a functionappropriate fashion. The details of that function- or application-specific reaction are outside the scope of
the USB Specification.

5.10.8 Buffering for Rate Matching
Given that there are multiple clocks that affect isochronous communication flows in the USB, buffering isrequired to rate match the communication flow across the USB. There must be buffer space available bothin the device per endpoint and on the host side on behalf of the client software. These buffers provide
space for data to accumulate until it is time for a transfer to move over the USB. Given the natural data
rates of the device, the maximum size of the data packets that move over the bus can also be calculated.
Figure 5-17 shows the equations used to determine buffer size on the device and host and maximum packet
size that must be requested to support a desired data rate. These equations allow a device and client
software design time-determined service clock rate (variable X), sample clock rate (variable C), and sample
size (variable S). The USB allows only one transaction per bus clock. These equations should provide
design information for selecting the appropriate packet size that an endpoint will report in its characteristic
information and the appropriate buffer requirements for the device/endpoint and its client software. Figure
5-14 shows actual buffer, packet, and clock values for a typical isochronous example.

71Universal Serial Bus Specification Revision 1.1

Isochronous Rate (Clock) Matching
By Buffering

X Service Clock

M = (2 * N * P) Byte Buffer
for 2 Services,N = (CEIL(1KHz / X)) packets
per service

1KHz Bus Clock

P = (CEIL(C / 1KHz) * S)
Byte Packets

C Sample Clock(S byte/sample)

B=2*P
Byte Buffer(2 Packets)

Figure 5-17. Packet and Buffer Size Formulas for Rate-Matched Isochronous Transfers
The USB data model assumes that devices have some natural sample size and rate. The USB supports thetransmission of packets that are multiples of sample size to make error recovery handling easier when
isochronous transactions are damaged on the bus. If a device has no natural sample size or if its samples
are larger than a packet, it should describe its sample size as being one byte. If a sample is split across adata packet, the error recovery can be harder when an arbitrary transaction is lost. In some cases, datasynchronization can be lost unless the receiver knows in what frame number each partial sample istransmitted. Furthermore, if the number of samples can vary due to clock correction (e.g., for a
non-derived device clock), it may be difficult or inefficient to know when a partial sample is transmitted.
Therefore, the USB does not split samples across packets.

72Universal Serial Bus Specification Revision 1.1

Chapter 6
MechanicalThis chapter provides the mechanical and electrical specifications for the cables, connectors, and cable
assemblies used to interconnect USB devices. The specification includes the dimensions, materials,
electrical, and reliability requirements. This chapter documents minimum requirements for the external
USB interconnect. Substitute material may be used as long as it meets these minimums.

6.1 Architectural Overview
The USB physical topology consists of connecting the downstream hub port to the upstream port of anotherhub or to a device. The USB can operate at two speeds. Full-speed, 12 Mb/s, requires the use of a shieldedcable with two power conductors and twisted pair signal conductors. Low-speed, 1.5 Mb/s, relaxes thecable requirement. Low-speed cable does not require shielding or twisted pair signal conductors.
The connectors are designed to be hot plugged. The USB Icon on the plugs provides tactile feedback
making it easy to obtain proper orientation.

6.2 Keyed Connector Proto col
To minimize end user termination problems, USB uses a “keyed connector” protocol. The physicaldifference in the Series “A” and “B” connectors insure proper end user connectivity. The “A” connector isthe principle means of connecting USB devices. All USB devices must have an “A” connector. The “B”
connector allows device vendors to provide a standard detachable cable. This facilitates end user cablereplacement. Figure 6-1 illustrates the keyed connector protocol.

Series "A" Connectors

Series "B" Connectors

 Series "A" plugs arealways oriented upstream
towards the Host System

 Series "B" plugs arealways oriented
downstream towards the
USB Device"A" Plugs(From theUSB Device)"B" Plugs(From theHost System)"A" Receptacles(Downstream Output
from the USB Host or
Hub)"B" Receptacles(Upstream Input to the
USB Device or Hub)Figure 6-1. Keyed Connector Protocol
The following list explains how the plugs and receptacles can be mated:

73Universal Serial Bus Specification Revision 1.1

Series “A” receptacle mates with a Series “A” plug. Electrically, Series “A” receptacles function as
outputs from host systems and/or hubs.

Series “A” plug mates with a Series “A” receptacle. The Series “A” plug always is oriented towards
the host system.



Series “B” receptacle mates with a Series “B” plug (male). Electrically, Series “B” receptacles
function as inputs to hubs or devices.



Series “B” plug mates with a Series “B” receptacle. The Series “B” plug is always oriented towards
the USB hub or device.

6.3 Cable
USB cable consists of four conductors, two power conductors and two signal conductors.
Full-speed cable consists of a signaling twisted pair, VBUS, GND, and an overall shield. Full-speed cablemust be marked to indicate suitability for USB usage (see Section 6.6.2). Full-speed cable may be usedwith either Low-speed or Full-speed devices. When Full-speed cable is used with Low-speed devices, thecable must meet all Low-speed requirements.
Low-speed cable does not require twisted signaling conductors or the overall shield.

6.4 Cable AssemblyThis specification describes three USB cable assemblies. Detachable cable, Full-speed captive cable, andLow-speed captive cable.
The color used for the cable assembly is vendor specific, recommended colors are White, Grey, or Black.

6.4.1 Detachable Cable Assemblies
Full-speed devices can utilize the “B” connector. This allows the device to have a detachable USB cable.
This eliminates the need to build the device with a hardwired cable and minimizes end user problems if
cable replacement is necessary.
Devices utilizing the “B” connector must be designed to work with worst case maximum length detachable
cable. Detachable cable assemblies may be used only on Full-speed devices. Using a Full-speed detachable
cable on a Low-speed device may exceed the maximum Low-speed cable length.
Figure 6-2 illustrates a detachable cable assembly.

74

Universal Serial Bus Specification Revision 1.1

8

7

6

5

4

3

2

1

IMPORTANT NOTICE: All detachable cable assemblies must be Full Speed.
H

H

G

A

B

A

B

G

Overmolded Series "A" Plug

Overmolded Series "B" Plug

(Always upstream towards the "host" system.)(Always downstream towards the USB Device.)

C

C

F

F

C

E

Detail A - A(Series "A" Plug)C
Detail C - C(Typical USB Shielded Cable)

Detail B - B(Series "B" Plug)E

Polyvinyl Chloride (PVC) Jacket> 65% Tinned Copper Braided Shield

11.5

10.5

15.7

Aluminum Metallized Polyester Inner Shield

7.5
1

2

3

28 AWG STC Drain Wire
2

1

3

4

4

D

D

12.0

27.0

C

12.0

Red (V B U S)

Green (D +)Black (Ground)White (D -)

C

32.0

All dimensions are in millimeters ( m m )unless otherwise noted.
Dimensions are TYPICAL and are forgeneral reference purposes only.

9.0
B

B

9.0

Optional Molded
Strain Relief
Series "A" Plug to Series "B" PlugUSB Detachable
Cable Assembly

A
SIZE

DATE

7

6

5

4

3

REV

N/A

2/98
A
SCALE: N/A
8

A

DRAWING NUMBER

C

SHEET
2

1 of 11

Figure 6-2. USB Detachable Cable Assembly75Universal Serial Bus Specification Revision 1.1

Detachable Cables must meet the following electrical requirements:


The cable must be terminated on one end with an overmolded Series “A” plug and the opposite end is
terminated with an overmolded Series “B” plug.

The cable must be rated for Full-speed.



The cable impedance must match the impedance of the Full-speed drivers. The drivers arecharacterized to drive specific cable impedance. Refer to Section 7.1.1 for details.



The maximum allowable cable length is determined by signal pair attenuation. Refer to Section 7.1.17
for details.



The maximum allowable cable length determined by the cable propagation delay. The USB utilizes anunterminated transmission scheme. Exceeding this limit will cause signaling reflections to interferewith data transmission. Refer to Section 7.1.14 for details.



Differences in propagation delay between the two signal conductors must be minimized. Refer to
Chapter 7.1.3 for details.



The GND lead provides a common ground reference between the upstream and downstream ports. The
maximum cable length is limited by the voltage drop across the GND lead. Refer to Section 7.2.2 fordetails. The minimum acceptable wire gauge is calculated assuming the attached device is high power

The VBUS lead provides power to the connected device. For detachable cables, the VBUS requirement
is the same as the GND lead.

6.4.2

Full-speed Captive Cable Assemblies

Full-speed captive cable assemblies may be used with either Full-speed or Low-speed devices. Assemblies
are considered captive if they are provided with a vendor-specific disconnect means. When using a Fullspeed captive cable on a Low-speed device the cable must meet all Low-speed requirements.
Figure 6-3 illustrates a Full-speed cable assembly.

76Universal Serial Bus Specification Revision 1.1

8

7

6

H

5

4

3

2

1

A

B

A

B

H

G

G

Overmolded Series "A" Plug
(Always upstream towards the "host" system.)

Detail A - A(Series "A" Plug)
F

F

15.7

Cut End(Always downstream towards the USB Device.)

7.5
1

2

3

4

Detail B - B (Typical Terminations)

E

12.0

Prepared Termination

Blunt Cut Termination

Polyvinyl Chloride (PVC) Jacket

Polyvinyl Chloride (PVC)
JacketBlunt Cut Termination(Length Dimension Point)

27.0

E

> 65% Tinned Copper Braided
ShieldMetallized
Mylar Inner Shield

D

D

28 AWG STC Drain Wire
Red (V B U S)
Black (Ground)Green (D +)White (D -)9.0

User Specified
Length Dimension Point

C

B

C

Optional Molded
Strain Relief

A l l d i m e n s i o n s a r e i n m i l l i m e t e r s (m m )unless otherwise note.

B

Dimensions are TYPICAL and are forgeneral reference purposes only.

Series "A" Plug to Cut End

USB Full Speed
Hardwired Cable Assembly A

A

SIZE

DATE

DRAWING NUMBER

A

2/98

N/A

SCALE: N/A
8

7

6

5

4

3

REV

C

SHEET
2

1 of 11

Figure 6-3. USB Full-speed Hardwired Cable Assembly77Universal Serial Bus Specification Revision 1.1

Full-speed Captive Cables must meet the following electrical requirements:


The cable must be terminated on one end with an overmolded Series “A” plug and the opposite end is
vendor specific. If the vendor specific interconnect is to be hot plugged it must meet the same
performance requirements as the USB “B” connector.



The cable must be rated for Full-speed.



The cable impedance must match the impedance of the Full-speed drivers. The drivers arecharacterized to drive specific cable impedance. Refer to Section 7.1.1 for details.



The maximum cable length is determined by the attenuation of the signal pair. Refer to Section 7.1.17
for details.



The maximum cable length is determined by the propagation delay though the cable. The USB utilizes
an unterminated transmission scheme, exceeding this limit will cause signaling reflections to interferewith data transmission. Refer to Section 7.1.14 for details.



Differences in propagation delay between the two signal conductors must be minimized. Refer to
Section 7.1.3 for details.



The GND lead provides a common reference between the upstream and downstream ports. The
maximum cable length is determined by the voltage drop across the GND lead. Refer to Section 7.2.2
for details. The minimum wire gauge is calculated using the worst case current consumption.

The VBUS lead provides power to the connected device. The minimum wire gauge is vendor specific.

6.4.3 Low-speed Captive Cable Assemblies
Assemblies are considered captive if they are provided with a vendor-specific disconnect means. Lowspeed cable may only be used on Low-speed devices.
Figure 6-4 illustrates a Low-speed cable assembly.

78

Universal Serial Bus Specification Revision 1.1

8

7

6

5

4

3

2

1

IMPORTANT NOTICE: For use in Low Speed applications only.
H

H

G

A

B

A

B

G

Overmolded Series "A" Plug
(Always upstream towards the "host" system.)F

F

Detail A - A(Series "A" Plug)
Cut End(Always downstream towards the USB Device.)

15.7
7.5
1

E

2

3

4

E

Detail B - B (Typical Terminations)
12.0

Blunt Cut Termination

Prepared Termination

Polyvinyl Chloride (PVC)
JacketBlunt Cut Termination(Length Dimension Point)

Polyvinyl Chloride (PVC) Jacket

D

Red (V B U S)
Black (Ground)D

27.0Green (D +)White (D -)User Specified
Length Dimension Point

C

C

9.0

Optional Molded
Strain Relief

BB

A l l d i m e n s i o n s a r e i n m i l l i m e t e r s (m m )unless otherwise noted.
Dimensions are TYPICAL and are forgeneral reference purposes only.

Series "A" Plug to Cut End

USB Low Speed
Hardwired Cable Assembly A

A

SIZE

DATE

8

7

6

5

4

3

REV

DRAWING NUMBER

N/A

2/98
A
SCALE: N/A

C

SHEET
2

1 of 11

Figure 6-4. USB Low-speed Hardwired Cable Assembly

79Universal Serial Bus Specification Revision 1.1

Low-speed Captive Cables must meet the following electrical requirements:


The cable must be terminated on one end with an overmolded Series “A” plug and the opposite end is
vendor specific. If the vendor specific interconnect is to be hot plugged it must meet the same
performance requirements as the USB “B” connector.



Low-Speed drivers are characterized for operation over a range of capacitive loads. This value includes
all sources of capacitance on the D+ and D-lines, not just the cable. Cable selection must insure that
total load capacitance falls between specified minimum and maximum values. If the desired
implementation does not meet the minimum requirement, additional capacitance needs to be added tothe device. Refer to section 7.1.1.2 for details.



The maximum Low-speed cable length determined by the rise and fall times of Low-speed signaling.
This forces Low-speed cable to be significantly shorter then Full-speed. Refer to Section 7.1.1.2 fordetails.



Differences in propagation delay between the two signal conductors must be minimized. Refer to
Section 7.1.3 for details.



The GND lead provides a common reference between the upstream and downstream ports. The
maximum cable length is determined by the voltage drop across the GND lead. Refer to Section 7.2.2
for details. The minimum wire gauge is calculated using the worst case current consumption.

The VBUS lead provides power to the connected device. The minimum wire gauge is vendor specific.

6.4.4 Prohibited Cable Assemblies
USB is optimized for ease of use. The expectation it that if the device can be plugged in it will work.
By specification, the only conditions that prevent a USB device from being successfully utilized arelack of power, lack of bandwidth, and excessive topology depth. These conditions are well understoodby the system software.
Non-acceptable cables may work in some situations but they cannot be guaranteed to work in all
instances.


Extension cable
Cables that provide a Series “A” plug with a series “A” receptacle or a Series “B” plug with a
Series “B” receptacle. This allows multiple cable segments to be connected together, possibly
exceeding the maximum permissible cable length.

Cable assembly that violates USB topology rules
A cable with both ends terminated in either Series “A” plugs or Series “B” receptacles. This cableallows two downstream ports to be directly connected.
Note: This prohibition does not prevent using a USB device to provide a bridge between two USB
busses.



Low-speed detachable cable
Detachable cables must be Full-speed. Low-speed devices are prohibited from using detachable
cables. Detachable cable is Full-speed rated, using a long Full-speed cable exceeds the capacitive
load of Low-speed.

6.5

Connector Mechanica l Configuration and Material Requirements
The USB Icon is used to identify USB plugs and the receptacles. Figure 6-5 illustrates the USB Icon

80

Universal Serial Bus Specification Revision 1.1

All dimensions are ± 5%
L
Dia:1.67 L

Dia:1.33 L

0.33 LL

Dia:L
1.50 L

Dia:L

0.33 LL
0.5 L

1.50 L

Dia:L

Dia:L

L
L

Dia:1.33 L
1.67 L

0.33 LL

2.33 L
3.75 L
5.00 L
5.17 L
6.25 L
8.00 L

Figure 6-5. USB Icon

6.5.1 USB Icon Location
The USB Icon is embossed, in a recessed area, on the topside of the USB plug. This provides easy user
recognition and facilitates alignment during the mating process. The USB Icon and Manufacture’s logo
should not project beyond the overmold surface. The USB Icon is required, while the Manufacture’s logo isrecommended, for both Series “A” and “B” plug assemblies. The USB Icon is also located adjacent to each
receptacle. Receptacles should be oriented to allow the Icon on the plug to be visible during the matingprocess. Figure 6-6 illustrates the typical plug orientation.

Top View
Optional Top"Locator Detail"A

A

Locator
HeightApproximately
0.6mm(0.024")

Manufacturer's
Engraved Logo

Engraved USB
Icon

1
2

Locator Width
Approximately
0.5mm(0.020")0.6mm (0.024") Max
Manufacturer's
Logo
Engraving Recess

Overmolding
0.6mm (0.024")
Max
USB IconEngraving Recess

3
4

Optional Top"Locator Detail"Section A - A(Plug Cross-Section)

Figure 6-6. Typical USB Plug Orientation81

Universal Serial Bus Specification Revision 1.1

6.5.2 USB Connector Termination Data
Table 6-1 provides the standardized contact terminating assignments by number and electrical value for
Series “A” and Series “B” connectors.
Table 6-1. USB Connector Termination Assignment

Contact
NumberSignal Name

Typical Wiring
Assignment

1

VBUS

Red

2

D-

White

3

D+

Green

4

GND

Black

Shell

ShieldDrain Wire

6.5.3 Series “A” and Series “B” Receptacles
Electrical and mechanical interface configuration data for Series "A" and Series "B" receptacles are shownin Figure 6-7 and Figure 6-8. Also, refer to Figure 6-12, Figure 6-13, and Figure 6-14 at the end of thischapter for typical PCB receptacle layouts.

82Universal Serial Bus Specification Revision 1.1

8

7

6

5

4

3

2

1

USB Series "A" Receptacle Interface
12.50 ± 0.10

HC

A
8.88 ± 0.20

R 0.64 ± 0.13 (Typical)

1.84 ± 0.05

B Center Line

8.38 ± 0.08

H

11.10 ± 0.10

R 0.32 ± 0.13 (Typical)

B

0.50 ± 0.10
G

G

2

1

3 0 0 ± 2 0 (2)34

5.12 ± 0.10
0.38 ± 0.13
F

F

Center Line of 5.12
4.98 ± 0.25

0.64 ± 0.13 (8)Receptacle Contact

1.00 ± 0.05 (2)Contact Point

3.50 ± 0.05 (2)1.00 ± 0.05 (4)Printed Circuit Board

C Center Line

4.13 REF
E

E

All dimensions are in millimeters m( m ) unlessotherwise noted.

USB Series "A" Receptacle and Plug
Mating Features

D

D

Fully Mated Series "A"
Receptacle and Plug0.50 ± 0.10 (2)Overmold Boot

3 0 0 ± 2 0 (2)CC

8.0 MAX

B

B

Receptacle Flange
12.67 MINInterface and Mating Drawing

Allow a minimum spacing of 2.67mm between
the face of the receptacle and the plug
overmold boot.

1
A

Series "A" Receptacle
SIZE

DATE

N/A

2/98
A
SCALE: N/A
8

7

6

5

4

3

SHEET
2

A

REV

DRAWING NUMBER

C
1 of 11

Figure 6-7. USB Series "A" Receptacle Interface and Mating Drawing

83Universal Serial Bus Specification Revision 1.1

8

7

6

5

4

3

2

1

USB Series "B" Receptacle InterfaceAH

5.60 + 0.10

C

8.38 + 0.08
Receptacle Contact

H8.45 + 0.10

8.88 + 0.20

1.63 + 0.05 (2)45 0 + 0.5 0 (2)B

30 0 + 2 0 (4)G

2

7.78 + 0.10

1

B Center LineG

3.18 + 0.05
34

0.80 + 0.08

3.67 + 0.08
0.38 + 0.13 (4)F

R 0.38 (6)Contact Point
1.00 + 0.05 (4)4.98 + 0.25

F

3.67 Center Line
1.25 + 0.10 (4)C Center Line
Receptacle Housing
30 0 + 2 0 (2)EE

B Center LineAll dimensions are in millimeters (m m )unless otherwise noted.

0.50 + 0.10 (2)DD

Receptacle Shell

USB Series "B" Receptacle and Plug Mating Features

Overmold BootC

Overmold BootC

10.5 MAX

11.5 MAX

B

B

2.67 MIN

1Interface and Mating Drawing

Receptacle Shell
Fully Mated Plug and Receptacle

USB Series "B" Receptacle

A

Allow a minimum spacing of 2.67mm between theface of the receptacle and the plug overmold boot.

1
8

7

6

5

4

SIZE

DATE

DRAWING NUMBER

A

2/98

N/A

SCALE:

N/A

3

SHEET
2

Figure 6-8. USB Series "B" Recptacle Interface and Mating Drawing

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Universal Serial Bus Specification Revision 1.1

6.5.3.1 Receptacle Injection M olded Thermoplastic Insulator Material
Minimum UL 94-V0 rated, thirty percent (30%) glass-filled polybutylene terephthalate (PBT) or
polyethylene terephthalate (PET) or better.
Typical Colors: Black, Gray and Natural.
Flammability Characteristics: UL 94-V0 rated.
Flame Retardant Package must meet or exceed the requirements for UL, CSA, VDE, et cetera.
Oxygen Index (LOI): Greater than 21%. ASTM D 2863.

6.5.3.2

Receptacle Shell Mate rials

Substrate Material: 0.30 + 0.05 mm phosphor bronze, nickel silver or other copper based high strength
materials.
Plating:1.

Underplate: Optional. Minimum 1.00 micrometers (40 microinches) Nickel. In addition,
manufacturer may use a copper underplate beneath the nickel.

2.

Outside: Minimum 2.5 micrometers (100 microinches) Bright Tin or Bright Tin-Lead.

6.5.3.3

Receptacle Contact M aterials

Substrate Material: 0.30 + 0.05 mm minimum half-hard phosphor bronze or other the high strength copper
based material.
Plating: Contacts are to be selectively plated.
A. Option I
1.

Underplate: Minimum 1.25 micrometers (50 microinches) Nickel. Copper over base material
is optional.

2.

Mating Area: Minimum 0.05 micrometers (2 microinches) Gold over a minimum of 0.70
micrometers (28 microinches) Palladium.

3.

Solder Tails: Minimum 3.8 micrometers (150 microinches) Bright Tin-Lead over theunderplate.

B. Option II
1.

Underplate: Minimum 1.25 micrometers (50 microinches) Nickel. Copper over base material
is optional.

2.

Mating Area: Minimum 0.05 micrometers (2 microinches) Gold over a minimum of 0.75
micrometers (30 microinches) Palladium-Nickel.

3.

Solder Tails: Minimum 3.8 micrometers (150 microinches) Bright Tin-Lead over theunderplate.

C. Option III
1.

Underplate: Minimum 1.25 micrometers (50 microinches) Nickel. Copper over base material
is optional.

2.

Mating Area: Minimum 0.75 micrometers (30 microinches) Gold.

3.

Solder Tails: Minimum 3.8 micrometers (150 microinches) Bright Tin-Lead over theunderplate.

85

Universal Serial Bus Specification Revision 1.1

6.5.4 Series “A” and Series “B” Plugs
Electrical and mechanical interface configuration data for Series "A" and Series "B" plugs are shown in
Figure 6-9 and Figure 6-10.

86Universal Serial Bus Specification Revision 1.1

87

8.0 MAX5

12.00 ± 0.10

B
H

6

4

3

1

11.75 MIN

R 0.64 + 0.13 TypicalH

3 0 0 ± 20 Typical

0.15 ± 0.10 Typical

0.315 ± 0.03 Typical2

4.50 ± 0.10
3 0 0 ± 20
4

3

2

1

G

Plug Contact

8.0 MAXAA

UL 94-V0 Plug Housing
11.75 MIN

A

1

16.0 MAX
4.0 MAXG

0.38 ± 0.13

1.95 ± 0.05

2.00 ± 0.13 (4)5.16 ± 0.10

B Center Line

1.00 ± 0.05 (4)F

2.50 ± 0.05 (2)B Center LineE

2.00 ± 0.05 (2)B

Section A - A

Overmold Boot

FE

2.50 ± 0.13 (4)BOvermold Boot
1

D

8.65 ± 0.19
7.41 ± 0.31
6.41 ± 0.31

4.2 MINGOLD PLATE AREA

Overall connector and cable assembly
length is measured from Datum 'A' of
the Series "A" Plug to Datum 'A' of theSeries "B" Plug or to the blunt end
termination.

D

All dimensions are in millimeters m( m)
unless otherwise noted.

CC

1.0 ± 0.05 (2)3.5 ± 0.05 (2)9.70 ± 0.13

Section B - B
B

B

Interface Drawing
A

USB Series "A" Plug

0.16 ± 0.15
SIZE

0.13 ± 0.13
8

7

6

DATE

N/A

2/98
A
SCALE: N/A

5

4

3

SHEET

2

A
REV

DRAWING NUMBER

C
1 of 11

Figure 6-9. USB Series "A" Plug Interface Drawing

87Universal Serial Bus Specification Revision 1.1

8

7

6

5

4

3

2

1

8.00 ± 0.10

HH

5.83 ± 0.10

C

10.5 MAX

0.38 MAXG

45 0 ± 0.5 0 (2)

1

4

F

30 0 ± 2 0 Typical

B

Overmold Boot

1.46 ± 0.10

23.29 ± 0.05
7.26 ± 0.10

30.80 ± 0.05

Center Lineof 2.85G

30 0 ± 2 0 (2)

A

F

A

2.85 ± 0.13 (2)C Center Line

B Center Line

11.75 MIN
11.5 MAX

3.70 ± 0.13

A

1

B
E

E

Overmold Boot

Section A - A

0.25 ± 0.05
6.41 ± 0.31

DD

4.20 MINGold Plate Area

B
8.65 ± 0.19
1C

1.16 MAX

7.41 ± 0.31

1.25 ± 0.10 (4)Overall connector and cable assembly lengthis measured from Datum 'A' of the Series "B"
Plug to Datum 'A' of the Series "A" Plu g or
the blunt end termination.

C

4.67 ± 0.10
All dimensions are in millimeters (m m )unless otherwise noted.

C Center Line

Section B - B
B

B

0.13 ± 0.13
Typical0.16 ± 0.15
TypicalInterface Drawing

USB Series "B" Plug

A

8

7

6

5

4

SIZE

DATE

DRAWING NUMBER

A

2/98

N/A

SCALE:

N/A

3

Figure 6-10. USB Series “B” Plug Interface Drawing

88

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2

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Universal Serial Bus Specification Revision 1.1

6.5.4.1

Plug Injection Molded Thermoplastic Insulator Material

Minimum UL 94-V0 rated, thirty percent (30%) glass-filled polybutylene terephthalate (PBT) or
polyethylene terephthalate (PET) or better.
Typical Colors: Black, Gray and Natural.
Flammability Characteristics: UL 94-V0 rated.
Flame Retardant Package must meet or exceed the requirements for UL, CSA and VDE.
Oxygen Index (LOI): 21%. ASTM D 2863.

6.5.4.2

Plug Shell Materials

Substrate Material: 0.30 + 0.05 mm phosphor bronze, nickel silver or other suitable material.
Plating:
A. Underplate: Optional. Minimum 1.00 micrometers (40 microinches) nickel. In addition,
manufacturer may use a copper underplate beneath the nickel.
B. Outside: Minimum 2.5 micrometers (100 microinches) bright tin or bright tin-lead.

6.5.4.3

Plug (Male) Contact M aterials

Substrate Material. 0.30 + 0.05 mm half-hard phosphor bronze.
Plating. Contacts are to be selectively plated.
A. Option I
1.

Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.

2.

Mating Area: Minimum 0.05 micrometers (2 microinches) gold over a minimum of 0.70
micrometers (28 microinches) palladium.

3.

Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over theunderplate.

B. Option II
1.

Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.

2.

Mating Area: Minimum 0.05 micrometers (2 microinches) gold over a minimum of 0.75
micrometers (30 microinches) palladium-nickel.

3.

Wire Crimp/Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead overthe underplate.

C. Option III
1.

Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.

2.

Mating Area: Minimum 0.75 micrometers (30 microinches) gold.

3.

Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over theunderplate.

89Universal Serial Bus Specification Revision 1.1

6.6 Cable Mechanical Con figuration and Material Requirements
Full-speed and Low-speed cables differ in data conductor arrangement and shielding. Low-speed cable
does not require twisted data conductors or a shield. Figure 6-11 shows the typical Full-speed cable
construction.
Polyvinyl Chloride (PVC) Jacket

Non-Twisted Power Pair:
Red:
V BUS
Black: Power Ground

Outer Shield > 65% Interwoven
Tinned Copper Braid

W
R

B

Inner Shield Aluminum
Metallized PolyesterG

28 AWG Tinned
Copper Drain Wire

Twisted Signaling Pair:
White:
DGreen: D+

Figure 6-11. Typical Full-speed Cable Construction

6.6.1 Description
Full-speed cable consists of one 28 to 20 AWG non-twisted power pair and one 28 AWG twisted data pairwith an aluminum metallized polyester inner shield, 28 AWG stranded tinned copper drain wire, > 65%
tinned copper wire interwoven (braided) outer shield and PVC outer jacket.
Low-speed cable does not require the data pair be twisted or a shield and drain wire.

90

Universal Serial Bus Specification Revision 1.1

6.6.2

Construction

Raw materials used in the fabrication of this cable shall be of such quality that the fabricated cable iscapable of meeting or exceeding the mechanical and electrical performance criteria of the most current
USB Specification Revision, and all applicable domestic and international safety/testing agency
requirements, e.g., UL, CSA, BSA, NEC, et cetera, for electronic signaling and power distribution cables in
its category.
Table 6-2. Power Pair

American Wire
Gauge (AWG)
28
26
24
22
20

Nominal Conductor
Outer Diameter

Stranded Tinned
Conductors

0.381 mm (0.015”)

7 x 36

0.406 mm (0.016”)

19 x 400.483 mm (0.019”)

7 x 340.508 mm (0.020”)

19 x 380.610 mm (0.024”)

7 x 320.610 mm (0.024”)

19 x 360.762 mm (0.030”)

7 x 300.787 mm (0.031”)

19 x 34

0.890 mm (0.035”)

7 x 280.931 mm (0.037”)

19 x 32

Note: Minimum conductor construction shall be stranded tinned copper.
Non-Twisted Power Pair:
A. Wire Gauge: Minimum 28 AWG or as specified by the user contingent upon the specified cable
length. Refer to Table 6-2.
B. Wire Insulation: Semirigid polyvinyl chloride (PVC).
1.

Nominal Insulation Wall Thickness: 0.25 mm (0.010”).

2.

Typical Power (VBUS) Conductor: Red Insulation.

3.

Typical Ground Conductor: Black Insulation.

Signal Pair:
A. Wire Gauge: 28 AWG minimum. Refer to Table 6-3.
Table 6-3. Signal Pair

American Wire
Gauge (AWG)
28

Nominal Conductor
Outer Diameter

Stranded Tinned
Conductors

0.381 mm (0.015”)

7 x 36

0.406 mm (0.016”)

19 x 4091

Universal Serial Bus Specification Revision 1.1

Note: Minimum conductor construction shall be stranded tinned copper
B. Wire Insulation: High-density polyethylene (HDPE), alternately foamed polyethylene or foamed
polypropylene.
1.

Nominal Insulation Wall Thickness: 0.31 mm (0.012”).

2.

Typical Data Plus (+) Conductor: Green Insulation.

3.

Typical Data Minus (-) Conductor: White Insulation.

C. Nominal Twist Ratio (not required for Low-speed): One full twist every 60 mm (2.36”) to 80 mm(3.15”).
Aluminum Metallized Polyester Inner Shield (not required for Low-speed):
A. Substrate Material: Polyethylene terephthalate (PET) or equivalent material.
B. Metallizing: Vacuum deposited aluminum.
C. Assembly:
1.

The aluminum metallized side of the inner shield shall be positioned facing out to ensure
direct contact with the drain wire.

2.

The aluminum metallized inner shield shall over lap by approximately one-quarter turn.

Drain Wire (not required for Low-speed):
A. Wire Gauge: Minimum 28 AWG stranded tinned copper (STC) non-insulated. Refer to Table
6-4.
Table 6-4. Drain Wire Signal Pair

American Wire
Gauge (AWG)
28

Nominal Conductor
Outer Diameter

Stranded Tinned
Conductors

0.381 mm (0.015”)

7 x 36

0.406 mm (0.016”)

19 x 40

Interwoven (Braided) Tinned Copper Wire (ITCW) Outer Shield (not required for Low-speed):
A. Coverage Area: Minimum 65%.
B. Assembly. The interwoven (braided) tinned copper wire outer shield shall encase the aluminum
metallized PET shielded power and signal pairs and shall be in direct contact with the drain wire.
Outer Polyvinyl Chloride (PVC) Jacket:
A. Assembly: The outer PVC jacket shall encase the fully shielded power and signal pairs and shallbe in direct contact with the tinned copper outer shield.
B. Nominal Wall Thickness: 0.64 mm (0.025”).
Marking: The cable shall be legibly marked using contrasting color permanent ink.
A. Minimum marking information for Full-speed cable shall include:
USB SHIELDED <Gauge/2C + Gauge/2C> UL CM 75o C — UL Vendor ID
B. Minimum marking information for Low-speed cable shall include:
USB specific marking is not required for Low-speed cable.

92

Universal Serial Bus Specification Revision 1.1

Nominal Fabricated Cable Outer Diameter:This is a nominal value and may vary slightly from manufacturer to manufacturer as function of the
conductor insulating materials and conductor specified. Refer to Table 6-5.
Table 6-5. Nominal Cable Diameter

6.6.3

Shielded USB
Cable Configuration

Nominal Outer
Cable Diameter

28/28

4.06 mm (0.160”)

28/26

4.32 mm (0.170”)

28/24

4.57 mm (0.180”)

28/22

4.83 mm (0.190”)

28/20

5.21 mm (0.205”)

Electrical Characteristics

All electrical characteristics shall be measured at or referenced to +20o C (68o F).
Voltage Rating: 30 Vrms maximum.
Conductor Resistance: Conductor resistance shall be measured in accordance with ASTM-D-4566 Section
13. Refer to Table 6-6.
Conductor Resistance Unbalance (Pairs): Conductor resistance unbalance between two (2) conductors ofany pair shall not exceed five percent (5%) when measured in accordance with ASTM-D-4566 Section 15.
Table 6-6. Conductor ResistanceAmericanWire Gauge (AWG)

6.6.4

Ohms () / 100 Meters
Maximum

28

23.20 26

14.60 24

9.09 

22

5.74 

20

3.58 

Cable Environmental Characteristics

Temperature Range:
A. Operating Temperature Range: 0o C to +50o C.
B. Storage Temperature Range: -20o C to +60o C.
C. Nominal Temperature Rating: +20o C.
Flammability: All plastic materials used in the fabrication of this product shall meet or exceed therequirements of NEC Article 800 for communications cables Type CM (Commercial).

93

Universal Serial Bus Specification Revision 1.1

6.6.5

Listing

The product shall be UL listed per UL Subject 444, Class 2, Type CM for Communications Cable
Requirements.

6.7 Electrical, Mechanical and Environmental Compliance Standards
Table 6-7 lists the minimum test criteria for all USB cable, cable assemblies and connectors
Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards

Test Description

Test Procedure
EIA 364-18

Visual and Dimensional
Inspection

Visual, dimensional and functional
inspection in accordance the USB
quality inspection plans.
EIA 364-21

Insulation Resistance94

1,000 M minimum.

The dielectric must withstand 500
VAC for one minute at sea level.

The object of this test procedure isto detail a test method to prove thata USB connector can operatesafely at its rated voltage andwithstand momentary overpotentials due to switching, surges
and/or other similar phenomena.
EIA 364-23

Low Level
Contact ResistanceMust meet or exceed therequirements specified by themost current version of Chapter 6of the USB Specification.

The object of this test procedure isto detail a standard method toassess the insulation resistance ofUSB connectors. This test
procedure is used to determine theresistance offered by the insulationmaterials and the various seals of aconnector to a DC potential
tending to produce a leakage ofcurrent through or on the surface of
these members.
EIA 364-20

Dielectric
Withstanding Voltage

Performance Requirement

The object of this test is to detail astandard method to measure theelectrical resistance across a pair ofmated contacts such that theinsulating films, if present, will not
be broken or asperity melting will
not occur.

30 m maximum when measured
at 20 mV maximum open circuit
at 100 mA. Mated test contacts
must be in a connector housing.

Universal Serial Bus Specification Revision 1.1

Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards (Continued)

Test Description

Contact Current
RatingTest Procedure

Performance Requirement

EIA 364-70 — Method B

1.5 A at 250 VAC minimum
when measured at an ambientO
temperature of 25 C. Withpower applied to the contacts, theO T shall not exceed +30 C at
any point in the USB connector
under test.

The object of this test procedure isto detail a standard method toassess the current carrying capacityof mated USB connector contacts.
EIA 364-30

Contact Capacitance

The object of this test is to detail astandard method to determine thecapacitance between conductive
elements of a USB connector.
EIA 364-13

Insertion Force

10 Newtons minimum at a
maximum rate of 12.5 mm(0.492”) per minute

The object of this test is to detail astandard method for determining
the mechanical forces required forextracting a USB connector.
EIA 364-09

Durability

35 Newtons maximum at a
maximum rate of 12.5 mm(0.492”) per minute

The object of this test is to detail astandard method for determining
the mechanical forces required for
inserting a USB connector.
EIA 364-13

Extraction Force

2 pF maximum unmated percontact

1,500 insertion/extraction cycles
at a maximum rate of 200 cycles
per hourThe object of this test procedure isto detail a uniform test method for
determining the effects caused bysubjecting a USB connector to theconditioning action of insertionand extraction, simulating the
expected life of the connectors.
Durability cycling with a gauge isintended only to produce
mechanical stress. Durability
performed with mating
components is intended to produce
both mechanical and wear stress.

95

Universal Serial Bus Specification Revision 1.1

Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards (Continued)

Test Description

Test Procedure
EIA 364-38
Test Condition A

Cable Pull-Out

Test Condition H

The object of this test procedure isto detail a standard method toassess the ability of a USB
connector to withstand specified
severity of mechanical shock.

EIA 364-28
Test Condition V Test Letter A

Random Vibration

96

After the application of a steadystate axial load of 40 Newtons forone minute

The object of this test procedure isto detail a standard method for
determining the holding effect of a
USB plug cable clamp withoutcausing any detrimental effects
upon the cable or connector
components when the cable issubjected to inadvertent axial
tensile loads.
EIA 364-27

Physical Shock

Performance Requirement

This test procedure is applicable to
USB connectors that may, inservice, be subjected to conditions
involving vibration. Whether a
USB connector has to function
during vibration or merely tosurvive conditions of vibration
should be clearly stated by thedetailed product specification. In
either case, the relevant
specification should always
prescribe the acceptableperformance tolerances.

No discontinuities of 1 S or
longer duration when mated USB
connectors are subjected to 11 ms
duration 30 Gs half-sine shock
pulses. Three shocks in eachdirection applied along threemutually perpendicular planes fora total of 18 shocks

No discontinuities of 1 S or
longer duration when mated USB
connectors are subjected to 5.35
Gs RMS. 15 minutes in each of
three mutually perpendicularplanesUniversal Serial Bus Specification Revision 1.1

Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards (Continued)

Test Description

Test Procedure
EIA 364-32
Test Condition I

Thermal Shock

Test Condition A Method III

The object of this test procedure isto detail a standard test method forthe evaluation of the properties ofmaterials used in USB connectors
as they are influenced by theeffects of high humidity and heat.
EIA 364-52

Solderability

The object of this test procedure isto detail a uniform test method for
determining USB connector
solderability. The test procedure
contained herein utilizes the solder
dip technique. It is not intended totest or evaluate solder cup, solder
eyelet, other hand-soldered type or
SMT type terminations.
UL 94 V-0

Flammability

OO

10 Cycles –55 C and +85 C. The
USB connectors under test must
be mated

The object of this test is todetermine the resistance of a USB
connector to exposure at extremesof high and low temperatures andto the shock of alternate exposures
to these extremes, simulating theworst case conditions for storage,transportation and application.
EIA 364-31

Humidity Life

Performance Requirement

This procedure is to ensure
thermoplastic resin compliance to
UL flammability standards.

168 Hours minimum (seven (7)complete cycles). The USB
connectors under test shall betested in accordance with EIA
364-31

USB contact solder tails shallpass 95% coverage after one hour
steam aging as specified in
Category 2

The manufacturer will require its
thermoplastic resin vendor to
supply a detailed C of C witheach resin shipment. The C of C
shall clearly show the resin’s UL
listing number, lot number, datecode, et cetera.

97

Universal Serial Bus Specification Revision 1.1

Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards (Continued)

Test Description

Cable Imedance
Only required for Full-speed

98

Test Procedure

Performance Requirement

The object of this test is to insure
the signal conductors have theproper impedance.

Impedance must be in the range
specified in Table 7-9 (ZO).

1.

Connect the Time Domain
Reflectometer (TDR) outputs
to the impedance/delay/skew
test fixture (Note 1). Use
separate 50 Ohm cables for theplus (or true) and minus (orcomplement) outputs. Set the
TDR head to differential TDR
mode.

2.

Connect the Series "A" plug ofthe cable to be tested to thetext fixture, leaving the otherend open-circuited.

3.

Define a waveform composedof the difference between thetrue and complement
waveforms, to allow
measurement of differential
impedance.

4.

Measure the minimum andmaximum impedances foundbetween the connector and theopen circuited far end of thecable.

Universal Serial Bus Specification Revision 1.1

Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards (Continued)

Test Description

Test Procedure
The object of this test is to insurethat adequate signal strength ispresented to the receiver tomaintain a low error rate.
1.

Connect the Network
Analyzer output port (port 1)
to the input connector on theattenuation test fixture (Note2).

2.

Connect the Series “A” plugof the cable to be tested to thetest fixture, leaving the otherend open-circuited.

3.

Calibrate the network analyzer
and fixture using theappropriate calibration
standards, over the desired
frequency range.

4.

Follow the method listed in
Hewlett Packard Application
Note 380-2 to measure theopen-ended response of thecable.

5.

Short circuit the Series “B”
end (or bare leads end, if a
captive cable), and measure
the short-circuit response.

6.

Using the software in H-PApp. Note 380-2 orequivalent, calculate the cableattenuation, accounting forresonance effects in the cableas needed.

Signal Pair Attenuation
Only required for Full-speed

Performance Requirement
Refer to Section 7.1.17 forfrequency range and allowableattenuation.

99

Universal Serial Bus Specification Revision 1.1

Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards (Continued)

Test Description

Test Procedure
The purpose of the test is to verify
the end to end propagation of thecable.
1.

2.

Connect the cable to be tested
to the test fixture. If
detachable, plug bothconnectors in to the matching
fixture connectors. If captive,
plug the series “A” plug into
the matching fixture
connector, and solder the
stripped leads on the other end
to the test fixture.

3.

Measure the propagation delayof the test fixture byconnecting a short piece ofwire across the fixture from
input to output, and recording
the delay.

4.

Remove the short piece ofwire and re-measure thepropagation delay. Subtractfrom it the delay of the test
fixture measured in theprevious step.

Propagation Delay

100

Connect one output of the
TDR sampling head to the D+
and D- inputs of the
impedance/delay/skew test
fixture (Note 1). Use one 50
cable for each signal, and set
the TDR head to differential
TDR mode.

Performance Requirement
Full-speed
See Section 7.1.1.1, Section
7.1.4, Section 7.1.16 and Table
7-9 (TFSCBL)
Low-speed
See Section 7.1.1.2, Section
7.1.16 and Table 7-9 (TLSCBL)

Universal Serial Bus Specification Revision 1.1

Table 6-7. USB Electrical, Mechanical and Environmental Compliance Standards (Continued)

Test Description

Test Procedure

Performance Requirement

This test insures that the signal on
both the D+ and D- lines arrive at
the receiver at the same time.

Propagation skew must meetthe requirements as listed in
Section 7.1.3.

1.

Connect the TDR to the
fixture with test sample
cable, as in the previous
section.

2.

Measure the difference indelay for the two conductors
in the test cable. Use theTDR cursors to find theopen-circuited end of eachconductor (where theimpedance goes infinite),and subtract the timedifference between the twovalues.

Propagation Delay Skew

The purpose of this test is to insure
the distributed inter-wire
capacitance is less then the lumped
capacitance specified by the Lowspeed transmit driver.
1.

Connect the one lead of the
Impedance Analyzer to the D+
pin on the
impedance/delay/skew fixture(Note 1), and the other lead to
the D- pin.

2.

Connect the series "A" plug tothe fixture, with the series “B”
end leads open-circuited.

3.

Set the Impedance Analyzer toa frequency of 100 kHz, tomeasured the capacitance.

Capacitive Load
Only required for Low-speed

Note1:

See Section 7.1.1.2 and Table
7-7 (CLINUA)

Impedance, propagation delay and skew test fixture.
This fixture will be used with the TDR for measuring the time domain performance of the cable under test.
The fixture impedance should be matched to the equipment, typically 50. Coaxial connectors should be
provided on the fixture for connection from the TDR.

Note 2: Attenuation text fixtureThis fixture provides a means of connection from the network analyzer to the Series "A" plug. Since USB
signals are differential in nature and operate over balanced cable, a transformer or balun (North Hills
NH13734 or equivalent) is ideally used. The transformer converts the unbalanced (also known as singleended) signal from the signal generator which is typically a 50 output, to the balanced (also known as
differential) and likely different impedance loaded presented by the cable. A second transformer or balun

101

Universal Serial Bus Specification Revision 1.1

should be used on the other end of the cable under test, to convert the signal back to unbalanced from of thecorrect impedance to match the network analyzer.

6.7.1 Applicable Documents
American National Standard/Electronic Industries Association
ANSI/EIA-364-C (12/94)

Electrical Connector/Socket Test Procedures
Including Environmental Classifications

American Standard Test Materials
ASTM-D-4565

Physical and Environmental Performance Properties
of Insulation and Jacket for Telecommunication
Wire and Cable, Test Standard Method

ASTM-D-4566

Electrical Performance Properties of Insulation andJacket for Telecommunication Wire and Cable, Test
Standard Method

Underwriters’ Laboratory, Inc.

6.8

UL STD-94

Test for Flammability of Plastic materials for Parts
in Devices and Appliances

UL Subject-444

Communication Cables

USB Grounding
The shield must be terminated to the connector plug for completed assemblies. The shield and chassis
are bonded together. The user selected grounding scheme for USB devices and cables must be consistent
with accepted industry practices and regulatory agency standards for safety and EMI/ESD/RFI.

6.9 PCB Reference Drawin gs
The following drawings describe typical receptacle PCB interfaces. This is included for information
purposes only.

102

Universal Serial Bus Specification Revision 1.1

8

7

6

5

4



3

2

1

  

H

H

16.0 REF

6.5 REF
13.1 REF

13.9 REF

2.80 + 0.10
2.0 REF

G

G

14.3 REF

10.3 REF
3.8 REF

F

F

15.0 R E F

7.6 REF

12.5 + 0.10

R 0.64 + 0.13 Typical (2)11.1 + 0.10

E

1

2

3

6.00 + 0.10

1.84 + 0.05
4E

5.12 + 0.10

9.0 REF
10.7 REF

2.56 + 0.05
Thermoplastic Insulator UL 94-V0
D

2.50 + 0.05

1.0 + 0.05 Wide - Selectively Plated Contact (4)2.50 + 0.05

D

2.00 + 0.05

NOTES:

Ø 0.92 + 0.10 (4)7.00 + 0.10
C

1. Critical Dimensions are TOLERANCED
and should not be deviated.

2.00 + 0.10

C

2. Dimensions that are labeled R E F are
typical dimensions and may vary frommanufacturer to manufacturer.
2.71 + 0.10
B

3. A l l d i m e n s i o n s a r e i n m i l l i m e t e r s (m m ) u n l e s sotherwise noted.

13.14 + 0.10

B

Ø 2.30 + 0.10 (2)Printed Circuit Board (PCB) Layout

Single Pin-Type

Series "A" Receptacle

A

8

7

6

5

4

SIZE

DATE

DRAWING NUMBER

A

2/98

N/A

SCALE:

N/A

3

SHEET
2

A
REV

C
1 of 11

Figure 6-12. Single Pin-Type Series "A" Receptacle

103Universal Serial Bus Specification Revision 1.1

8

7

6

5

4

H

3

2

1

1

2

3

4

1

2

3

4

H

15.60 REF
14.70 ± 0.10

GG

3.70 R E F

13.78 + 0.10
12.30 REF

F

2.62 ± 0.05

F

11.01 ± 0.10

5.70 R E F
2.00 REF
12.50 + 0.10

3.07 ± 0.10 (2)7.00 ± 0.10

2.00 ± 0.10

2.50 ± 0.10

2.50 ± 0.10

EE

2.62 ± 0.05

5.68 ± 0.10

DD

Ø 0.92 ± 0.10 (8)10.28 ± 0.20

Ø 2.3 ± 0.10 (4)11.10 REF
10.30 R E FCConnector Front Edge

16.95 REF

C

Printed Circuit Board (PCB) Layout

NOTES:
B

1. Critical Dimensions are TOLERANCED
and should not be deviated.



2. Dimensions that are labeled R E F are
typical dimensions and may vary frommanufacturer to manufacturer.
A  

Dual Pin-Type

Series "A" Receptacle
3. All dimensions are in millimeters ( m m )unless otherwise noted.
8

7

6

5

SIZE

DATE

N/A

4

3

SHEET
2

A

REV

DRAWING NUMBER

A
2/98
SCALE: N/A

Figure 6-13. Dual Pin-Type Series "A" Receptacle

104

B

C
1 of 11

Universal Serial Bus Specification Revision 1.1

8

7

6

5

4

3

2

1

1.0 + 0.05 Wide - Selectively Plated Contacts (4) ....
H

H

Thermoplastic Insulator UL 94-V0 ....

2

7.78 + 0.10

111.50 R E F

GG

5.60 + 0.10

3

4

F

F

3.01 + 0.10

2.71 + 0.10

3.50 R E F
8.45 + 0.10

2.50 + 0.10

2.00 + 0.10

4.71 + 0.10

EE

12.04 + 0.10
2.50 + 0.10
12.00 R E F

4.77 + 0.10
2.00 + 0.10

DD

2.71 + 0.10

10.30 R E F
CC

16.00 R E F
Ø 0.92 + 0.1 (4)NOTES:

B

Ø 2.30 + 0.1 (2)1. Critical Dimensions are TOLERANCED
and should not be deviated.
2. Dimensions that are labeled REF are
typical dimensions and may vary frommanufacturer to manufacturer.

A

Printed Circuit Board (PCB) LayoutB  

Single Pin-Type

Series "B" Receptacle

3. All dimensions are in millimeters ( m m )unless otherwise noted.

SIZE

DATE

DRAWING NUMBER

N/A

2/98
A
SCALE: N/A
8

7

6

5

4

3

SHEET
2

A

REV

C
1 of 11

Figure 6-14. Single Pin-Type Series "B" Receptacle

105Universal Serial Bus Specification Revision 1.1

106Universal Serial Bus Specification Revision 1.1

Chapter 7
ElectricalThis chapter describes the electrical specification for the USB. It contains signaling, power distribution,and physical layer specifications.

7.1 Signaling
The signaling specification for the USB is described in the following subsections.

7.1.1 USB Driver Characteristics
The USB uses a differential output driver to drive the USB data signal onto the USB cable. The static
output swing of the driver in its low state must be below VOL (max) of 0.3V with a 1.5k load to 3.6V andin its high state must be above the VOH (min) of 2.8V with a 15k load to ground as listed in Table 7-5.
Full-speed drivers have more stringent requirements, as described in Section 7.1.1.1. The output swings
between the differential high and low state must be well-balanced to minimize signal skew. Slew rate
control on the driver is required to minimize the radiated noise and cross talk. The driver’s outputs must
support three-state operation to achieve bi-directional half-duplex operation.
USB devices must be capable of withstanding continuous exposure to the waveforms shown in Figure 7-1
while in any drive state. These waveforms are applied directly into each USB data pin from a voltage
source with an output impedance of 39. The open-circuit voltage of the source shown in Figure 7-1 isbased on the expected worst-case overshoot and undershoot.
It is recommended that these DC and AC stresses be used as qualification criteria against which the longterm reliability of each device is evaluated.

Evaluation Setup
D+ or D- pin
on USB connector
nearest device

USB
Device

60nS
(min)
4.6V
4-20nsRSRC

V
RSRC = 39 2%-1.0V
166.7ns(6MHz)The signal produced by the voltage generator may be
distorted when observed at the data pin due to input
protection devices possibly incorporated in the USB
device.

Figure 7-1. Maximum Input Waveforms for USB Signaling
A USB device must be able to withstand a continuous short circuit of D+ and D- to VBUS, GND, other dataline, or the cable shield at the connector. The device must not be damaged when presented with a driving
signal that provides a duty cycle of 50% transmit and 50% receive. The transmit phase consists of asymmetrical signal that toggles between drive high and drive low. This requirement must be met for max
value of VBUS.

107

Universal Serial Bus Specification Revision 1.1

7.1.1.1 Full-speed (12Mb/s) Driver Characteristics
A full-speed USB connection is made through a shielded, twisted pair cable with a characteristic
impedance (Z0) of 90 15% and a maximum one-way delay of 26ns. The impedance of each of thedrivers (ZDRV) must be between 28 and 44 i.e. within the grey area in Figure 7-3.
For a CMOS implementation, the driver impedance will typically be realized by a CMOS driver with an
impedance significantly less than this resistance with a discrete series resistor making up the balance asshown in Figure 7-2. The series resistor RS is included in the buffer impedance requirement shown in
Figure 7-3. In the rest of the chapter, references to the buffer assume a buffer with the series impedance
unless stated otherwise.

Buffer Output Imped. (ZBUF)
TxD+

RS

OE

Identical
CMOS
Buffers

TxD-

D+ (28 to 44 Equiv. Imped.)D- (28 to 44 Equiv. Imped.)
RS
Figure 7-2. Example Full-speed CMOS Driver Circuit

The buffer impedance must be measured for driving high as well as driving low. Figure 7-3 shows thecomposite V/I characteristics for the full-speed drivers with included series damping resistor (RS). The
characteristics are normalized to the steady-state, unloaded output swing of the driver. The normalized
driver characteristics are found by dividing the measured voltages and currents by the actual swing of thedriver under test. The normalized V/I curve for the driver must fall entirely inside the shaded region. The
V/I region is bounded by the minimum driver impedance above and the maximum driver impedance
below. The minimum drive region is intersected by a constant current region of |6.1VOH|mA when driving
low and -|6.1VOH|mA when driving high. This is the minimum current drive level necessary to ensure thatthe waveform at the receiver crosses the opposite single-ended switching level on the first reflection.
When testing, the current into or out of the device need not exceed 10.71*VOH mA and the voltage
applied to D+/D- need not exceed 0.3*VOH for the drive low case and need not drop below 0.7*VOH for
the drive high case.

108

Universal Serial Bus Specification Revision 1.1

drive low
IOUT
(mA)

Slope = 1/28
Test Limit

10.71 * |VOH|

6.1 * |VOH|

Slope = 1/44

2.32
0
0

0.3V

0.27*VOH

0.3*VOH

VOH

VOUT (Volts)

0drive highSlope = 1/44

-6.1*|VOH|

Test Limit-10.71 * |VOH|

Slope = 1/28

IOUT
(mA)

0

VOUT (Volts)

0.7*VOH 0.73*VOH

VOH

Figure 7-3. Full-speed Buffer V/I Characteristics

109

Universal Serial Bus Specification Revision 1.1

Figure 7-4 shows the full-speed driver signal waveforms.
One BitTime(12Mb/s)

DriverSignal Pins

VSS
One-Way
Trip CableDelay

VIH (min)
Signal pins pass
input spec levels
after one cabledelay

ReceiverSignal Pins
VIL (max)
VSS

Figure 7-4. Full-speed Signal Waveforms

7.1.1.2 Low-speed (1.5Mb/s) Driver Characteristics
A low-speed device must have a captive cable with the Series A connector on the plug end. The
combination of the cable and the device must have a single-ended capacitance of no less than 200pF and no
more than 450pF on the D+ or D- lines.
The propagation delay (TLSCBL) of a low-speed cable must be less than 18ns. This is to ensure that thereflection occurs during the first half of the signal rise/fall, which allows the cable to be approximated by alumped capacitance.
One BitTime(1.5Mb/s)

Signal pins
pass output
spec levels
with minimalreflections andringing

VIH (min)DriverSignal Pins
VIL (max)
VSS

Figure 7-5. Low-speed Driver Signal Waveforms

7.1.2 Data Signal Rise and Fall
The output rise time and fall times are measured between 10% and 90% of the signal (Figure 7-6). Riseand fall time requirements apply to differential transitions as well as to transitions between differential andsingle-ended signaling.

110Universal Serial Bus Specification Revision 1.1

The rise and fall times for full-speed buffers are measured with the load shown in Figure 7-7. The rise andfall times must be between 4ns and 20ns, and matched to within 10% to minimize RFI emissions andsignal skew. The transitions must be monotonic.
The rise and fall times for low-speed buffers are measured with the load shown in Figure 7-8. The
capacitive load shown in Figure 7-8 is representative of the worst-case load allowed by the specification.
A downstream port is allowed 150pF of input/output capacitance (CIND). A low-speed device (including
cable) may have a capacitance of as little as 200pF and as much as 450pF. This gives a range of 200pF to600pF as the capacitive load that a downstream low-speed buffer might encounter. Upstream buffers on
low-speed devices must be designed to drive the capacitance of the attached cable plus an additional
150pF. If a low-speed buffer is designed for an application where the load capacitance is known to fall in adifferent range, the test load can be adjusted to match the actual application. Low-speed buffers on hosts
and hubs that are attached to USB receptacles must be designed for the 200pF to 600pF range. The riseand fall time must be between 75ns and 300ns for any balanced, capacitive test load. In all cases, the edges
must be matched to within 20% to minimize RFI emissions and signal skew. The transitions must be
monotonic.
For both full-speed and low-speed signaling, the crossover voltage (VCRS) must be between 1.3V and
2.0V.
This specification does not require matching signal swing matching to any greater degree than describedabove. However, when signaling, it is preferred that the average voltage on the D+ and D- lines should beconstant. This means that the amplitude of the signal swing on both D+ and D- should be the same; thelow and high going transition should begin at the same time and change at the same rate; and the crossovervoltage should be the same when switching to a J or K. Deviations from signal matching will result incommon-mode noise that will radiate and affect the ability of devices and systems to pass tests that aremandated by government agencies.

Rise Time

Fall Time
90%

90%

VCRS
10%
Differential
Data Lines

10%

tF

tR

Figure 7-6. Data Signal Rise and Fall Time
Full-speed
Buffer

RS
TxD+
CL
RS
TxDCL
CL = 50pF

Figure 7-7. Full-speed Load

111

Universal Serial Bus Specification Revision 1.1

Low-speed
BufferLow-speed
Buffer

RS

RS

TxD+

TxD+

RS

CL 3.6V
1.5K

TxD-



CL
TxD-

CL

CL

CL = 200pF to 600pF

CL = 50pF to 150pF

Low-speed downstream port load

15K



RS
15K



Low-speed upstream port load

Figure 7-8. Low-speed Port Loads
Note: the CL for low-speed port load only represents the range of loading that might be added when the
low-speed device is attached to a hub. The low-speed buffer must be designed to drive the load of its
attached cable plus CL. A low-speed buffer design that can drive the downstream port test load would becapable of driving any legitimate upstream load.

7.1.2.1 Driver Usage
The upstream ports (towards the host) of all hubs and full-speed functions must use full-speed drivers.
The upstream hub port transmits data at both full- and low-speed data rates. However, the signaling
always uses full-speed signaling conventions and edge rates (refer to Figure 7-13 Upstream Full Speed Port
Transceiver and Table 7-1 Signaling Levels). Transmission of low-speed data does not change the driver’s
characteristics. The upstream port of low-speed functions must use a low-speed driver.
External downstream ports of all hubs (including the host), are required to be capable of both driver
characteristics, such that any type of device can be plugged in to these ports. When the transceiver is in
full-speed mode it uses full-speed signaling conventions and edge rates. In low-speed it uses low-speed
signaling conventions and edge rates (refer to Figure 7-14 Downstream Port Transceiver and Table 7-1
Signaling Levels).

7.1.3 Cable Skew
The maximum skew introduced by the cable between the differential signaling pair (i.e., D+ and D(TSKEW)) must be less than 400pS and is measured as described in Section 6.7.

7.1.4 Receiver Characteristics
A differential input receiver must be used to accept the USB data signal. The receiver must feature an
input sensitivity (VDI) of at least 200mV when both differential data inputs are in the differential common
mode range (VCM) of 0.8V to 2.5V, as shown in Figure 7-9.
In addition to the differential receiver, there must be a single-ended receiver for each of the two data lines.
The receivers must have a switching threshold between 0.8V (VIL) and 2.0V (VIH). It is recommended thatthe single-ended receivers incorporate hysteresis to reduce their sensitivity to noise.
Both D+ and D- may temporarily be less than Vih(min) during differential signal transitions. This period
can be up to 14ns (TFST) for full-speed transitions and up to 210ns (TLST) for low-speed transitions. Logicin the receiver must ensure that that this is not interpreted as an SE0.

112

Universal Serial Bus Specification Revision 1.1

Differential Input Voltage Range

Differential Output
CrossoverVoltage Range
-1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

4.6

Input Voltage Range (volts)

Figure 7-9. Differential Input Sensitivity Range

7.1.5 Device Speed Identification
The USB is terminated at the hub and function ends as shown in Figure 7-10 and Figure 7-11. Full-speed
and low-speed devices are differentiated by the position of the pull-up resistor on the downstream end of
the cable:Full-speed devices are terminated as shown in Figure 7-10 with the pull-up resistor on the D+ line.
Low-speed devices are terminated as shown in Figure 7-11 with the pull-up resistor on the D- line.
The pull-down terminators on downstream ports are resistors of 15k 5% connected to ground.

The design of the pull-up resistor must ensure that the signal levels satisfy the requirements specified inTable 7-1. In order to facilitate bus state evaluation that may be performed at the end of a reset, the design
must be able to pull-up D+ or D- from 0V to VIH (min) within the minimum reset relaxation time of 2.5s.
A device that has a detachable cable must use a 1.5k 5% resistor tied to a voltage source between 3.0V
and 3.6V (VTERM) to satisfy these requirements. Devices with captive cables may use alternative
termination means. However, the Thevenin resistance of any termination must be no less than 900.
Note: Thevenin resistance of termination does not include the 15k 5% resistor on host/hub.

The voltage source on the pull-up resistor must be derived from or controlled by the power supplied on the
USB cable such that when VBUS is removed, the pull-up resistor does not supply current on the data line towhich it is attached.

R pu
D+

D+
Full-speed or
R pd
Low-speed USB
TransceiverD- 15%
R =15K  5%
R =1.5K   5%

Full-speed USB
TransceiverD-

Z 0=90

R pd

pd

Host orHub Portpu

Hub Upstream Portor
Full-speed Function

Figure 7-10. Full-speed Device Cable and Resistor Connections

113

Universal Serial Bus Specification Revision 1.1

Rpu
D+ Low-speed USB
TransceiverD+

Full-speed or
Low-speed USB
Rpd
Transceiver
DRpd
Host orHub Port

Rpd=15K
Rpu=1.5K 5%

D- Slow Slew Rate
Buffers 5% (nominal)
Low-speed Function

Figure 7-11. Low-speed Device Cable and Resistor Connections

7.1.6 Input Characteristics

The input impedance of D+ or D- without termination should be  300 k (ZINP). The input capacitance ofa port is measured at the connector pins. Upstream and downstream ports are allowed different values ofcapacitance. The maximum capacitance (differential or single-ended) (CIND) allowed on a downstream
port of a hub or host is 150pF on D+ or D-. This is comprised of up to 75pF of lumped capacitance toground on each line at the transceiver and in the connector, and an additional 75pF capacitance on eachconductor in the transmission line between the receptacle and the transceiver. The transmission line
between the receptacle and RS must be 90 15%).
The maximum capacitance on an upstream port of a full-speed device with a detachable cable (CINUB) is
100pF on D+ or D- . This is comprised of up to 75 pF of lumped capacitance to ground on each line at thetransceiver and in the connector, and an additional 25pF capacitance on each conductor in the transmission
line between the receptacle and the transceiver. The difference in capacitance between D+ and D- must be
less than 10%.
For full-speed devices with captive cables, the device itself may have up to 75pF of lumped capacitance toground on on D+ and D-. The cable accounts for the remainder of the input capacitance .
A low-speed device is required to have a captive cable. The input capacitance of the low-speed device will
include the cable. The maximum single-ended or differential input capacitance of a low-speed device is
450pF (CLINUA).

For devices with captive cables, the single-ended input capacitance must be consistent with the termination
scheme used. The termination must be able to charge the D+ or D- line from 0V to VIH (min) within 2.5s.
The capacitance on D+/D- includes the single-ended input-capacitance of the device (measured from thepins on the connector on the cable) and the 150pF of input capacitance of the host/hub.
An implementation may use small capacitors at the transceiver for purposes of edge rate control. The sumof the capacitance of the added capacitor (CEDGE) , the transceiver, and the trace connecting capacitor andtransceiver to RS must not exceed 75pF (either single-ended or differential) and the capacitance must bebalanced to within 10%. The added capacitor, if present, must be placed between the transceiver pins andRS (see Figure 7-12).
Use of ferrite beads on the D+ or D- lines of full-speed devices is discouraged.

114

Universal Serial Bus Specification Revision 1.1

TxD+

RS
CEDGE

TxD-

RS
CEDGE

Figure 7-12. Placement of Optional Edge Rate Control Capacitors

7.1.7 Signaling Levels
Table 7-1 summarizes the USB signaling levels. The source is required to drive the levels specified in thesecond column and the target is required to identify the correct bus state when it sees the levels in thethird column. (Target receivers can be more sensitive as long as they are within limits specified in the
fourth column).

115

Universal Serial Bus Specification Revision 1.1

Table 7-1. Signaling Levels
Bus State

Signaling Levels
At originating source
connector (at end of bit time)

At final target connector
Required

Acceptable

Differential “1”D+ > VOH (min) and D- < VOL (max)(D+) - (D-)  200mVand D+  VIH (min)(D+) - (D-)  200mV

Differential “0”D- > VOH (min) and D+ < VOL (max)(D-) - (D+) > 200mVand D-  VIH (min)(D-) - (D+) > 200mV

Single-ended 0 (SE0)

D+ and D- < VOL (max)

D+ and D- < VIL (max)

D+ and D- < VIH (min)Low-speed

Differential “0”Differential “0”Full-speed

Differential “1”Differential “1”Low-speed

Differential “1”Differential “1”Full-speed

Differential “0”Differential “0”Data J state:

Data K state:

Idle state:

N.A.
D- > VIHZ (min) andLow-speed

Full-speed

D- > VIHZ (min) andD+ < VIL (max)

D+ < VIH (min)D+ > VIHZ (min) andD+ > VIHZ (min) and

D- < VIL (max)D- < VIH (min)Resume state

Data K state

Data K state

Start-of-Packet (SOP)

Data lines switch from Idle to K stateEnd-of-Packet (EOP)4

SE0 for approximately 2 bit times1
followed by a J for 1 bit time3

SE0 for  1 bit time2
followed by a J statefor 1 bit time

Disconnect(at downstream port)

N.A.

SE0 for 2.5sConnect(at downstream port)

N.A.

Idle for 2ms

Idle for 2.5 s

Reset

D+ and D-  VOL (max) for 10ms

D+ and D-  VIL (max)for 10ms

D+ and D-  VIL (max)for 2.5 s

SE0 for  1 bit time2
followed by a J state

Note 1: The width of EOP is defined in bit times relative to the speed of transmission. (Specification EOP widths are
given in Table 7-5 and Table 7-6.)
Note 2: The width of EOP is defined in bit times relative to the device type receiving the EOP. The bit time isapproximate.
Note 3: The width of the J state following the EOP is defined in bit times relative to the buffer edge rate. The J state froma low-speed buffer must be a low-speed bit time wide and from a full-speed buffer, a full-speed bit time wide.
Note 4: The keep-alive is a low-speed EOP.

116

Universal Serial Bus Specification Revision 1.1

The J and K data states are the two logical levels used to communicate differential data in the system.
Differential signaling is measured from the point where the data line signals cross over. Differential data
signaling is not concerned with the level at which the signals cross, as long as the crossover voltage meetsthe requirements in Section 7.1.2. Note that, at the receiver, the Idle and Resume states are logicallyequivalent to the J and K states respectively.
As shown in Table 7-1, the J and K states for full-speed signaling are inverted from those for low-speed
signaling. The sense of data, idle, and resume signaling is set by the type of device that is being attached toa port. If a full-speed device is attached to a port, that segment of the USB uses full-speed signaling
conventions (and fast rise and fall times), even if the data being sent across the data lines is at the lowspeed data rate. The low-speed signaling conventions shown in Table 7-1 (plus slow rise and fall times)
are used only between a low-speed device and the port to which it is attached.

 

3.0V V 3.6V
1.5K or equivalent

D+
RxD

DDifferential Receiver

RxD+

Single-ended Receivers
RxD-

TxD+
OE

Output Buffers

TxD-

Figure 7-13. Upstream Full-speed Port TransceiverD+
RxD

DDifferential Receiver

RxD+

Single-ended Receivers
RxD-

TxD+

Output Buffers

OE
Speed
TxD-

15K 

Note: additional logic is required
to invert signal polarity on
data in/out when low-speed
devices are attached.

Figure 7-14. Downstream Port Transceiver117

Universal Serial Bus Specification Revision 1.1

7.1.7.1 Connect and Disconnect Signaling
When no function is attached to the downstream port of the host or hub, the pull-down resistors present
there will cause both D+ and D- to be pulled below the single-ended low threshold of the host or hub port
when that port is not being driven by the hub. This creates an SE0 state on the downstream port. A
disconnect condition (TDDIS) is indicated if the host or hub is not driving the data lines and an SE0 persists
on a downstream port for more than 2.5s (see Figure 7-15).
A connect condition (TDCNN) will be detected when the hub detects that one of the data lines is pulled
above its VIH threshold for more than 2.5s (see Figure 7-16 and Figure 7-17).
Hubs may optionally determine the speed of the attached device by sampling the state of the bus
immediately before driving SE0 to indicate a reset condition to the device. Alternatively, the hub may float
the bus after driving reset and perform bus state evaluation after 2.5s as shown in Figure 7-18.
All signaling levels given in Table 7-1 are set for this bus segment (and this segment alone) once the speedof the attached device is determined. The mechanics of speed detection are described in Section 11.8.2.
D+/DVIHZ (min)

VIL
D-/D+
VSS

 2.5S
DeviceDisconnected

Disconnect
Detected

Figure 7-15. Disconnect Detection

D+
VIH

DVSS

 2.5S
DeviceConnected

Connect
Detected

Figure 7-16. Full-speed Device Connect Detection118

Universal Serial Bus Specification Revision 1.1

DVIH

D+
VSS

 2.5s

DeviceConnected

Connect
Detected

Figure 7-17. Low-speed Device Connect Detection
VOH(min)
VIH(min)Differential
data lines

VIL
VIL(max)VOL(max)
VSS
Diff 1 orDiff 0 10ms 2.5s

SE0
Port driving
reset

Reset completed
and bus is floated

Bus evaluated after reset:
Diff 1 = FS connected
Diff 0 = LS connected
SE0 = Disconnected

Figure 7-18. Bus State Evaluation after reset (optional)
Because USB components may be hot plugged, and hubs may implement power switching, it is necessary
to comprehend the delays between power switching and/or device attach and when the device’s internal
power has stabilized. Figure 7-19 shows all the events associated with both turning on port power with adevice connected and hot-plugging a device. There are six delays and a sequence of events that are definedby this specification.

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Hub port
power OK

Reset Recovery
Time

Attach Detected

Hub port
power-on4.01V

VBUS

t4

t5

USB System Software
reads device speed

VIH(min)
VIH
D+
or
D-

t1

100ms
t2

10ms
100ms
t3

t6

Figure 7-19. Power-on and Connection Events Timingt1This is the amount of time required for the hub port power switch to operate. This delay is afunction of the type of hub port switch. Hubs report this time in the hub descriptor (see Section
11.15.2.1), which can be read via a request to the Hub Controller (see Section 11.16.2.4). If adevice were plugged into a non-switched or already-switched on port, t1 is equal to zero.

t2 (TSIGATT)This is the maximum time from when VBUS is up to valid level (4.01V) to when a device
has to signal attach. t2 represents the time required for the device’s internal power rail to stabilize
and for D+ or D- to reach VIH (min) at the hub. t2 must be less than 100ms for all hub and device
implementations. (This requirement only applies if the device is drawing power from the bus).

t3 (TATTDB) This is a debounce interval with a minimum duration of 100ms that is provided by the USB
System Software. It ensures that the electrical and mechanical connection is stable before software
attempts to reset the attached device. The interval starts when the USB System Software is notified
of a connection detection. The interval restarts if there is a disconnect. The debounce interval
ensures that power is stable at the device for at least 100ms before any requests will be sent to thedevice.

t4 (T2SUSP) Anytime a device observes no bus activity, it must obey the rules of going into suspend (seeSection 7.1.7.4).

t5 (TDRST) This is the period of time hubs drive reset to a device. Refer to Section 11.5.1.5 for details.
t6 (TRSTRCY) The USB System Software guarantees a minimum of 10ms for reset recovery. Device
response to any bus transactions addressed to the default device address during the reset recovery
time is undefined.

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Universal Serial Bus Specification Revision 1.1

7.1.7.2 Data Signaling
Data transmission within a packet is done with differential signals.
The start of a packet (SOP) is signaled by the originating port by driving the D+ and D- lines from the Idle
state to the opposite logic level (K state). This switch in levels represents the first bit of the SYNC field.
Hubs must limit the change in the width of the first bit of SOP when it is retransmitted to less than 5ns.
Distortion can be minimized by matching the nominal data delay through the hub with the output enable
delay of the hub.
The SE0 state is used to signal an end-of-packet (EOP). EOP will be signaled by driving D+ and D- to theSE0 state for two bit times followed by driving the lines to the J state for one bit time. The transition fromthe SE0 to the J state defines the end of the packet at the receiver. The J state is asserted for one bit time
and then both the D+ and D- output drivers are placed in their high-impedance state. The bus termination
resistors hold the bus in the Idle state. Figure 7-20 shows the signaling for start and end of a packet.
VOH(min)
VIH(min)

VIL(max)VOL(max)

VSS Bus Idle
SOP

First Bitof Packet

Last Bitof Packet

Bus Driven to
J State at endof EOPBus
SE0
Floatsportionof EOPBus Idle

VOH(min)
VIH(min)

VIL(max)VOL(max)

VSS
Figure 7-20. Packet Voltage Levels

121Universal Serial Bus Specification Revision 1.1

7.1.7.3 Reset Signaling
A hub signals reset to a downstream port by driving an extended SE0 at the port. After the reset isremoved, the device will be in the Default state (refer to Section 9.1).
The reset signaling can be generated on any Hub or Host Controller port by request from the USB System
Software. The reset signaling must be driven for a minimum of 10ms (TDRST). After the reset, the hub
port will transition to the Enabled state (refer to Section 11.5). Host Controllers and the USB System
Software must ensure that resets issued to the root ports drive reset long enough to overwhelm any
concurrent resume attempts by downstream devices. Resets from root ports should be nominally 50ms
(TDRSTR). It is not required that this be 50ms of continuous Reset signaling. However, if the reset is notcontinuous, the interval(s) between signaling reset must be less than 3ms (TRHRSI).
A device seeing an SE0 on its upstream port for more than 2.5s (TDETRST) may treat that signal as areset. The reset must have taken effect before the reset signaling ends.
Hubs will propagate traffic to a newly reset port after the port is in the Enabled state. The device attachedto this port must recognize this bus activity and keep from going into the Suspend state.
Hubs must be able to accept all hub requests and devices must be able to accept a SetAddress() request(refer to Section 11.16.2 and Section 9.4 respectively) after the reset recovery time 10ms (TRSTRCY) after
the reset is removed. Failure to accept this request may cause the device not to be recognized by the USB
system software. Hubs and devices must complete commands within the times specified in Chapter 9 andChapter 11.
Reset must wake a device from the Suspend state.

7.1.7.4 Suspending
All devices must support the Suspend state. Devices can go into the Suspend state from any powered state.
They begin the transition to the Suspend state after they see a constant Idle state on their upstream bus lines
for more than 3.0ms. The device must actually be suspended, drawing only suspend current from the bus
after no more than 10ms of bus inactivity on all its ports. Any bus activity on the upstream port will keep adevice out of the Suspend state. In the absence of any other bus traffic, the SOF token (refer to
Section 8.4.2) will occur once per frame to keep full-speed devices from suspending. In the absence of any
low-speed traffic, low-speed devices will see at least one keep-alive (defined in Table 7-1) in every framein which an SOF occurs, which keeps them from suspending. Hubs generate this keep-alive as described in
Section 11.8.4.1.
While in the Suspend state, a device must continue to provide power to its D+ (full-speed) or D- (lowspeed) pull-up resistor to maintain an idle so that the upstream hub can maintain the correct connectivity
status for the device.

7.1.7.4.1 Global Suspend
Global suspend is used when no communication is desired anywhere on the bus and the entire bus is placedin the Suspend state. The host signals the start of global suspend by ceasing all its transmissions (including
the SOF token). As each device on the bus recognizes that the bus is in the Idle state for the appropriate
length of time, it goes into the Suspend state.

7.1.7.4.2 Selective Suspend
Segments of the bus can be selectively suspended by sending the command
SetPortFeature(PORT_SUSPEND) to the hub port to which that segment is attached. The suspended port
will block activity to the suspended bus segment and devices on that segment will go into the Suspend state
after the appropriate delay as described above.

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Universal Serial Bus Specification Revision 1.1

Section 11.5 describes the port Suspend state and its interaction with the port state machine. Suspend isfurther described in Section 11.9.

7.1.7.5 Resume
If a device is in the Suspend state, its operation is resumed when any non-idle signaling is received on its
upstream port. Additionally, the device can signal the system to resume operation if its remote wakeup
capability has been enabled by the USB System Software. Resume signaling is used by the host or a
device to bring a suspended bus segment back to the active condition. Hubs play an important role in the
propagation and generation of resume signaling. The following description is an outline of a general
global resume sequence. A complete description of the resume sequence, the special cases caused by
selective suspend, and the role of the hub are given in Section 11.9.
The host may signal resume (TDRSMDN) at any time. It must send the resume signaling for at least 20ms
and then end the resume signaling with a standard, low-speed EOP (two low-speed bit times of SE0
followed by a J). The 20ms of resume signaling ensures that all devices in the network that are enabled tosee the resume are awakened. The EOP tears down the connectivity established by the resume signaling
and prepares the hubs for normal operation. After resuming the bus, the host must begin sending bus
traffic (at least the SOF token) within 3ms to keep the system from going back into the Suspend state.
A device with remote wakeup capability may not generate resume signaling unless the bus has been
continuously in the Idle state for 5ms (TWTRSM). This allows the hubs to get into their Suspend state andprepare for propagating resume signaling. The remote wakeup device must hold the resume signaling forat least 1ms but for no more than 15ms (TDRSMUP). At the end of this period, the device stops driving thebus (puts its drivers into the high-impedance state and does not drive the bus to the J state).
If the hub upstream of a remote wakeup device is suspended, it will propagate the resume signaling to its
upstream port and to all of its enabled downstream ports, including the port that originally signaled theresume. The hub must begin this rebroadcast (TURSM) of the resume signaling within 100s of receiving
the original resume. The resume signal will propagate in this manner upstream until it reaches the host or a
non-suspended hub (refer to Section 11.9), which will reflect the resume downstream and take control ofresume timing. This hub is termed the controlling hub. Intermediate hubs (hubs between the resume
initiator and the controlling hub) drive resume (TDRSMUP) on their upstream port for at least 1ms during
which time they also continue to drive resume on enabled downstream ports. An intermediate hub willstop driving resume on the upstream port and reverse the direction of connectivity from upstream todownstream within 15ms after first asserting resume on its upstream port. When all intermediate hubs
have reversed connectivity, resume is being driven from the controlling hub through all intermediate hubs
and to all enabled ports. The controlling hub must rebroadcast the resume signaling within 100s(TURSM) and ensures that resume is signaled for at least 20ms (TDRSMDN). The hub may then begin
normal operation by terminating the resume process as described above.
The USB System Software must provide a 10ms resume recovery time (TRSMRCY) during which it will not
attempt to access any device connected to the affected (just-activated) bus segment.
Port connects and disconnects can also cause a hub to send a resume signal and awaken the system. These
events will cause a hub to send a resume signal only if the hub has been enabled as a remote-wakeup
source. Refer to Section 11.4.4 for more details.
Refer to Section 7.2.3 for a description of power control during suspend and resume.

7.1.8 Data Encoding/Decoding
The USB employs NRZI data encoding when transmitting packets. In NRZI encoding, a “1” is represented
by no change in level and a “0” is represented by a change in level. Figure 7-21 shows a data stream andthe NRZI equivalent. The high level represents the J state on the data lines in this and subsequent figures
showing NRZI encoding. A string of zeros causes the NRZI data to toggle each bit time. A string of ones
causes long periods with no transitions in the data.
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Universal Serial Bus Specification Revision 1.1





J
K



 

 







 



 

  


Figure 7-21. NRZI Data Encoding

7.1.9 Bit Stuffing
In order to ensure adequate signal transitions, bit stuffing is employed by the transmitting device whensending a packet on USB (see Figure 7-22 and Figure 7-24). A zero is inserted after every six consecutive
ones in the data stream before the data is NRZI encoded, to force a transition in the NRZI data stream.
This gives the receiver logic a data transition at least once every seven bit times to guarantee the data andclock lock. Bit stuffing is enabled beginning with the Sync Pattern and throughout the entire transmission.
The data “one” that ends the Sync Pattern is counted as the first one in a sequence. Bit stuffing by thetransmitter is always enforced, without exception. If required by the bit stuffing rules, a zero bit will beinserted even if it is the last bit before the end-of-packet (EOP) signal.
The receiver must decode the NRZI data, recognize the stuffed bits, and discard them. If the receiver sees
seven consecutive ones anywhere in the packet, then a bit stuffing error has occurred and the packet shouldbe ignored. The time interval just before an EOP is a special case. The last data bit before the EOP canbecome stretched by hub switching skews. This is known as dribble and can lead to the case illustrated in
Figure 7-23, which shows where dribble introduces a sixth bit that does not require a bit stuff. Therefore,
the receiver must accept a packet for which there are up to six full bit times at the port with no transitions
prior to the EOP.

Data Encoding Sequence:
Raw Data
Sync Pattern

Packet Data
Stuffed Bit

Bit Stuffed Data
Sync Pattern

Packet Data
Six Ones

NRZI
Idle
Encoded Data

Sync PatternFigure 7-22. Bit Stuffing

124

Packet Data

Universal Serial Bus Specification Revision 1.1

Transmitted
Data

0

0

1

1

1

1

1

EOP

EOP

J

CRC
Data fromTransmitter

Acceptable
Extra Bit,
No Error

EOP

CRC
Data at
Receiver

Extra
bit

EOP

Figure 7-23. Illustration of Extra Bit Preceding EOPPower UpNo Packet
Transmission

Idle
Begin Packet Transmission
Reset Bit
Counter to 0

Get Next
Bit

=0

Bit
Value?

=1

Increment
the Counter

No

Counter= 6?

Yes

Insert a
Zero Bit

Reset Bit
Counter to 0No

Is Packet
Transfer
Done?

Yes

Figure 7-24. Flow Diagram for Bit Stuffing

125Universal Serial Bus Specification Revision 1.1

7.1.10 Sync Pattern
The NRZI bit pattern shown in Figure 7-25 is used as a synchronization pattern and is prefixed to eachpacket. This pattern is equivalent to a data pattern of seven zeroes followed by a one (80H).
 











Figure 7-25. Sync Pattern

7.1.11 Data Signaling Rate
The full-speed data rate is nominally 12.000Mb/s. The data-rate tolerance for host, hub, and full-speedfunctions is ±0.25% (2,500ppm). The accuracy of the Host Controller’s data rate must be known andcontrolled to better than ±0.05% (500ppm). This tolerance includes inaccuracies from all sources:







Initial frequency accuracy
Crystal capacitive loading
Supply voltage on the oscillator
Temperature
Aging.

The low-speed data rate is nominally 1.50Mb/s. The permitted data-rate tolerance for low-speed functionsis 1.5% (15,000ppm). This tolerance includes inaccuracies from all sources:







Initial frequency accuracy
Crystal capacitive loading
Supply voltage on the oscillator
Temperature
Aging.

This tolerance allows the use of resonators in low cost, low-speed devices.

7.1.12 Frame Interval and Frame Interval Adjustment
The USB defines a frame interval to be 1.000ms ±500ns long. The frame interval is measured from any
point in an SOF token in one frame to the same point in the SOF token of the next frame.
The Host Controller must be able to adjust the frame interval. There are two possible components to theframe interval adjustment. If the host’s data rate clock is not exactly 12.000Mb/s, then the initial ±0.05%
frame interval accuracy can be met by changing the default number of bits per frame from the nominal of
12,000. A Host Controller component that has a range of possible clock-source values may have to makethis initial frame count a programmable value. An additional adjustment of 15 full-speed bit times isrequired to allow the host to synchronize to an external time reference. During normal bus operation,
software may not change the frame interval by more than one full-speed bit time every six frames. If noadjustment is being made, the frame interval repeatability (difference in frame interval between two
successive frames) must be less than 0.5 full-speed bit times (TRFI). If an adjustment is being made theframe interval repeatability must be less than 1.5 full-speed bit times (TRFIADJ).

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Universal Serial Bus Specification Revision 1.1

Hubs and certain full-speed functions need to track the frame interval. They also are required to havesufficient frame timing adjustment to compensate for their own frequency tolerance and track the host’s15 full-speed bit time variability.

7.1.13 Data Source Signaling
This section covers the timing characteristics of data produced and sent from a port (the data source).
Section 7.1.14 covers the timing characteristics of data that is transmitted through the Hub Repeater sectionof a hub. In this section, TPERIOD is defined as the actual period of the data rate that can have a range as
defined in Section 7.1.11.

7.1.13.1 Data Source Jitter
The source of data can have some variation (jitter) in the timing of edges of the data transmitted. The time
between any set of data transitions is N * TPERIOD  jitter time, where ‘N’ is the number of bits between
the transitions. The data jitter is measured with the same load used for maximum rise and fall times and ismeasured at the crossover points of the data lines, as shown in Figure 7-26.

Crossover
PointsDifferential
Data Lines
JitterConsecutive
Transitions

Integer multiples of TPERIOD
PairedTransitions

Figure 7-26. Data Jitter TaxonomyFor full-speed transmissions, the jitter time for any consecutive differential data transitions must bewithin 2.0ns and within 1.0ns for any set of paired (JK-to-next JK transition or KJ-to-next KJtransition) differential data transitions.
For low-speed transmissions, the jitter time for any consecutive differential data transitions must bewithin 25ns and within 10ns for any set of paired differential data transitions.

These jitter numbers include timing variations due to differential buffer delay and rise and fall time
mismatches, internal clock source jitter, and noise and other random effects.

7.1.13.2 EOP Width
The width of the SE0 in the EOP is approximately 2 * TPERIOD. The SE0 width is measured with the sameload used for maximum rise and fall times and is measured at the same level as the differential signal
crossover points of the data lines (see Figure 7-27).

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Universal Serial Bus Specification Revision 1.1

TPERIOD
Data
Crossover
Level

Differential
Data Lines

SE0 for
EOP
Width
Figure 7-27. SE0 for EOP Width TimingFor full-speed transmissions, the SE0 for EOP width from the transmitter must be between 160ns and175ns.
For low-speed transmissions, the transmitter’s SE0 for EOP width must be between 1.25s and 1.50s.

These ranges include timing variations due to differential buffer delay and rise and fall time mismatches
and to noise and other random effects.
A receiver must accept any valid EOP. Receiver design should note that the single-ended input thresholdvoltage can be different from the differential crossover voltage and the SE0 transitions will in general be
asynchronous to the clock encoded in the NRZI stream.



A full-speed EOP may have the SE0 interval reduced to as little as 82ns (TFEOPR) and a low-speed
SE0 interval may be as short as 670ns (TLEOPR).

A hub may tear down connectivity if it sees an SE0 of at least TFST or TLST followed by a transition to theJ state. A hub must tear down connectivity on any valid EOP.

7.1.14 Hub Signaling Timings
The propagation of a full-speed, differential data signal through a hub is shown in Figure 7-28. The
downstream signaling is measured without a cable connected to the port and with the load used formeasuring rise and fall times. The total delay through the upstream cable and hub electronics must be a
maximum of 70ns (THDD1). If the hub has a USB detachable cable, then the delay (THDD2) through hub
electronics and the associated transmission line must be a maximum of 44ns to allow for a worst-case cabledelay of 26ns (TFSCBL). The delay through this hub is measured in both the upstream and downstream
directions, as shown in Figure 7-28B, from data line crossover at the input port to data line crossover at theoutput port.

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Universal Serial Bus Specification Revision 1.1

Upstream End
of Cable

Data Line
Crossover
Point

Downstream
Port
50% Point of
Initial Swing

VSS

50% Point of
Initial Swing
Hub Delay
Downstream
70ns (max)DownstreamPort

Hub Delay
Upstream
70ns (max)Upstream End
of Cable

Data Line
Crossover
Point

VSS

A. Downstream Hub Delay
upstream end of cable

B. Upstream Hub Delay

upstream port

Host orHub

downstream port

Hub

plug

receptacle

Function

downstream signaling
upstream signaling

C. Measurement Points

Figure 7-28. Hub Propagation Delay of Full-speed Differential Signals
Low-speed propagation delay for differential signals is measured in the same fashion as for full-speed
signaling. The maximum low-speed hub delay is 300ns (TLHDD). This allows for the slower low-speed
buffer propagation delay and rise and fall times. It also provides time for the hub to re-clock the low-speed
data in the upstream direction.
When the hub acts as a repeater, it must reproduce the received, full-speed signal accurately on its outputs.
This means that for differential signals, the propagation delays of a J-to-K state transition must match
closely to the delays of a K-to-J state transition. For full-speed propagation, the maximum difference
allowed between these two delays (THDJ1) (see Figure 7-28 and Figure 7-42) for a hub plus cable is 3.0ns.
Similarly, the difference in delay between any two J-to-K or K-to-J transitions through a hub (THDJ2) mustbe less than 1.0ns. For low-speed propagation in the downstream direction, the corresponding allowablejitter (TLDHJ1) is 45ns and (TLDHJ2) 15ns, respectively. For low-speed propagation in the upstream
direction, the allowable jitter is 45ns in both cases (TLUHJ1 and TLUHJ2).
An exception to this case is the skew that can be introduced in the Idle-to-K state transition at SOP (TFSOP
and TLSOP) (refer to Section 7.1.7.2). In this case, the delay to the opposite port includes the time to enable
the output buffer. However, the delays should be closely matched to the normal hub delay and the
maximum additional delay difference over a normal J-to-K transition is 5.0ns. This limits the maximum
distortion of the first bit in the packet.
Note: because of this distortion of the SOP transition relative to the next K-to-J state transition, the first
SYNC field bit should not be used to synchronize the receiver to the data stream.
The EOP must be propagated through a hub in the same way as the differential signaling. The propagation
delay for sensing an SE0 must be no less than the greater of the J-to-K, or K-to-J differential data delay (toavoid truncating the last data bit in a packet), but not more than 15ns greater than the larger of thesedifferential delays at full-speed and 200ns at low-speed (to prevent creating a bit stuff error at the end of
the packet). EOP delays are shown in Figure 7-43.
Because the sense levels for the SE0 state are not at the midpoint of the signal swing, the width of SE0
state will be changed as it passes through each hub. A hub may not change the width of the SE0 state in a129

Universal Serial Bus Specification Revision 1.1
full-speed EOP by more than 15ns (TFHESK), as measured by the difference of the leading edge andtrailing edge delays of the SE0 state (see Figure 7-43). An SE0 from a low-speed device has long rise andfall times and is subject to greater skew, but this conditions exists only on the cable from the low-speed
device to the port to which it is connected. Thereafter, the signaling uses full-speed buffers and their fasterrise and fall times. The SE0 from the low-speed device cannot be changed by more than 300ns (TLHESK)
as it passes through the hub to which the device is connected. This time allows for some signal
conditioning in the low-speed port to reduce its sensitivity to noise.

7.1.15 Receiver Data Jitter
The data receivers for all types of devices must be able to properly decode the differential data in thepresence of jitter. The more of the bit cell that any data edge can occupy and still be decoded, the morereliable the data transfer will be. Data receivers are required to decode differential data transitions that
occur in a window plus and minus a nominal quarter bit cell from the nominal (centered) data edge
position. (A simple 4X over-sampling state machine DPLL can be built that satisfies these requirements.)
This requirement is derived in Table 7-2 and Table 7-3. The tables assume a worst-case topology of fivehubs between the host and device and the worst-case number of seven bits between transitions. The
derived numbers are rounded up for ease of specification.
Jitter will be caused by the delay mismatches discussed above and by mismatches in the source anddestination data rates (frequencies). The receive data jitter budgets for full- and low-speed are given in
Table 7-2 and Table 7-3. These tables give the value and totals for each source of jitter for bothconsecutive (next) and paired transitions. Note that the jitter component related to the source or destination
frequency tolerance has been allocated to the appropriate device (i.e., the source jitter includes bit shifts
due to source frequency inaccuracy over the worst-case data transition interval). The output driver jittercan be traded off against the device clock accuracy in a particular implementation as long as the jitter
specification is met.
The low-speed jitter budget table has an additional line in it because the jitter introduced by the hub towhich the low-speed device is attached is different from all the other devices in the data path. The
remaining devices operate with full-speed signaling conventions (though at low-speed data rate).

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Universal Serial Bus Specification Revision 1.1

Table 7-2. Full-speed Jitter Budget
Jitter Source

Full-speed
Next Transition
Each (ns)

Paired Transition

Total (ns)Each (ns) Total (ns)

Source Driver Jitter

2.0

2.0

1.0

1.0

Source Frequency Tolerance (worst-case)

0.21/bit

1.5

0.21/bit

3.0

Source Jitter Total

3.5

Hub Jitter

3.0

15.0

Jitter Specification

4.0
1.0

5.0

18.5

Destination Frequency Tolerance

0.21/bit

Receiver Jitter Budget

1.5

9.0
0.21/bit

3.0

20.0

12.0

Table 7-3. Low-speed Jitter Budget
Jitter Source

Low-speed Upstream
Next Transition

Paired Transition

Each (ns) Total (ns) Each (ns) Total (ns)Function Driver Jitter

25.0

25.0

10.0

10.0

Function Frequency Tolerance (worst-case)10.0/bit

70.0

10.0/bit

140.0

Source (Function) Jitter Total

95.0

150.0

Hub with Low-speed Device Jitter

45.0

45.0

45.0

45.0

Remaining (full-speed) Hubs' Jitter

3.0

12.0

1.0

4.0

Jitter Specification
Host Frequency Tolerance

152.0
1.7/bitHost Receiver Jitter Budget

12.0

199.0
1.7/bit

164.0

24.0
223.0

Low-speed Downstream
Next Transition

Paired Transition

Each (ns) Total (ns) Each (ns) Total (ns)
Host Driver Jitter

2.0

2.0

1.0

1.0

Host Frequency Tolerance (worst-case)

1.7/bit

12.0

1.7/bit24.0

Hub with Low-speed Device Jitter

45.0

45.0

15.0

15.0

Remaining (full-speed) Hubs' Jitter

3.0

12.0

1.0

4.0

Source (Host) Jitter Total

14.0

Jitter Spec
Function Frequency Tolerance
Function Receiver Jitter Budget

25.0

71.0
10.0/bit

70.0
141.0

44.0
10.0/bit

140.0
184.0

Note: this table describes the host transmitting at low-speed data rate using full-speed signaling toa low-speed device through the maximum number of hubs. When the host is directly connected to
the low-speed device, then it uses low-speed data rate and low-speed signaling, and the host has tomeet the source jitter listed in the “Jitter Specification” row.

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Universal Serial Bus Specification Revision 1.1

7.1.16 Cable Delay
Except (in certain cases) for the SOP, only one data transition is allowed on a USB cable at a time. A fullspeed signal edge has to propagate to the far end of the cable, return, and settle within one full-speed bit
time. Therefore, the maximum total one-way signal propagation delay allowed is 30ns. The allocation forcable delay is 26ns. A maximum delay of 3ns is allowed from a Host or Hub Controller downstream port
to its exterior downstream connector, while a maximum delay of 1ns is allowed from the upstream
connector to the upstream port of any device. For a standard USB detachable cable, the cable delay ismeasured from the Series A connector pins to the Series B connector pins and is no more than 26ns. For
other cables, the delay is measured from the series A connector to the point where the cable is connected tothe device.
The maximum one-way data delay on a full-speed cable is measured as shown in Figure 7-29.
One-way cable delay for low-speed cables must be less than 18ns. It is measured as shown in
Figure 7-30.
Traces on Board
Host/Hub

Hub/Device

Downstream
Port

UpstreamPort

3ns (max)

A-Connector

B-Connector

Total One-Way
Propagation Delay
30ns (max)

1ns (max)

Driver End
of Cable
50% Point of Initial Swing
VSS
ReceiverEnd ofCable

One Way Cable
Delay 26ns
(max)

Data Line
Crossover
Point at input of
B-connector

VSS

Figure 7-29. Full-speed Cable Delay
Traces on Board
Host/Hub
Downstream
Port

Low-speedDevice

A-Connector + cable

18nS (max)One-way Propagation Delay
Figure 7-30. Low-speed Cable Delay132

Universal Serial Bus Specification Revision 1.1

7.1.17 Cable Attenuation
The allowable attenuation of the signal pair (D+, D-) for full speed signaling per cable is listed in Table
7-4. The cable attenuation measurement is defined in Section 6.7.
Table 7-4. Signal Attenuation
Frequency (MHz)

Attenuation (maximum) dB/cable

0.064

0.08

0.256

0.11

0.512

0.13

0.772

0.15

1.000

0.20

4.000

0.39

8.000

0.57

12.0000.67

24.0000.95

48.0001.35

96.0001.9

7.1.18 Bus Turn-around Time and Inter-packet Delay
Inter-packet delays are measured from the SE0-to-J transition at the end of the EOP to the J-to-K transition
that starts the next packet.
A device is required to allow two bit times of inter-packet delay. The delay is measured at the responding
device with a bit time defined in terms of the response. This provides adequate time for the device sending
the EOP to drive J for one bit time and then turn off its output buffers.
The host must provide at least two bit times of J after the SE0 of an EOP and the start of a new packet(TIPD). If a function is expected to provide a response to a host transmission, the maximum inter-packet
delay for a function or hub with a detachable (TRSPIPD1) cable is 6.5 bit times measured at the Series B
receptacle. If the device has a captive cable, the inter-packet delay (T RSPIPD2) must be less than 7.5 bittimes as measured at the Series-A plug. These timings apply to both full-speed and low-speed devices andthe bit times are referenced to the data rate of the packet.
The maximum inter-packet delay for a host response is 7.5 bit times, measured at the host’s port pins.
There is no maximum inter-packet delay between packets in unrelated transactions.

7.1.19 Maximum End-to-end Signal Delay
A device expecting a response to a transmission will invalidate the transaction if it does not see the start-ofpacket (SOP) transition within the timeout period after the end of the transmission (after the SE0-to-J statetransition in the EOP). This can occur between an IN token and the following data packet or between a133

Universal Serial Bus Specification Revision 1.1

data packet and the handshake packet (refer to Chapter 8). The device expecting the response will not timeout before 16 bit times but will timeout before 18 bit times (measured at the data pins of the device fromthe SE0-to-J transition at the end of the EOP). The host will wait at least 18 bit times for a response to start
before it will start a new transaction.
Figure 7-31 depicts the configuration of six signal hops (cables) that results in allowable worst-case signal
delay. The maximum propagation delay from the upstream end of a hub’s cable to any downstream port
connector is 70ns.

Host
Controller

Hub 5

Hub 4

Hub 3

Hub 2

Cable Delay + Hub Delay  70ns (each)

Hub 1

Function

Propagation Delay  30ns

Figure 7-31. Worst-case End to End Signal Delay Model

7.2 Power Distribution
This section describes the USB power distribution specification.

7.2.1 Classes of Devices
The power source and sink requirements of different device classes can be simplified with the introductionof the concept of a unit load. A unit load is defined to be 100mA. The number of unit loads a device candraw is an absolute maximum, not an average over time. A device may be either low-power at one unitload or high-power, consuming up to five unit loads. All devices default to low-power. The transition tohigh-power is under software control. It is the responsibility of software to ensure adequate power isavailable before allowing devices to consume high-power.
The USB supports a range of power sourcing and power consuming agents; these include the following:










134

Root port hubs: Are directly attached to the USB Host Controller. Hub power is derived from thesame source as the Host Controller. Systems that obtain operating power externally, either AC or DC
must supply at least five unit loads to each port. Such ports are called high-power ports. Batterypowered systems may supply either one or five unit loads. Ports that can supply only one unit load aretermed low-power ports.
Bus-powered hubs: Draw all of their power for any internal functions and downstream ports from
VBUS on the hub’s upstream port. Bus-powered hubs may only draw up to one unit load upon powerup, and five unit loads after configuration. The configuration power is split between allocations to thehub, any non-removable functions and the external ports. External ports in a bus-powered hub can
supply only one unit load per port regardless of the current draw on the other ports of that hub. The
hub must be able to supply this port current when the hub is in the Active or Suspend state.
Self-powered hubs: Power for the internal functions and downstream ports does not come from
VBUS. However, the USB interface of the hub may draw up to one unit load from its upstream VBUS
to allow the interface to function when the remainder of the hub is powered down. Hubs that obtain
operating power externally (from the USB) must supply five unit loads to each port. Battery-powered
hubs may supply either one or five unit loads per port.
Low-power bus-powered functions: All power to these devices comes from VBUS. They may draw
no more than one unit load at any time.
High-power bus-powered functions: All power to these devices comes from VBUS. They must draw
no more than one unit load upon power-up and may draw up to five unit loads after being configured.

Universal Serial Bus Specification Revision 1.1



Self-powered functions: May draw up to one unit load from VBUS to allow the USB interface tofunction when the remainder of the function is powered down. All other power comes from anexternal (to the USB) source.

No device shall supply (source) current on VBUS at its upstream port at any time. From VBUS on its
upstream port, a device may only draw (sink) current. They may not provide power to the pull-up resistor
on D+/D- unless VBUS is present (see Section 7.1.5). On power-up, a device needs to ensure that its
upstream port is not driving the bus, so that the device is able to receive the reset signaling. Devices mustalso ensure that the maximum operating current drawn by a device is one unit load, until configured. Any
device that draws power from the bus must be able to detect lack of activity on the bus, enter the Suspend
state and reduce its current consumption from VBUS (refer to Section 7.2.3 and Section 9.2.5.1).

7.2.1.1 Bus-powered Hubs
Bus-powered hub power requirements can be met with a power control circuit such as the one shown in
Figure 7-32. Bus-powered hubs often contain at least one non-removable function. Power is always
available to the hub’s controller, which permits host access to power management and other configuration
registers during the enumeration process. A non-removable function(s) may require that its power beswitched, so that upon power-up the entire device (hub and non-removable functions) draws no more than
one unit load. Power switching on any non-removable function may be implemented either by removing
its power or by shutting off the clock. Switching on the non-removable function is not required if theaggregate power drawn by it and the Hub Controller is less than one unit load. However, as long as the
hub port associated with the function is in the Power-off state, the function must be logically reset and thedevice must appear to be not connected. The total current drawn by a bus-powered device is the sum of thecurrent to the Hub Controller, any non-removable function(s), and the downstream ports.
Figure 7-32 shows the partitioning of power based upon the maximum upstream current draw of five unit
loads: one unit load for the Hub Controller and the non-removable function, and one unit load for each of
the external downstream ports. If more than four external ports are required, then the hub will need to be
self-powered. If the non-removable function(s) and Hub Controller draw more than one unit load, then thenumber of external ports must be appropriately reduced. Power control to a bus-powered hub may requirea regulator. If present, the regulator is always enabled to supply the Hub Controller. The regulator canalso power the non-removable functions(s). Inrush current limiting must also be incorporated into theregulator subsystem.

Upstream
Data Port

Downstream
Data Ports

Hub Controller

Iportctrl

Upstream V BUS
5 unit loads

On/OffRegulator

Non-removable
Function

1 unit load - Iportctrl
On/OffSwitch1 unit load/port

Downstream VBUS

Figure 7-32. Compound Bus-powered HubPower to external downstream ports of a bus-powered hub must be switched. The Hub Controller suppliesa software controlled on/off signal from the host, which is in the “off” state when the device is powered up
or after reset signaling. When switched to the “on” state, the switch implements a soft turn-on function that135

Universal Serial Bus Specification Revision 1.1

prevents excessive transient current from being drawn from the upstream port. The voltage drop across the
upstream cable, connectors, and switch in a bus-powered hub must not exceed 350mV at maximum rated
current.

7.2.1.2 Self-powered Hubs
Self-powered hubs have a local power supply that furnishes power to any non-removable functions and to
all downstream ports, as shown in Figure 7-33. Power for the Hub Controller, however, may be supplied
from the upstream VBUS (a “hybrid” powered hub) or the local power supply. The advantage of supplying
the Hub Controller from the upstream supply is that communication from the host is possible even if thedevice’s power supply remains off. This makes it possible to differentiate between a disconnected and an
unpowered device. If the hub draw power for its upstream port from VBUS, it may not draw more than one
unit load.

Upstream VBUS
1 unit load (max)Regulator

Hub Controller.
.
.

Downstream
Data Ports

Upstream
Data Port

Local Power
SupplyRegulator

Non-removable
Function

On/Off.
.

Current Limit

5 unit loads/port

Current Limit

Downstream VBUS.
.

Figure 7-33. Compound Self-powered Hub
The number of ports that can be supported is limited only by the address capability of the hub and the
local supply.
Self-powered hubs may experience loss of power. This may be the result of disconnecting the power cord
or exhausting the battery. Under these conditions, the hub may force a re-enumeration of itself as a buspowered hub. This requires the hub to implement port power switching on all external ports. When poweris lost, the hub must ensure that upstream current does not exceed low-power. All the rules of a buspowered hub then apply.

7.2.1.2.1 Over-current Protection
The host and all self-powered hubs must implement over-current protection for safety reasons, and the hub
must have a way to detect the over-current condition and report it to the USB software. Should theaggregate current drawn by a gang of downstream ports exceed a preset value, the over-current protection
circuit removes or reduces power from all affected downstream ports. The over-current condition isreported through the hub to Host Controller, as described in Section 11.13.5. The preset value cannot
exceed 5.0 A and must be sufficiently above the maximum allowable port current such that transientcurrents (e.g. during power up or dynamic attach or reconfiguration) do not trip the over-current protector.

136

Universal Serial Bus Specification Revision 1.1

If an over-current condition occurs on any port, subsequent operation of the USB is not guaranteed, andonce the condition is removed, it may be necessary to reinitialize the bus as would be done upon power-up.
The over-current limiting mechanism must be resettable without user mechanical intervention. Polymeric
PTCs and solid-state switches are examples of methods, which can be used for over-current limiting.

7.2.1.3 Low-power Bus-powered Functions
A low-power function is one that draws up to one unit load from the USB cable when operational. Figure
7-34 shows a typical bus-powered, low-power function, such as a mouse. Low-power regulation can be
integrated into the function silicon. Low-power functions must be capable of operating with input VBUS
voltages as low as 4.40V, measured at the plug end of the cable.
Upstream
Data Port

Function

Upstream V BUS

Regulator

1 unit load (max)Figure 7-34. Low-power Bus-powered Function

7.2.1.4 High-power Bus-powered Functions
A function is defined as being high-power if, when fully powered, it draws over one but less than five unit
loads from the USB cable. A high-power function requires staged switching of power. It must first comeup in a reduced power state of less than one unit load. At bus enumeration time, its total power
requirements are obtained and compared against the available power budget. If sufficient power exists, theremainder of the function may be powered on. A typical high-power function is shown in Figure 7-35.
The function’s electronics have been partitioned into two sections. The function controller contains theminimum amount of circuitry necessary to permit enumeration and power budgeting. The remainder of thefunction resides in the function block. High-power functions must be capable of operating in their lowpower (one unit load) mode with an input voltage as low as 4.40V, so that it may be detected andenumerated even when plugged into a bus-powered hub. They must also be capable of operating at fullpower (up to five unit loads) with a VBUS voltage of 4.75V, measured at the upstream plug end of thecable.
Upstream
Data Port

Function Controller

Function
On/Off

1 unit load(max)

Upstream VBUS

Regulator

5 unit loads (max)Figure 7-35. High-power Bus-powered Function

7.2.1.5 Self-powered Functions
Figure 7-36 shows a typical self-powered function. The function controller is powered either from the
upstream bus via a low-power regulator or from the local power supply. The advantage of the former
scheme is that it permits detection and enumeration of a self-powered function whose local power supply isturned off. The maximum upstream power that the function controller can draw is one unit load, and the137Universal Serial Bus Specification Revision 1.1

regulator block must implement inrush current limiting. The amount of power that the function block may
draw is limited only by the local power supply. Because the local power supply is not required to powerany downstream bus ports, it does not need to implement current limiting, soft start, or power switching.

Upstream
Data Port

Function Controller

Upstream VBUS

Function

Regulator

1 unit load (max)Local Power
SupplyRegulator

Figure 7-36. Self-powered Function

7.2.2 Voltage Drop Budget
The voltage drop budget is determined from the following:









The voltage supplied by high-powered hub ports is 4.75V to 5.25V.
The voltage supplied by low-powered hub ports is 4.4V to 5.25V.
Bus-powered hubs can have a maximum drop of 350mV from their cable plug (where they attach to asource of power) to their output port connectors (where they supply power).
The maximum voltage drop (for detachable cables) between the A-series plug and B-series plug on
VBUS is 125mV (VBUSD).
The maximum voltage drop for all cables between upstream and downstream on GND is 125mV(VGNDD).
All hubs and functions must be able to provide configuration information with as little as 4.40V at theconnector end of their upstream cables. Only low-power functions need to be operational with thisminimum voltage.
Functions drawing more than one unit load must operate with a 4.75V minimum input voltage at theconnector end of their upstream cables.

Figure 7-37 shows the minimum allowable voltages in a worst-case topology consisting of a bus-powered
hub driving a bus-powered function.
Bus-powered
Hub

Host orPowered Hub
4.750V

4.735V4.640V

4.625V*4.400V

Low-power
Function
4.397V

4.378V

4.375V

4.500V

0.000V

0.015V

0.110V

0.125V
Referenced
to Source

4.350V
0.000V

0.003V

0.022V

0.025V

Referenced
to Hub*Under transient conditions, supply at hub can drop from 4.400V to 4.070V

Figure 7-37. Worst-case Voltage Drop Topology (Steady State)138

Universal Serial Bus Specification Revision 1.1

7.2.3 Power Control During Suspend/Resume
Suspend current is a function of unit load allocation. All USB devices initially default to low-power. Lowpower devices or high-power devices operating at low-power are limited to 500A of suspend current. If
the device is configured for high-power and enabled as a remote wakeup source, it may draw up to 2.5mA
during suspend. When computing suspend current, the current from VBUS through the bus pull-up andpull-down resistors must be included. Configured bus-powered hubs may also consume a maximum of
2.5mA, with 500A allocated to each available external port and the remainder available to the hub and its
internal functions. If a hub is not configured, it is operating as a low-power device and must limit its
suspend current to 500A.
While in the Suspend state, a device may briefly draw more than the average current. The amplitude of thecurrent spike cannot exceed the device power allocation 100mA (or 500mA). A maximum of 1.0 second isallowed for an averaging interval. The average current cannot exceed the average suspend current limit(ICCSH or ICCSL, see Table 7-5) during any 1.0s interval (TSUSAVG1). The profile of the current spike isrestricted so the transient response of the power supply (which may be an efficient, low-capacity, trickle
power supply) is not overwhelmed. The rising edge of the current spike must be no more than 100mA/s.
Downstream ports must be able to absorb the 500mA peak current spike and meet the voltage droop
requirements defined for inrush current during dynamic attach (see Section 7.2.4.1). Figure 7-38
illustrates a typical example profile for an averaging interval. If the supply to the pull-up resistor on D+/Dis derived from VBUS, then the suspend current will never go to zero because the pull-up and pull-downresistors will always draw power.

ICONFIGURED(max)Edge rate must
not exceed
100mA/ sCurrent Spike

ICCS(H|L)
I

0 mA
time

Averaging Interval

Figure 7-38. Typical Suspend Current Averaging Profile
Devices are responsible for handling the bus voltage reduction due to the inductive and resistive effects ofthe cable. When a hub is in the Suspend state, it must still be able to provide the maximum current per port
(one unit load of current per port for bus-powered hubs and five unit loads per port for self-powered hubs).
This is necessary to support remote wakeup-capable devices that will power-up while the remainder of thesystem is still suspended. Such devices, when enabled to do remote wakeup, must drive resume signaling
upstream within 10ms of starting to draw the higher, non-suspend current. Devices not capable of remote
wakeup must draw the higher current only when not suspended.
When devices wakeup, either by themselves (remote wakeup) or by seeing resume signaling, they must
limit the inrush current on VBUS. The target maximum droop in the hub VBUS is 330mV. The device musthave sufficient on-board bypass capacitance or a controlled power-on sequence such that the current drawn
from the hub does not exceed the maximum current capability of the port at any time while the device iswaking up.

139

Universal Serial Bus Specification Revision 1.1

7.2.4 Dynamic Attach and Detach
The act of plugging or unplugging a hub or function must not affect the functionality of another device onother segments of the network. Unplugging a function will stop the transaction between that function andthe host. However, the hub to which this function was attached will recover from this condition and will
alert the host that the port has been disconnected.

7.2.4.1 Inrush Current Limiting
When a function or hub is plugged into the network, it has a certain amount of on-board capacitance
between VBUS and ground. In addition, the regulator on the device may supply current to its output bypass
capacitance and to the function as soon as power is applied. Consequently, if no measures are taken toprevent it, there could be a surge of current into the device which might pull the VBUS on the hub below its
minimum operating level. Inrush currents can also occur when a high-power function is switched into its
high-power mode. This problem must be solved by limiting the inrush current and by providing sufficient
capacitance in each hub to prevent the power supplied to the other ports from going out of tolerance. An
additional motivation for limiting inrush current is to minimize contact arcing, thereby prolonging
connector contact life.
The maximum droop in the hub VBUS is 330mV, or about 10% of the nominal signal swing from thefunction. In order to meet this requirement, the following conditions must be met:The maximum load (CRPB) that can be placed at the downstream end of a cable is 10F in parallelwith 44. The 10F capacitance represents any bypass capacitor directly connected across the VBUS
lines in the function plus any capacitive effects visible through the regulator in the device. The 44
resistance represents one unit load of current drawn by the device during connect.
If more bypass capacitance is required in the device, then the device must incorporate some form of
VBUS surge current limiting, such that it matches the characteristics of the above load.
The hub downstream port VBUS power lines must be bypassed (CHPB) with no less than 120F of lowESR capacitance per hub. Standard bypass methods should be used to minimize inductance andresistance between the bypass capacitors and the connectors to reduce droop. The bypass capacitors
themselves should have a low dissipation factor to allow decoupling at higher frequencies.

The upstream port of a hub is also required to meet the above requirements. Furthermore, a bus-powered
hub must provide additional surge limiting in the form of a soft-start circuit when it enables power to its
downstream ports.
A high-power bus-powered device that is switching from a lower power configuration to a higher power
configuration must not cause droop > 330 mV on the VBUS at its upstream hub . The device can meet thisby ensuring that changes in the capacitive load it presents do not exceed 10F.
Signal pins are protected from excessive currents during dynamic attach by being recessed in the connector
such that the power pins make contact first. This guarantees that the power rails to the downstream device
are referenced before the signal pins make contact. In addition, the signal lines are in a high-impedancestate during connect, so that no current flows for standard signal levels.

7.2.4.2 Dynamic Detach
When a device is detached from the network with power flowing in the cable, the inductance of the cable
will cause a large flyback voltage to occur on the open end of the device cable. This flyback voltage is not
destructive. Proper bypass measures on the hub ports will suppress any coupled noise. The frequency
range of this noise is inversely dependent on the length of the cable, to a maximum of 60MHz for a onemeter cable. This will require some low capacitance, very low inductance bypass capacitors on each hub
port connector. The flyback voltage and the noise it creates is also moderated by the bypass capacitance on
the device end of the cable. Also, there must be some minimum capacitance on the device end of the cable140

Universal Serial Bus Specification Revision 1.1

to ensure that the inductive flyback on the open end of the cable does not cause the voltage on the device
end to reverse polarity. A minimum of 1.0F is recommended for bypass across VBUS.

7.3 Physical Layer
The physical layer specifications are described in the following subsections.

141

Universal Serial Bus Specification Revision 1.1

7.3.1 Regulatory Requirements
All USB devices should be designed to meet the applicable regulatory requirements.

7.3.2 Bus Timing/Electrical Characteristics
Table 7-5. DC Electrical Characteristics
Parameter

SymbolConditions

Min.

Max.

Units

Supply Voltage:
High-power Port

VBUS

Note 2, Section 7.2.1

4.75

5.25

V

Low-power Port

VBUS

Note 2, Section 7.2.1

4.40

5.25

V

High-power Hub Port (out)

ICCPRT

Section 7.2.1

500

mA

Low-power Hub Port (out)ICCUPT

Section 7.2.1

100

mA

High-power Function (in)ICCHPF

Section 7.2.1

500

mA

Low-power Function (in)ICCLPF

Section 7.2.1

100

mA

Unconfigured Function/Hub (in)ICCINIT

Section 7.2.1.4

100

mA

Suspended High-power Device

ICCSH

Section 7.2.3 ; Note 15

Suspended Low-power Device

ICCSL

Section 7.2.3

High (driven)

VIH

Note 4, Section 7.1.4

2.0High (floating)

VIHZ

Note 4, Section 7.1.4

2.7

Low

VIL

Note 4, Section 7.1.4

Differential Input Sensitivity

VDI|(D+)-(D-)|;
Figure 7-9; Note 4

0.2

Differential Common Mode
Range

VCM

Includes VDI range;
Figure 7-9; Note 4

0.8

2.5

V

Low

VOL

Note 4, 5, Section 7.1.1

0.0

0.3

V

High (Driven)

VOH

Note 4, 6, Section 7.1.1

2.8

3.6

V

Output Signal Crossover
Voltage

VCRS

Measured as in Figure
7-6; Note 10

1.3

2.0

V

Downstream Port Bypass
Capacitance (per hub)

CHPB

VBUS to GND, Section
7.2.4.1

Upstream Port Bypass
Capacitance

CRPB

VBUS to GND; Note 9,
Section 7.2.4.1

Supply Current:

2.5
500

mA

A

Input Levels:
V
3.6

V

0.8

V
V

Output Levels:

Decoupling Capacitance:

142

F

120
1.0

10.0

F

Universal Serial Bus Specification Revision 1.1

Table 7-5. DC Electrical Characteristics (Continued)
Parameter

SymbolConditions

Min.

Max.

Units

Input Capacitance:
Downstream Port

CIND

Note 2; Section 7.1.6

150

pF

Upstream Port (w/o cable)

CINUB

Note 3; Section 7.1.6

100

pF

Transceiver edge rate control
capacitance

CEDGE

Section 7.1.6

75

pF

Bus Pull-up Resistor on
Upstream Port

RPU

1.5k 5%
Section 7.1.5

1.425

1.575

k

Bus Pull-down Resistor on
Downstream Port

RPD

15k 5%
Section 7.1.5

14.25

15.75

k

Input impedance exclusive ofpullup/pulldown

ZINP

Section 7.1.6

300

Termination voltage for
upstream port pullup (RPU)

VTERM

Section 7.1.5

Terminations:

3.0

k
3.6

V

143

Universal Serial Bus Specification Revision 1.1

Table 7-6. Full-speed Source Electrical Characteristics
Parameter

SymbolConditions

Min.

Max.

Units

Driver Characteristics:
Rise Time

TFR

Figure 7-6; Figure 7-7

4

20

ns

Fall Time

TFF

Figure 7-6; Figure 7-7

4

20

ns

Differential Rise and Fall Time
Matching

TFRFM(TFR/TFF) Note 10, Section
7.1.2

90

111.11%

Driver Output Resistance

ZDRVSection 7.1.1.1

28

44



Full-speed Data RateTFDRATE

Average bit rate, Section
7.1.11

11.9700

12.0300

Mb/s

Frame Interval

TFRAME

Section 7.1.12

0.99951.0005ms

Consecutive Frame Interval
JitterTRFI

No clock adjustment

42

ns

Consecutive Frame Interval
Jitter

TRFIADJWith clock adjustment

126

ns

Clock Timings:

Full-speed Data Timings:
Source Jitter Total (including
frequency tolerance):
To Next Transition
For Paired Transitions

TDJ1
TDJ2

Source Jitter for Differential
Transition to SE0 Transition

TFDEOP

Receiver Jitter:

Note 7, 8, 12, 10;
Measured as in Figure
7-39;

Note 8; Figure 7-40;
Note 11

-3.5
-4

3.5
4

ns
ns

-2

5

ns

-18.5
-9

18.5
9

ns
ns

175

ns

Note 8; Figure 7-41

To Next Transition
For Paired Transitions

TJR1
TJR2

Source SE0 interval of EOP

TFEOPT

Figure 7-40

160

Receiver SE0 interval of EOP

TFEOPR

Note 13; Section 7.1.13.2;
Figure 7-40

82

Width of SE0 interval during
differential transition

TFST

Section 7.1.4

144

ns
14

ns

Universal Serial Bus Specification Revision 1.1

Table 7-7. Low-speed Source Electrical Characteristics
Parameter

SymbolConditions

Min.

Max.

Units

Driver Characteristics:
Transition Time:
Rise Time
Fall Time

TLR
TLF

Measured as in Figure 7-6

75
75

300
300

ns
ns

Rise and Fall Time Matching

TLRFM(TLR/TLF) Note 10

80

125

%

Upstream Port(w/cable, low-speed only)

CLINUA

Note 1; Section 7.1.6

200

450

pF

TLDRATE

Section 7.1.11

Clock Timings:
Low-speed Data Rate

1.4775

1.5225

Mb/s

Low-speed Data Timings:
Upstream port source Jitter
Total (including frequency
tolerance):

Note 7, 8; Figure 7-39
TUDJ1
TUDJ2

-95
-150

95
150

ns
ns

-40

100

ns

-75
-45

75
45

ns
ns

-25
-14

25
14

ns
ns

To Next Transition
For Paired Transitions
Upstream port source Jitter for
Differential Transition to SE0
Transition
Upstream port differential
Receiver Jitter:
To Next Transition
For Paired Transitions
Downstream port source Jitter
Total (including frequency
tolerance):

TLDEOP

Note 8; Figure 7-40;
Note 11
Note 8; Figure 7-41

TDJR1
TDJR2
Note 7, 8; Figure 7-39
TDDJ1
TDDJ2

To Next Transition
For Paired Transitions
Downstream port source Jitterfor Differential Transition to SE0
Transition

Note 8; Figure 7-40;
Note 11

Downstream port Differential
Receiver Jitter:

Note 8; Figure 7-40

To Next Transition
For Paired Transitions

ns

-152
-200

TUJR1
TUJR2

Source SE0 interval of EOP

TLEOPT

Figure 7-40

Receiver SE0 interval of EOP

TLEOPR

Note 13; Section 7.1.13.2;
Figure 7-40

Width of SE0 interval during
differential transition

TLST

Section 7.1.4

1.25

152
200
1.50

670

ns
ns
µs
ns

210

ns

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Universal Serial Bus Specification Revision 1.1

Table 7-8. Hub/Repeater Electrical Characteristics
Parameter

SymbolConditions

Min.

Max.

Units

Full-speed Hub Characteristics (as measured at connectors):
Driver Characteristics:(Refer to Table 7-6)

Upstream port anddownstream ports
configured as full-speed

Hub Differential Data Delay:

Note 7, 8(with cable)(without cable)

THDD1
THDD2

Hub Differential Driver Jitter:(including cable)
To Next Transition
For Paired Transitions

Figure 7-42A
Figure 7-42B

70
44

ns
ns

-3
-1

3
1

ns
ns

Note 7, 8; Figure 7-42,
Section 7.1.14
THDJ1
THDJ2

Data Bit Width Distortion after SOP

TFSOP

Note 8; Figure 7-42

-5

5

ns

Hub EOP Delay Relative to THDDTFEOPD

Note 8; Figure 7-43

0

15

ns

Hub EOP Output Width Skew

TFHESK

Note 8; Figure 7-43

-15

15

ns

300

ns

Low-speed Hub Characteristics (as measured at connectors):
Downstream ports
configured as low-speed

Driver Characteristics:(Refer to Table 7-7)
Hub Differential Data Delay

TLHDD

Hub Differential Driver Jitter(including cable):

Note 7, 8; Figure 7-42
Note 7, 8; Figure 7-42

Downstream port :To Next Transition
For Paired Transitions

TLDHJ1
TLDHJ2

-45
-15

45
15

ns
ns

TLUHJ1TLUHJ2

-45
-45

45
45

ns
ns

Upstream port:
To Next Transition
For Paired Transitions
Data Bit Width Distortion after SOP

TLSOP

Note 8; Figure 7-42

-60

60

ns

Hub EOP Delay Relative to THDDTLEOPD

Note 8; Figure 7-43

0

200

ns

Hub EOP Output Width SkewTLHESK

Note 8; Figure 7-43

-300

+300

ns

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Universal Serial Bus Specification Revision 1.1

Table 7-9. Cable Characteristics (Note 14)
Parameter

SymbolConditions

Min

Max

Units

VBUS Voltage drop fordetachable cables

VBUSD

Section 7.2.2

125

mV

GND Voltage drop (for all
cables)VGNDDSection 7.2.2

125

mV

Differential Cable Impedance(full-speed)

ZO(90 15%);

103.5



26
18

ns
ns

Cable Delay (one way)
Full-speed
Low-speed

76.5

Section 7.1.16
TFSCBLTLSCBL

Cable Skew

TSKEW

Section 7.1.3

400

ps

Unmated Contact Capacitance

CUC

Section 6.7

2

pF

Note 1: Measured at A plug
Note 2: Measured at A receptacle
Note 3: Measured at B receptacle
Note 4: Measured at A or B connector

Note 5: Measured with RL of 1.425k to 3.6V

Note 6: Measured with RL of 14.25k to GND
Note 7: Timing difference between the differential data signals
Note 8: Measured at crossover point of differential data signals
Note 9: The maximum load specification is the maximum effective capacitive load allowed that meets the target
VBUS drop of 330mV

Note 10: Excluding the first transition from the Idle state
Note 11: The two transitions should be a (nominal) bit time apart
Note 12: For both transitions of differential signaling
Note 13: Must accept as valid EOP
Note 14: Single-ended capacitance of D+ or D- is the capacitance of D+/D- to all other conductors and, if present,
shield in the cable. I.e., to measure the single-ended capacitance of D+, short D-, VBUS, GND and theshield line together and measure the capacitance of D+ to the other conductors.
Note 15: For high power devices (non-hubs) when enabled for remote wakeup

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Table 7-10. Hub Event Timings
Event Description

SymbolConditions

Min

Time to detect a downstream port
connect event
Awake Hub
Suspended Hub

TDCNN

Section 11.5 and Section
7.1.7.1

Time to detect a disconnect eventat a downstream port:
Awake Hub
Suspended Hub

TDDIS

Duration of driving resume to adownstream port; Only from acontrolling hub

TDRSMDN

Nominal; Section 7.1.7.5
and Section 11.5

Time from detecting downstream
resume to rebroadcast.

TURSM

Section 7.1.7.5

Duration of driving reset to adownstream port

TDRST

Only for a
SetPortFeature(PORT_RESET) request;
Section 7.1.7.3 andSection 11.5

10Overall duration of driving reset todownstream port, root hub

TDRSTR

only for root hubs;
Section 7.1.7.3

50

Maximum interval between reset
segments used to create TDRSTRTRHRSI

only for root hubs; eachreset pulse must be oflength TDRST; Section
7.1.7.3

Time to evaluate device speed
after reset

TDSPDEV

Optional
Section 11.8.2

Time to detect a long K fromupstream

TURLKTime to detect a long SE0 fromupstream

Max

Unit

2.5
2.5

2000
12000

s
s

2
2

2.5
10000.0ssSection 7.1.7.1

20

ms

100

µs

20

ms

ms

3

ms

2.5

1000

s

Section 11.6.1

2.5

100

s

TURLSE0

Section 11.6.1

2.5

10000

s

Duration of repeating SE0
upstream

TURPSE0

Section 11.6.2

23

FS bittimes

Duration of sending SE0 upstream
after EOF1

TUDEOP

Optional
Section 11.6.2

2

FS bittimes

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Universal Serial Bus Specification Revision 1.1

Table 7-11. Device Event Timings
Parameter

SymbolTime from internal power good todevice pulling D+/D- beyond VIHZ(min) (signaling attach)

TSIGATT

Figure 7-19

100

ms

Debounce interval provided by
USB system software after attach

TATTDB

Figure 7-19

100

ms

Maximum time a device can draw
power >suspend power when bus
is continuously in idle state

T2SUSP

Section 7.1.7.4

10

ms

Maximum duration of suspend
averaging interval

TSUSAVGI

Section 7.2.3

1sPeriod of idle bus before device
can initiate resume

TWTRSM

Device must be
remote-wakeup
enabled. Section
7.1.7.5

5Duration of driving resume
upstream

TDRSMUP

Section 7.1.7.5

1Resume Recovery TimeTRSMRCY

Provided by USB
System Software;
Section 7.1.7.5

10Time to detect a reset fromupstream

TDETRST

Section 7.1.7.3

2.5

Reset Recovery Time

TRSTRCY

Section 7.1.7.3

TIPD

Section 7.1.18

Inter-packet delay for device
response w/detachable cable

TRSPIPD1

Section 7.1.18

6.5

bit
times

Inter-packet delay for device
response w/captive cable

TRSPIPD2

Section 7.1.18

7.5

bit
times

SetAddress() Completion TimeTDSETADDR

Section 9.2.6.3

50

ms

Time to complete standard
request with no data

TDRQCMPLTND

Section 9.2.6.4

50

ms

Inter-packet Delay

Conditions

Min

Max

Units

ms

15

ms

ms

10000

s

10

ms

2

bit
times

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Universal Serial Bus Specification Revision 1.1

Table 7-11. Device Event Timings (Continued)
Parameter

SymbolTime to deliver first andsubsequent (except last) data forstandard request

TDRETDATA1Time to deliver last data forstandard request

TDRETDATAN

150

Conditions

Min

Max

Units

Section 9.2.6.4

500

ms

Section 9.2.6.4

50msUniversal Serial Bus Specification Revision 1.1

7.3.3 Timing Waveforms

TPERIOD
Differential
Data Lines

Crossover
PointsConsecutive
Transitions
N * TPERIOD + TxDJ1PairedTransitions
N * TPERIOD + TxDJ2Figure 7-39. Differential Data Jitter

TPERIOD
Differential
Data Lines

Crossover Point
Extended

Crossover
Point

Diff. Data-toSE0 Skew
N * TPERIOD + TxDEOP

Source EOP Width:

TFEOPT
TLEOPT

Receiver EOP Width: TFEOPR,
TLEOPR

Figure 7-40. Differential-to-EOP Transition Skew and EOP Width

TPERIOD
Differential
Data Lines
TxJR

TxJR1

TxJR2

Consecutive
Transitions
N * TPERIOD + TxJR1
PairedTransitions
N * TPERIOD + TxJR2

Figure 7-41. Receiver Jitter Tolerance

TPERIOD is the data rate of the receiver that can have the range as defined in Section 7.1.11

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Universal Serial Bus Specification Revision 1.1

Upstream
End of
Cable

Upstream
Port of hub

Crossover
Point

50% Point of
Initial Swing
VSS

VSS
Downstream
Port of hub

Hub Delay
Downstream
THDD1

Hub Delay
Downstream
THDD2

Downstream
Port of hub

50% Point of
Initial Swing

VSS

VSS

A. Downstream Hub Delay with Cable

Downstream
Port of hub

B. Downstream Hub Delay without Cable

Crossover
Point

VSS
Upstream
Port or End
of Cable

Hub Delay
Upstream
THDD1
THDD2

Crossover
Point

VSS

C. Upstream Hub Delay with or without Cable
Hub Differential Jitter:
T HDJ1 = T HDDx(J) - T HDDx(K) or T HDDx(K) - THDDx(J) Consecutive Transitions
T HDJ2 = T HDDx(J) - T HDDx(J) or T HDDx(K) - T HDDx(K) Paired Transitions
Bit after SOP Width Distortion (same as data jitter for SOP and next J transition):
T FSOP = T HDDx(next J) - T HDDx(SOP)
Low-speed timings are determined in the same way for:T LHDD, T LDHJ1, T LDJH2, T LUHJ1, T LUJH2, and T LSOP

Figure 7-42. Hub Differential Delay, Differential Jitter, and SOP Distortion
Note: Measurement locations referenced in Figure 7-42 and Figure 7-43 are specified in Figure 7-28

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Universal Serial Bus Specification Revision 1.1

Upstream
End ofCable
VSS

50% Point of
Initial Swing

Crossover
Point
Extended

Upstream
Port of hub
TEOP-

VSS

TEOP+

TEOP-

Downstream
Port of hub

Downstream
Port of hub

VSS

VSS

A. Downstream EOP Delay with Cable

TEOP

B. Downstream EOP Delay without Cable

Crossover
Point
ExtendedDownstreamPortVSS
TEOPUpstream
Port orEnd of Cable

TEOP+

Crossover
Point
Extended

VSS

C. Upstream EOP Delay with or Without Cable
EOP Delay:
TFEOPD = TEOPy - THDDx(TEOPy means that this equation applies to TEOP- and TEOP+)EOP Skew:
TFHESK = TEOP+ - TEOPLow-speed timings are determined in the same way for:
TLEOPD and TLHESK

Figure 7-43. Hub EOP Delay and EOP Skew

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154Universal Serial Bus Specification Revision 1.1

Chapter 8
Protocol Layer
This chapter presents a bottom-up view of the USB protocol, starting with field and packet definitions.
This is followed by a description of packet transaction formats for different transaction types. Link layer
flow control and transaction level fault recovery are then covered. The chapter finishes with a discussionof retry synchronization, babble, and loss of bus activity recovery.

8.1 Bit Ordering
Bits are sent out onto the bus least-significant bit (LSb) first, followed by the next LSb, through to themost-significant bit (MSb) last. In the following diagrams, packets are displayed such that both individual
bits and fields are represented (in a left to right reading order) as they would move across the bus.

8.2 SYNC Field
All packets begin with a synchronization (SYNC) field, which is a coded sequence that generates a
maximum edge transition density. The SYNC field appears on the bus as IDLE followed by the binary
string “KJKJKJKK,” in its NRZI encoding. It is used by the input circuitry to align incoming data with thelocal clock and is defined to be eight bits in length. SYNC serves only as a synchronization mechanism
and is not shown in the following packet diagrams (refer to Section 7.1.10). The last two bits in the SYNC
field are a marker that is used to identify the end of the SYNC field and, by inference, the start of the PID.

8.3 Packet Field Formats
Field formats for the token, data, and handshake packets are described in the following section. Packet bit
definitions are displayed in unencoded data format. The effects of NRZI coding and bit stuffing have been
removed for the sake of clarity. All packets have distinct Start- and End-of-Packet delimiters. The Startof-Packet (SOP) delimeter is part of the SYNC field, and the End-of-Packet (EOP) delimiter is described in
Chapter 7.

8.3.1 Packet Identifier Field
A packet identifier (PID) immediately follows the SYNC field of every USB packet. A PID consists of afour-bit packet type field followed by a four-bit check field as shown in Figure 8-1. The PID indicates thetype of packet and, by inference, the format of the packet and the type of error detection applied to thepacket. The four-bit check field of the PID ensures reliable decoding of the PID so that the remainder ofthe packet is interpreted correctly. The PID check field is generated by performing a one’s complement of
the packet type field. A PID error exists if the four PID check bits are not complements of their respective
packet identifier bits.
(LSb)
PID

(MSb)
0

PID 1

PID 2

PID 3

PID 0

PID 1

PID

2

PID 3

Figure 8-1. PID Format

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Universal Serial Bus Specification Revision 1.1

The host and all functions must perform a complete decoding of all received PID fields. Any PID received
with a failed check field or which decodes to a non-defined value is assumed to be corrupted and it, as wellas the remainder of the packet, is ignored by the packet receiver. If a function receives an otherwise valid
PID for a transaction type or direction that it does not support, the function must not respond. For
example, an IN-only endpoint must ignore an OUT token. PID types, codings, and descriptions are listed
in Table 8-1.
Table 8-1. PID Types
PID Type

PID Name

Token

OUT

0001B

Address + endpoint number in host-to-function
transaction

IN

1001B

Address + endpoint number in function-to-hosttransaction

SOF

0101B

Start-of-Frame marker and frame number

SETUP

1101B

Address + endpoint number in host-to-function
transaction for SETUP to a control pipe

DATA0

0011B

Data packet PID even

DATA1

1011B

Data packet PID odd

ACK

0010B

Receiver accepts error-free data packet

NAK

1010B

Rx device cannot accept data or Tx device cannot send
data

STALL

1110B

Endpoint is halted or a control pipe request is notsupported.

PRE

1100B

Host-issued preamble. Enables downstream bus traffic
to low-speed devices.

Data

Handshake

SpecialPID[3:0]*

Description*Note: PID bits are shown in MSb order. When sent on the USB, the rightmost bit (bit 0) will be sent first.

PIDs are divided into four coding groups: token, data, handshake, and special, with the first twotransmitted PID bits (PID<0:1>) indicating which group. This accounts for the distribution of PID codes.

8.3.2 Address Fields
Function endpoints are addressed using two fields: the function address field and the endpoint field. A
function needs to fully decode both address and endpoint fields. Address or endpoint aliasing is not
permitted, and a mismatch on either field must cause the token to be ignored. Accesses to non-initialized
endpoints will also cause the token to be ignored.

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Universal Serial Bus Specification Revision 1.1

8.3.2.1 Address Field
The function address (ADDR) field specifies the function, via its address, that is either the source ordestination of a data packet, depending on the value of the token PID. As shown in Figure 8-2, a total of
128 addresses are specified as ADDR<6:0>. The ADDR field is specified for IN, SETUP, and OUT
tokens. By definition, each ADDR value defines a single function. Upon reset and power-up, a function’s
address defaults to a value of zero and must be programmed by the host during the enumeration process.
Function address zero is reserved as the default address and may not be assigned to any other use.
(MSb)

(LSb)
Addr 0Addr 1 Addr 2

Addr 3

Addr 4Addr 5Addr 6

Figure 8-2. ADDR Field

8.3.2.2 Endpoint Field
An additional four-bit endpoint (ENDP) field, shown in Figure 8-3 permits more flexible addressing offunctions in which more than one endpoint is required. Except for endpoint address zero, endpoint
numbers are function-specific. The endpoint field is defined for IN, SETUP, and OUT token PIDs only.
All functions must support a control pipe at endpoint number zero (the Default Control Pipe). Low-speed
devices support a maximum of three pipes per function: a control pipe at endpoint number zero plus two
additional pipes (either two control pipes, a control pipe and a interrupt endpoint, or two interrupt
endpoints). Full-speed functions may support up to the maximum of 16 endpoint numbers of any type.
(MSb)

(LSb)
Endp

0

Endp

1

Endp

2

Endp

3

Figure 8-3. Endpoint Field

8.3.3 Frame Number Field
The frame number field is an 11-bit field that is incremented by the host on a per-frame basis. The frame
number field rolls over upon reaching its maximum value of 7FFH, and is sent only in SOF tokens at thestart of each frame.

8.3.4 Data Field
The data field may range from zero to 1,023 bytes and must be an integral number of bytes. Figure 8-4
shows the format for multiple bytes. Data bits within each byte are shifted out LSb first.
(MSb)

(LSb)

D7

D0

Byte N-1

D1

D2

D3

D4

D5

D6

Byte N

(MSb)

(LSb)

D7

D0
Byte N+1

Figure 8-4. Data Field Format
Data packet size varies with the transfer type, as described in Chapter 5.

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Universal Serial Bus Specification Revision 1.1

8.3.5 Cyclic Redundancy Checks
Cyclic redundancy checks (CRCs) are used to protect all non-PID fields in token and data packets. In thiscontext, these fields are considered to be protected fields. The PID is not included in the CRC check of apacket containing a CRC. All CRCs are generated over their respective fields in the transmitter before bit
stuffing is performed. Similarly, CRCs are decoded in the receiver after stuffed bits have been removed.
Token and data packet CRCs provide 100% coverage for all single- and double-bit errors. A failed CRC isconsidered to indicate that one or more of the protected fields is corrupted and causes the receiver to ignore
those fields, and, in most cases, the entire packet.
For CRC generation and checking, the shift registers in the generator and checker are seeded with an allones pattern. For each data bit sent or received, the high order bit of the current remainder is XORed withthe data bit and then the remainder is shifted left one bit and the low-order bit set to zero. If the result ofthat XOR is one, then the remainder is XORed with the generator polynomial.
When the last bit of the checked field is sent, the CRC in the generator is inverted and sent to the checker
MSb first. When the last bit of the CRC is received by the checker and no errors have occurred, theremainder will be equal to the polynomial residual.
A CRC error exists if the computed checksum remainder at the end of a packet reception does not matchthe residual.
Bit stuffing requirements must be met for the CRC, and this includes the need to insert a zero at the end of
a CRC if the preceding six bits were all ones.

8.3.5.1 Token CRCs
A five-bit CRC field is provided for tokens and covers the ADDR and ENDP fields of IN, SETUP, and
OUT tokens or the time stamp field of an SOF token. The generator polynomial is:
5

2

G(X) = X + X + 1
The binary bit pattern that represents this polynomial is 00101B. If all token bits are received without
error, the five-bit residual at the receiver will be 01100B.

8.3.5.2 Data CRCs
The data CRC is a 16-bit polynomial applied over the data field of a data packet. The generating
polynomial is:
16

15

2

G(X) = X + X + X + 1
The binary bit pattern that represents this polynomial is 1000000000000101B. If all data and CRC bits arereceived without error, the 16-bit residual will be 1000000000001101B.

8.4 Packet Formats
This section shows packet formats for token, data, and handshake packets. Fields within a packet are
displayed in these figures in the order in which bits are shifted out onto the bus.

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Universal Serial Bus Specification Revision 1.1

8.4.1 Token Packets
Figure 8-5 shows the field formats for a token packet. A token consists of a PID, specifying either IN,
OUT, or SETUP packet type; and ADDR and ENDP fields. For OUT and SETUP transactions, the addressand endpoint fields uniquely identify the endpoint that will receive the subsequent Data packet. For IN
transactions, these fields uniquely identify which endpoint should transmit a Data packet. Only the hostcan issue token packets. IN PIDs define a Data transaction from a function to the host. OUT and SETUP
PIDs define Data transactions from the host to a function.
8 bits7 bits4 bits5 bits

PID

ADDR

ENDP

CRC5

Figure 8-5. Token Format
Token packets have a five-bit CRC that covers the address and endpoint fields as shown above. The CRC
does not cover the PID, which has its own check field. Token and SOF packets are delimited by an EOP
after three bytes of packet field data. If a packet decodes as an otherwise valid token or SOF but does not
terminate with an EOP after three bytes, it must be considered invalid and ignored by the receiver.

8.4.2 Start-of-Frame Packets
Start-of-Frame (SOF) packets are issued by the host at a nominal rate of once every 1.00ms 0.0005ms.
SOF packets consist of a PID indicating packet type followed by an 11-bit frame number field as illustrated
in Figure 8-6.
8 bitsPID

11 bits
Frame Number

5 bits
CRC5

Figure 8-6. SOF Packet
The SOF token comprises the token-only transaction that distributes an SOF marker and accompanying
frame number at precisely timed intervals corresponding to the start of each frame. All full-speedfunctions, including hubs, receive the SOF packet. The SOF token does not cause any receiving function
to generate a return packet; therefore, SOF delivery to any given function cannot be guaranteed. The SOF
packet delivers two pieces of timing information. A function is informed that an SOF has occurred when it
detects the SOF PID. Frame timing sensitive functions, which do not need to keep track of frame number(e.g., a hub), need only decode the SOF PID; they can ignore the frame number and its CRC. If a functionneeds to track frame number, it must comprehend both the PID and the time stamp. Full-speed devices thathave no particular need for bus timing information may ignore the SOF packet.

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Universal Serial Bus Specification Revision 1.1

8.4.3 Data Packets
A data packet consists of a PID, a data field containing zero or more bytes of data, and a CRC as shown in
Figure 8-7. There are two types of data packets, identified by differing PIDs: DATA0 and DATA1. Twodata packet PIDs are defined to support data toggle synchronization (refer to Section 8.6).
8 bitsPID

0-1023 bytes

16 bits

DATA

CRC16

Figure 8-7. Data Packet Format
Data must always be sent in integral numbers of bytes. The data CRC is computed over only the data fieldin the packet and does not include the PID, which has its own check field.

8.4.4 Handshake Packets
Handshake packets, as shown in Figure 8-8, consist of only a PID. Handshake packets are used to report
the status of a data transaction and can return values indicating successful reception of data, command
acceptance or rejection, flow control, and halt conditions. Only transaction types that support flow control
can return handshakes. Handshakes are always returned in the handshake phase of a transaction and may
be returned, instead of data, in the data phase. Handshake packets are delimited by an EOP after one byteof packet field. If a packet decodes as an otherwise valid handshake but does not terminate with an EOP
after one byte, it must be considered invalid and ignored by the receiver.
8 bitsPID

Figure 8-8. Handshake Packet
There are three types of handshake packets:

ACK indicates that the data packet was received without bit stuff or CRC errors over the data field and thatthe data PID was received correctly. ACK may be issued either when sequence bits match and the receiver
can accept data or when sequence bits mismatch and the sender and receiver must resynchronize to eachother (refer to Section 8.6 for details). An ACK handshake is applicable only in transactions in which data
has been transmitted and where a handshake is expected. ACK can be returned by the host for IN
transactions and by a function for OUT or SETUP transactions.



NAK indicates that a function was unable to accept data from the host (OUT) or that a function has no datato transmit to the host (IN). NAK can only be returned by functions in the data phase of IN transactions or
the handshake phase of OUT transactions. The host can never issue NAK. NAK is used for flow control
purposes to indicate that a function is temporarily unable to transmit or receive data, but will eventually beable to do so without need of host intervention.



STALL is returned by a function in response to an IN token or after the data phase of an OUT transaction(see Figure 8-9 and Figure 8-13). STALL indicates that a function is unable to transmit or receive data, orthat a control pipe request is not supported. The host is not permitted to return a STALL under any
condition.

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Universal Serial Bus Specification Revision 1.1

The STALL handshake is used by a device in one of two distinct occasions. The first case, known as“functional stall,” is when the Halt feature associated the endpoint is set. (The Halt feature is specified in
Chapter 9 of this document.) A special case of the functional stall is the “commanded stall.” Commanded
stall occurs when the host explicitly sets the endpoint’s Halt feature, as detailed in Chapter 9. Once afunction’s endpoint is halted, the function must continue returning STALL until the condition causing thehalt has been cleared through host intervention.
The second case, known as “protocol stall,” is detailed in Section 8.5.2. Protocol stall is unique to control
pipes. Protocol stall differs from functional stall in meaning and duration. A protocol STALL is returned
during the Data or Status stage of a control transfer, and the STALL condition terminates at the beginningof the next control transfer (Setup). The remainder of this section refers to the general case of a functional
stall.

8.4.5 Handshake Responses
Transmitting and receiving functions must return handshakes based upon an order of precedence detailedin Table 8-2 through Table 8-4. Not all handshakes are allowed, depending on the transaction type andwhether the handshake is being issued by a function or the host. Note that if an error occurs during thetransmission of the token to the function, the function will not respond with any packets until the next
token is received and successfully decoded.

8.4.5.1 Function Response to IN Transactions
Table 8-2 shows the possible responses a function may make in response to an IN token. If the function isunable to send data, due to a halt or a flow control condition, it issues a STALL or NAK handshake,
respectively. If the function is able to issue data, it does so. If the received token is corrupted, the function
returns no response.
Table 8-2. Function Responses to IN Transactions
Token Received
Corrupted

Function Tx
Endpoint Halt
FeatureFunction Can
Transmit Data

Action Taken

YesDon’t care

Don’t care

Return no response

No

Set

Don’t care

Issue STALL handshakeNo

Not setNo

Issue NAK handshakeNo

Not setYes

Issue data packet

8.4.5.2 Host Response to IN Transactions
Table 8-3 shows the host response to an IN transaction. The host is able to return only one type ofhandshake: ACK. If the host receives a corrupted data packet, it discards the data and issues no response.
If the host cannot accept data from a function, (due to problems such as internal buffer overrun) thiscondition is considered to be an error and the host returns no response. If the host is able to accept data
and the data packet is received error-free, the host accepts the data and issues an ACK handshake.

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Table 8-3. Host Responses to IN Transactions
Data Packet
Corrupted

Host Can
Accept Data

Handshake Returned by Host

Yes

N/A

Discard data, return no response

No

No

Discard data, return no response

No

Yes

Accept data, issue ACK

8.4.5.3 Function Response to an OUT Transaction
Handshake responses for an OUT transaction are shown in Table 8-4. Assuming successful token decode,a function, upon receiving a data packet, may return any one of the three handshake types. If the data
packet was corrupted, the function returns no handshake. If the data packet was received error-free and thefunction’s receiving endpoint is halted, the function returns STALL. If the transaction is maintaining
sequence bit synchronization and a mismatch is detected (refer to Section 8.6 for details), then the function
returns ACK and discards the data. If the function can accept the data and has received the data error-free,
it returns ACK. If the function cannot accept the data packet due to flow control reasons, it returns NAK.
Table 8-4. Function Responses to OUT Transactions in Order of Precedence
Data Packet
Corrupted

ReceiverHalt
FeatureSequence Bits
Match

Function Can
Accept Data

Handshake Returned
by Function

Yes

N/A

N/A

N/A

None

No

Set

N/A

N/A

STALL

No

Not set

No

N/A

ACK

No

Not set

Yes

Yes

ACK

No

Not set

Yes

No

NAK

8.4.5.4 Function Response to a SETUP Transaction
SETUP defines a special type of host-to-function data transaction that permits the host to initialize an
endpoint’s synchronization bits to those of the host. Upon receiving a SETUP token, a function must
accept the data. A function may not respond to a SETUP token with either STALL or NAK and thereceiving function must accept the data packet that follows the SETUP token. If a non-control endpoint
receives a SETUP token, it must ignore the transaction and return no response.

8.5 Transaction Formats
Packet transaction format varies depending on the endpoint type. There are four endpoint types: bulk,
control, interrupt, and isochronous.

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8.5.1 Bulk Transactions
Bulk transaction types are characterized by the ability to guarantee error-free delivery of data between thehost and a function by means of error detection and retry. Bulk transactions use a three-phase transaction
consisting of token, data, and handshake packets as shown in Figure 8-9. Under certain flow control andhalt conditions, the data phase may be replaced with a handshake resulting in a two-phase transaction inwhich no data is transmitted.
Idle

Token

Data

IN

DATA0/
DATA1

OUT

NAK

DATA0/
DATA1

STALL
Idle

Handshake
ACK

Data
Error

ACK

NAK

STALL

Data
Error
Idle

Host

Function

Figure 8-9. Bulk Transaction Format
When the host is ready to receive bulk data, it issues an IN token. The function endpoint responds by
returning either a data packet or, should it be unable to return data, a NAK or STALL handshake. NAK
indicates that the function is temporarily unable to return data, while STALL indicates that the endpoint ispermanently halted and requires USB System Software intervention. If the host receives a valid data
packet, it responds with an ACK handshake. If the host detects an error while receiving data, it returns nohandshake packet to the function.
When the host is ready to transmit bulk data, it first issues an OUT token packet followed by a data packet.
If the data is received without error by the function it will return one of three handshakes:



ACK indicates that the data packet was received without errors and informs the host that that it may
send the next packet in the sequence.



NAK indicates that the data was received without error but that the host should resend the data because
the function was in a temporary condition preventing it from accepting the data (e.g., buffer full).



If the endpoint was halted, STALL is returned to indicate that the host should not retry thetransmission because there is an error condition on the function.

If the data packet was received with a CRC or bit stuff error, no handshake is returned.

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Figure 8-10 shows the sequence bit and data PID usage for bulk reads and writes. Data packet
synchronization is achieved via use of the data sequence toggle bits and the DATA0/DATA1 PIDs. A bulk
endpoint’s toggle sequence is initialized to DATA0 when the endpoint experiences any configuration event(configuration events are explained in Sections 9.1.1.5 and 9.4.5). Data toggle on an endpoint is NOT
initialized as the direct result of a short packet transfer or the retirement of an IRP.
Bulk
Write
Bulk
Read

OUT (0)OUT (1)

DATA0

DATA1

IN (0)IN (1)DATA0...

OUT (0/1)
DATA0/1...

IN (0/1)

DATA1

DATA0/1

Figure 8-10. Bulk Reads and Writes
The host always initializes the first transaction of a bus transfer to the DATA0 PID with a configuration
event. The second transaction uses a DATA1 PID, and successive data transfers alternate for the remainderof the bulk transfer. The data packet transmitter toggles upon receipt of ACK, and the receiver toggles
upon receipt and acceptance of a valid data packet (refer to Section 8.6).

8.5.2 Control Transfers
Control transfers minimally have two transaction stages: Setup and Status. A control transfer may
optionally contain a Data stage between the Setup and Status stages. During the Setup stage, a SETUP
transaction is used to transmit information to the control endpoint of a function. SETUP transactions are
similar in format to an OUT, but use a SETUP rather than an OUT PID. Figure 8-11 shows the SETUP
transaction format. A SETUP always uses a DATA0 PID for the data field of the SETUP transaction. The
function receiving a SETUP must accept the SETUP data and respond with ACK, if the data is corrupted,
discard the data and return no handshake.
Idle

Token

SETUP

Data

DATA0

Handshake

ACK

Idle

Host

Function

Figure 8-11. Control SETUP Transaction
The Data stage, if present, of a control transfer consists of one or more IN or OUT transactions and follows
the same protocol rules as bulk transfers. All the transactions in the Data stage must be in the same
direction (i.e., all INs or all OUTs). The amount of data to be sent during the data phase and its direction
are specified during the Setup stage. If the amount of data exceeds the prenegotiated data packet size, the164Universal Serial Bus Specification Revision 1.1

data is sent in multiple transactions (INs or OUTs) that carry the maximum packet size. Any remaining
data is sent as a residual in the last transaction.
The Status stage of a control transfer is the last operation in the sequence. A Status stage is delineated by achange in direction of data flow from the previous stage and always uses a DATA1 PID. If, for example,
the Data stage consists of OUTs, the status is a single IN transaction. If the control sequence has no Data
stage, then it consists of a Setup stage followed by a Status stage consisting of an IN transaction.
Figure 8-12 shows the transaction order, the data sequence bit value, and the data PID types for control
read and write sequences. The sequence bits are displayed in parentheses.
Setup
Stage

Control
Write
Control
Read

No-data
Control

Data
Stage

SETUP (0)OUT (1)

OUT (0)

DATA0

DATA1

DATA0

SETUP (0)IN (1)

DATA0

DATA1

Setup
Stage

StatusStage

SETUP (0)IN (1)

DATA0

DATA1

IN (0)
DATA0

Status
Stage

...
...

OUT (0/1)

IN (1)

DATA0/1

DATA1

IN (0/1)

OUT (1)

DATA0/1

DATA1

Figure 8-12. Control Read and Write Sequences
When a STALL handshake is sent by a control endpoint in either the Data or Status stages of a control
transfer, a STALL handshake must be returned on all succeeding accesses to that endpoint until a SETUPPID is received. The endpoint is not required to return a STALL handshake after it receives a subsequent
SETUP PID.

8.5.2.1 Reporting Status Results
The Status stage reports to the host the outcome of the previous Setup and Data stages of the transfer.
Three possible results may be returned:The command sequence completed successfully.

The command sequence failed to complete.

The function is still busy completing command.

Status reporting is always in the function-to-host direction. The Table 8-5 summarizes the type of
responses required for each. Control write transfers return status information in the data phase of the Status
stage transaction. Control read transfers return status information in the handshake phase of a Status stagetransaction, after the host has issued a zero-length data packet during the previous data phase.

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Table 8-5. Status Stage Responses
Status Response

Control Write Transfer

Control Read Transfer(sent during data phase)(send during handshake phase)

Function completesZero-length data packet

ACK handshake

Function has an error

STALL handshake

STALL handshake

Function is busy

NAK handshake

NAK handshake

For control reads, the host sends an OUT token to the control pipe to initiate the Status stage. The host
may only send a zero-length data packet in this phase but the function may accept any length packet as avalid status inquiry. The pipe’s handshake response to this data packet indicates the current status. NAK
indicates that the function is still processing the command and that the host should continue the Status
stage. ACK indicates that the function has completed the command and is ready to accept a newcommand. STALL indicates that the function has an error that prevents it from completing the command.
For control writes, the host sends an IN token to the control pipe to initiate the Status stage. The function
responds with either a handshake or a zero-length data packet to indicate its current status. NAK indicatesthat the function is still processing the command and that the host should continue the Status stage; returnof a zero-length packet indicates normal completion of the command; and STALL indicates that thefunction cannot complete the command. The function expects the host to respond to the data packet in the
Status stage with ACK. If the function does not receive ACK, it remains in the Status stage of thecommand and will continue to return the zero-length data packet for as long as the host continues to send
IN tokens.
If during a Data stage a command pipe is sent more data or is requested to return more data than was
indicated in the Setup stage (see Section 8.5.2.2), it should return STALL. If a control pipe returns STALL
during the Data stage, there will be no Status stage for that control transfer.

8.5.2.2 Variable-length Data StageA control pipe may have a variable-length data phase in which the host request more data than is containedin the specified data structure. When all of the data structure is returned to the host, the function should
indicate that the Data stage is ended by returning a packet that is shorter than the MaxPacketSize for thepipe. If the data structure is an exact multiple of wMaxPacketSize for the pipe, the funtion will return azero-length packet to indicate the end of the Data stage.

8.5.2.3 Error Handling on the Last Data Transaction
If the ACK handshake on an IN transaction is corrupted, the function and the host will temporarily disagree
on whether the transaction was successful. If the transaction is followed by another IN, the toggle retry
mechanism will detect the mismatch and recover from the error. If the ACK was on the last IN of a Data
stage, the toggle retry mechanism cannot be used and an alternative scheme must be used.
The host that successfully received the data of the last IN will send ACK., Later, the host will issue an
OUT token to start the Status stage of the transfer. If the function did not receive the ACK that ended theData stage, the function will interpret the start of the Status stage as verification that the host successfullyreceived the data. Control writes do not have this ambiguity. If an ACK handshake on an OUT getscorrupted, the host does not advance to the Status stage and retries the last data instead. A detailed analysisof retry policy is presented in Section 8.6.4.

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8.5.2.4 STALL Handshakes Returned by Control Pipes
Control pipes have the unique ability to return a STALL handshake due to function problems in control
transfers. If the device is unable to complete a command, it returns a STALL in the Data and/or Status
stages of the control transfer. Unlike the case of a functional stall, protocol stall does not indicate an error
with the device. The protocol stall condition lasts until the receipt of the next SETUP transaction and thefunction will return STALL in response to any IN or OUT transaction on the pipe until the SETUP
transaction is received. In general, protocol stall indicates that the request or its parameters is notunderstood by the device and thus provides a mechanism for extending USB requests.
A control pipe may also support functional stall as well, but this is not recommended. This is adegenerative case, because a functional stall on a control pipe indicates that it has lost the ability tocommunicate with the host. If the control pipe does support functional stall, then it must possess a Haltfeature, which can be set or cleared by the host. Chapter 9 details how to treat the special case of a Haltfeature on a control pipe. A well-designed device will associate all of its functions and Halt features with
non-control endpoints. The control pipes should be reserved for servicing USB requests.

8.5.3 Interrupt Transactions
Interrupt transactions may consist of IN or OUT transfers. Upon receipt of an IN token, a function may
return data, NAK, or STALL. If the endpoint has no new interrupt information to return (i.e., no interrupt
is pending), the function returns a NAK handshake during the data phase. If the Halt feature is set for theinterrupt endpoint, the function will return a STALL handshake. If an interrupt is pending, the function
returns the interrupt information as a data packet. The host, in response to receipt of the data packet, issues
either an ACK handshake if data was received error-free or returns no handshake if the data packet wasreceived corrupted. Figure 8-13 shows the interrupt transaction format.
Idle

Token

Data

IN

DATA0/
DATA1

OUT

NAK

DATA0/
DATA1

STALL
Idle

Handshake
ACK

Data
Error

ACK

NAK

STALL

Data
Error
Idle

Host

Function

Figure 8-13. Interrupt Transaction Format
When an endpoint is using the interrupt transfer mechanism for actual interrupt data, the data toggle
protocol must be followed. This allows the function to know that the data has been received by the hostand the event condition may be cleared. This “guaranteed” delivery of events allows the function to only
send the interrupt information until it has been received by the host rather than having to send the interrupt
data every time the function is polled and until the USB System Software clears the interrupt condition.

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When used in the toggle mode, an interrupt endpoint is initialized to the DATA0 PID by any configuration
event on the endpoint and behaves the same as the bulk transactions shown in Figure 8-10.
An interrupt endpoint may also be used to communicate rate feedback information for certain types ofisochronous functions. When used in this mode, the data toggle bits should be changed after each data
packet is sent to the host without regard to the presence or type of handshake packet. This capability is
supported only for interrupt IN endpoints.

8.5.4 Isochronous Transactions
Isochronous (ISO) transactions have a token and data phase, but no handshake phase, as shown in Figure8-14. The host issues either an IN or an OUT token followed by the data phase in which the endpoint (forINs) or the host (for OUTs) transmits data. ISO transactions do not support a handshake phase or retry
capability.
Idle

IN

OUT

DATA0/

DATA0

Token

Data

Idle
Host

Function

See Note Below

Figure 8-14. Isochronous Transaction Format
Note: a device or Host Controller should be able to accept either DATA0 or DATA1. A device or Host
Controller should only send DATA0.
ISO transactions do not support toggle sequencing.

8.6 Data Toggle Synchronization and Retry
The USB provides a mechanism to guarantee data sequence synchronization between data transmitter andreceiver across multiple transactions. This mechanism provides a means of guaranteeing that thehandshake phase of a transaction was interpreted correctly by both the transmitter and receiver.
Synchronization is achieved via use of the DATA0 and DATA1 PIDs and separate data toggle sequence
bits for the data transmitter and receiver. Receiver sequence bits toggle only when the receiver is able toaccept data and receives an error-free data packet with the correct data PID. Transmitter sequence bits
toggle only when the data transmitter receives a valid ACK handshake. The data transmitter and receivermust have their sequence bits synchronized at the start of a transaction. The synchronization mechanism
used varies with the transaction type. Data toggle synchronization is not supported for ISO transfers.

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8.6.1 Initialization via SETUP Token
Control transfers use the SETUP token for initializing host and function sequence bits. Figure 8-15 shows
the host issuing a SETUP packet to a function followed by an OUT transaction. The numbers in the circles
represent the transmitter and receiver sequence bits. The function must accept the data and return ACK.
When the function accepts the transaction, it must set its sequence bit so that both the host’s and function’ssequence bits are equal to one at the end of the SETUP transaction.
Host

Device

SETUP

Rx
(X)

Tx
(X-1)
DATA0

Tx
(1)

Acceptdata

Rx(X->1)

ACK

Rx
(1)

Tx
(1)

Figure 8-15. SETUP Initialization

8.6.2 Successful Data Transactions
Figure 8-16 shows the case where two successful transactions have occurred. For the data transmitter, thismeans that it toggles its sequence bit upon receipt of ACK. The receiver toggles its sequence bit only if it
receives a valid data packet and the packet’s data PID matches the current value of its sequence bit. The
transmitter only toggles its sequence bit after it receives and ACK to a data packet.
During each transaction, the receiver compares the transmitter sequence bit (encoded in the data packet
PID as either DATA0 or DATA1) with its receiver sequence bit. If data cannot be accepted, the receivermust issue NAK and the sequence bits of both the transmitter and receiver remain unchanged. If data canbe accepted and the receiver’s sequence bit matches the PID sequence bit, then data is accepted and thesequence bit is toggled. Two-phase transactions in which there is no data packet leave the transmitter andreceiver sequence bits unchanged.
DATA1

DATA0

Tx
(0)

Acceptdata

Rx(0->1)

Tx
(1)

Rx
(1)

Transfer iRx(1->0)

ACK

ACK

Tx
(0->1)

Accept
data

Rx
(0)

Tx
(1->0)

Transfer i + 1

Figure 8-16. Consecutive Transactions

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8.6.3 Data Corrupted or Not Accepted
If data cannot be accepted or the received data packet is corrupted, the receiver will issue a NAK or
STALL handshake, or timeout, depending on the circumstances, and the receiver will not toggle its
sequence bit. Figure 8-17 shows the case where a transaction is NAKed and then retried. Any non-ACK
handshake or timeout will generate similar retry behavior. The transmitter, having not received an ACK
handshake, will not toggle its sequence bit. As a result, a failed data packet transaction leaves thetransmitter’s and receiver’s sequence bits synchronized and untoggled. The transaction will then be retried
and, if successful, will cause both transmitter and receiver sequence bits to toggle.
DATA0

DATA0

Reject
data

Tx
(0)

Accept
data

Tx
(0)

Rx
(0->0)

Rx(0->1)

ACK

NAK

Rx
(0)

Tx
(0->0)

Rx(1)

Tx(0->1)

Transfer i

Retry
Transfer i

Figure 8-17. NAKed Transaction with Retry

8.6.4 Corrupted ACK Handshake
The transmitter is the last and only agent to know for sure whether a transaction has been successful, due to
its receiving an ACK handshake. A lost or corrupted ACK handshake can lead to a temporary loss ofsynchronization between transmitter and receiver as shown in Figure 8-18. Here the transmitter issues avalid data packet, which is successfully acquired by the receiver; however, the ACK handshake iscorrupted.
DATA0

Tx
(0)

DATA0

Acceptdata

Rx(0->1)

Tx(0)

Failed ACK

Transfer i

Rx
(1)

Tx
(1)

ACK

Rx
(1)

Tx
(0->0)

Ignore
data

DATA1

ACK

Rx
(1)

Tx
(0->1)

Transfer i(retried)Rx(1->0)

Tx(1->0)

Rx(0)

Transfer i + 1

Figure 8-18. Corrupted ACK Handshake with RetryAt the end of transaction i, there is a temporary loss of coherency between transmitter and receiver, as
evidenced by the mismatch between their respective sequence bits. The receiver has received good data,
but the transmitter does not know whether it has successfully sent data. On the next transaction, thetransmitter will resend the previous data using the previous DATA0 PID. The receiver’s sequence bit andthe data PID will not match, so the receiver knows that it has previously accepted this data. Consequently,
it discards the incoming data packet and does not toggle its sequence bit. The receiver then issues ACK,
which causes the transmitter to regard the retried transaction as successful. Receipt of ACK causes the170Universal Serial Bus Specification Revision 1.1

transmitter to toggle its sequence bit. At the beginning of transaction i+1, the sequence bits have toggled
and are again synchronized.
The data transmitter must guarantee that any retried data packet is identical (same length and content) as
that sent in the original transaction. If the data transmitter is unable, because of problems such as a buffer
underrun condition, to transmit the identical amount of data as was in the original data packet, it must abort
the transaction by generating a bit stuffing violation. This causes a detectable error at the receiver andguarantees that a partial packet will not be interpreted as a good packet. The transmitter should not try toforce an error at the receiver by sending a known bad CRC. A combination of a bad packet with a “bad”
CRC may be interpreted by the receiver as a good packet.

8.6.5 Low-speed Transactions
The USB supports signaling at two speeds: full-speed signaling at 12.0Mb/s and low-speed signaling at
1.5Mb/s. Hubs disable downstream bus traffic to all ports to which low-speed devices are attached during
full-speed downstream signaling. This is required both for EMI reasons and to prevent any possibility thata low-speed device might misinterpret downstream a full-speed packet as being addressed to it.
Figure 8-19 shows an IN low-speed transaction in which the host issues a token and handshake andreceives a data packet.
Hub disables lowspeed port outputs

Hub enables lowspeed port outputs
Preamblesent at full-speed

SYNC

PID

Token sent at low-speed

Hub setup

SYNC

PID

ENDP

...

EOP

Data packet sent at low-speed

SYNC

PID

Preamblesent at full-speed

SYNC

PID

DATA

CRC

EOP

Hub disables lowspeed port outputs

Hub enables lowspeed port outputs
Handshake sent at low-speed

Hub setup

SYNC

PID

EOP

Figure 8-19. Low-speed Transaction
All downstream packets transmitted to low-speed devices require a preamble. The preamble consists of a
SYNC followed by a PRE PID, both sent at full-speed. Hubs must comprehend the PRE PID; all otherUSB devices may ignore it and treat it as undefined. After the end of the preamble PID, the host must wait
at least four full-speed bit times during which hubs must complete the process of enabling the repeater
function on ports that are connected to low-speed devices. During this hub setup interval, hubs must drive
their full-speed and low-speed ports to their respective Idle states. Hubs must be ready to repeat low-speed
signaling on low-speed ports before the end of the hub setup interval. Low-speed connectivity rules are
summarized below:1.

Low-speed devices are identified during the connection process and the hub ports to which they areconnected are identified as low-speed.

2.

All downstream low-speed packets must be prefaced with a preamble (sent at full-speed), which turns
on the output buffers on low-speed hub ports.

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3.

Low-speed hub port output buffers are turned off upon receipt of EOP and are not turned on again
until a preamble PID is detected.

4.

Upstream connectivity is not affected by whether a hub port is full- or low-speed.

Low-speed signaling begins with the host issuing SYNC at low-speed, followed by the remainder of thepacket. The end of the packet is identified by an End-of-Packet (EOP), at which time all hubs tear down
connectivity and disable any ports to which low-speed devices are connected. Hubs do not switch ports for
upstream signaling; low-speed ports remain enabled in the upstream direction for both low-speed and fullspeed signaling.
Low-speed and full-speed transactions maintain a high degree of protocol commonality. However, lowspeed signaling does have certain limitations which include:

Data payload is limited to eight bytes, maximum

Only interrupt and control types of transfers are supported

The SOF packet is not received by low-speed devices.

8.7 Error Detection and Recovery
The USB permits reliable end-to-end communication in the presence of errors on the physical signaling
layer. This includes the ability to reliably detect the vast majority of possible errors and to recover fromerrors on a transaction-type basis. Control transactions, for example, require a high degree of datareliability; they support end-to-end data integrity using error detection and retry. Isochronous transactions,by virtue of their bandwidth and latency requirements, do not permit retries and must tolerate a higher
incidence of uncorrected errors.

8.7.1 Packet Error Categories
The USB employs three error detection mechanisms: bit stuff violations, PID check bits, and CRCs. Bit
stuff violations are defined in Section 7.1.9. PID errors are defined in Section 8.3.1. CRC errors aredefined in Section 8.3.5.
With the exception of the SOF token, any packet that is received corrupted causes the receiver to ignore it
and discard any data or other field information that came with the packet. Table 8-6 lists error detection
mechanisms, the types of packets to which they apply, and the appropriate packet receiver response.
Table 8-6. Packet Error Types
Field

Error

ActionPID

PID Check, Bit Stuff

Ignore packet

Address

Bit Stuff, Address CRC

Ignore token

Frame Number

Bit Stuff, Frame Number CRC

Ignore Frame Number field

Data

Bit Stuff, Data CRC

Discard data

8.7.2 Bus Turn-around Timing
Neither the device nor the host will send an indication that a received packet had an error. This absence of
positive acknowledgement is considered to be the indication that there was an error. As a consequence of
this method of error reporting, the host and USB function need to keep track of how much time has elapsed
from when the transmitter completes sending a packet until it begins to receive a response. This time is172

Universal Serial Bus Specification Revision 1.1

referred to as the bus turn-around time. The timer starts counting on the SE0-to-‘J’ transition of the EOP
strobe and stops counting when the Idle-to-‘K’ SOP transition is detected. Both devices and the hostrequire turn-around timers. The device bus turn-around time is defined by the worst case round trip delay
plus the maximum device response delay (refer to Section 7.1.18). If a response is not received within thisworst case timeout, then the transmitter considers that the packet transmission has failed. USB devices
timeout no sooner than 16 bit times and no latter than 18 bit times after the end of the previous EOP. If thehost wishes to indicate an error condition via a timeout, it must wait at least 18 bit times before issuing thenext token to ensure that all downstream devices have timed out.
As shown in Figure 8-20, the device uses its bus turn-around timer between token and data or data andhandshake phases. The host uses its timer between data and handshake or token and data phases.
If the host receives a corrupted data packet, it must wait before sending out the next token. This wait
interval guarantees that the host does not attempt to issue a token immediately after a false EOP.
OUT/SETUP

Data
device waits

host waits

Data

IN
host waits

Handshake

Handshake
device waits

Figure 8-20. Bus Turn-around Timer Usage

8.7.3 False EOPs
False EOPs must be handled in a manner which guarantees that the packet currently in progress completes
before the host or any other device attempts to transmit a new packet. If such an event were to occur, it
would constitute a bus collision and have the ability to corrupt up to two consecutive transactions.
Detection of false EOP relies upon the fact that a packet into which a false EOP has been inserted willappear as a truncated packet with a CRC failure. (The last 16 bits of the packet will have a very low
probability of appearing to be a correct CRC.)
The host and devices handle false EOP situations differently. When a device sees a corrupted data packet,
it issues no response and waits for the host to send the next token. This scheme guarantees that the device
will not attempt to return a handshake while the host may still be transmitting a data packet. If a false EOP
has occurred, the host data packet will eventually end, and the device will be able to detect the next token.
If a device issues a data packet that gets corrupted with a false EOP, the host will ignore the packet and notissue the handshake. The device, expecting to see a handshake from the host, will timeout.
If the host receives a corrupted data packet, it assumes that a false EOP may have occurred and waits for 16
bit times to see if there is any subsequent upstream traffic. If no bus transitions are detected within the 16
bit interval and the bus remains in the Idle state, the host may issue the next token. Otherwise, the hostwaits for the device to finish sending the remainder of its packet. Waiting 16 bit times guarantees two
conditions:

The first condition is to make sure that the device has finished sending its packet. This is guaranteed
by a timeout interval (with no bus transitions) greater than the worst case six-bit time bit stuff interval.

The second condition is that the transmitting device’s bus turn-around timer must be guaranteed to
expire.

Note that the timeout interval is transaction speed sensitive. For full-speed transactions, the host must wait
16 full-speed bit times; for low-speed transactions, it must wait 16 low-speed bit times.

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If the host receives a data packet with a valid CRC, it assumes that the packet is complete and need notdelay in issuing the next token.

8.7.4 Babble and Loss of Activity Recovery
The USB must be able to detect and recover from conditions which leave it waiting indefinitely for an EOPor which leave the bus in something other than the Idle state at the end of a frame.



Loss of activity (LOA) is characterized by SOP followed by lack of bus activity (bus remains driven toa ‘J’ or ‘K’) and no EOP at the end of a frame.

Babble is characterized by an SOP followed by the presence of bus activity past the end of a frame.

LOA and babble have the potential to either deadlock the bus or force out the beginning of the next frame.
Neither condition is acceptable, and both must be prevented from occurring. As the USB component
responsible for controlling connectivity, hubs are responsible for babble/LOA detection and recovery. All
USB devices that fail to complete their transmission at the end of a frame are prevented from transmitting
past a frame’s end by having the nearest hub disable the port to which the offending device is attached.
Details of the hub babble/LOA recovery mechanism appear in Section 11.8.1.

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Chapter 9
USB Device Framework
A USB device may be divided into three layers:The bottom layer is a bus interface that transmits and receives packets.

The middle layer handles routing data between the bus interface and various endpoints on the device.
An endpoint is the ultimate consumer or provider of data. It may be thought of as a source or sink for
data.



The top layer is the functionality provided by the serial bus device; for instance, a mouse or ISDN
interface.

This chapter describes the common attributes and operations of the middle layer of a USB device. These
attributes and operations are used by the function-specific portions of the device to communicate throughthe bus interface and ultimately with the host.

9.1

USB Device States
A USB device has several possible states. Some of these states are visible to the USB and the host, whileothers are internal to the USB device. This section describes those states.

9.1.1 Visible Device States
This section describes USB device states that are externally visible (see Figure 9-1). Table 9-1 summarizes
the visible device states.
Note: USB devices perform a reset operation in response to reset signaling on the upstream port. When
reset signaling has completed, the USB device is reset.

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Attached

Hub Reset
Hubor
Deconfigured Configured

PoweredPower
Interruption

Bus
Inactive

Suspended

Bus Activity

Reset

Default
Reset

Bus
Inactive

Suspended

Bus Activity
Address
Assigned

AddressBus
Inactive

Suspended

Bus Activity
DeviceDeviceDeconfigured Configured

Configured

Bus
Inactive
Bus Activity

Figure 9-1. Device State Diagram

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Table 9-1. Visible Device States
Attached

Powered Default

Address

Configured

Suspended

State

No

--

--

--

--

--

Device is not attached to
the USB. Other attributes
are not significant.

Yes

No

--

--

--

--

Device is attached to theUSB, but is not powered.
Other attributes are notsignificant.

Yes

Yes

No

--

--

--

Device is attached to the
USB and powered, but
has not been reset.

Yes

Yes

Yes

No

--

--

Device is attached to the
USB and powered andhas been reset, but has
not been assigned aunique address. Device
responds at the default
address.

Yes

Yes

Yes

Yes

No

--

Device is attached to theUSB, powered, has been
reset, and a uniquedevice address has been
assigned. Device is notconfigured.

Yes

Yes

Yes

Yes

Yes

No

Device is attached to theUSB, powered, has been
reset, has a uniqueaddress, is configured,and is not suspended.
The host may now use
the function provided by
the device.

Yes

Yes

--

--

--

Yes

Device is, at minimum,
attached to the USB andis powered and has notseen bus activity for 3 ms.
It may also have a uniqueaddress and be
configured for use.
However, because thedevice is suspended, thehost may not use thedevice’s function.

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9.1.1.1 Attached
A USB device may be attached or detached from the USB. The state of a USB device when it is detachedfrom the USB is not defined by this specification. This specification only addresses required operations
and attributes once the device is attached.

9.1.1.2 Powered
USB devices may obtain power from an external source and/or from the USB through the hub to which
they are attached. Externally powered USB devices are termed self-powered. Although self-powered
devices may already be powered before they are attached to the USB, they are not considered to be in thePowered state until they are attached to the USB and VBUS is applied to the device.
A device may support both self-powered and bus-powered configurations. Some device configurations
support either power source. Other device configurations may be available only if the device is selfpowered. Devices report their power source capability through the configuration descriptor. The current
power source is reported as part of a device’s status. Devices may change their power source at any time;e.g., from self- to bus-powered. If a configuration is capable of supporting both power modes, the power
maximum reported for that configuration is the maximum the device will draw from VBUS in either mode.
The device must observe this maximum, regardless of its mode. If a configuration supports only one
power mode and the power source of the device changes, the device will lose its current configuration andaddress and return to the Powered state. If a device is self-powered and its current configuration requires
more than 100mA, then if the device switches to being bus-powered, it must return to the Address state.
Self-powered hubs that use VBUS to power the Hub Controller are allowed to remain in the Configured
state if local power is lost. Refer to Section 11.14 for details.
A hub port must be powered in order to detect port status changes, including attach and detach. Buspowered hubs do not provide any downstream power until they are configured, at which point they will
provide power as allowed by their configuration and power source. A USB device must be able to beaddressed within a specified time period from when power is initially applied (refer to Chapter 7). After anattachment to a port has been detected, the host may enable the port, which will also reset the device
attached to the port.

9.1.1.3 Default
After the device has been powered, it must not respond to any bus transactions until it has received a resetfrom the bus. After receiving a reset, the device is then addressable at the default address.

9.1.1.4 Address
All USB devices use the default address when initially powered or after the device has been reset. EachUSB device is assigned a unique address by the host after attachment or after reset. A USB device
maintains its assigned address while suspended.
A USB device responds to requests on its default pipe whether the device is currently assigned a uniqueaddress or is using the default address.

9.1.1.5 Configured
Before a USB device’s function may be used, the device must be configured. From the device’s
perspective, configuration involves writing a non-zero value to the device configuration register.
Configuring a device or changing an alternate setting causes all of the status and configuration values
associated with endpoints in the affected interfaces to be set to their default values. This includes setting
the data toggle of any endpoint using data toggles to the value DATA0.

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9.1.1.6 Suspended
In order to conserve power, USB devices automatically enter the Suspended state when the device has
observed no bus traffic for a specified period (refer to Chapter 7). When suspended, the USB device
maintains any internal status, including its address and configuration.
All devices must suspend if bus activity has not been observed for the length of time specified in
Chapter 7. Attached devices must be prepared to suspend at any time they are powered, whether they havebeen assigned a non-default address or are configured. Bus activity may cease due to the host entering asuspend mode of its own. In addition, a USB device shall also enter the Suspended state when the hub port
it is attached to is disabled. This is referred to as selective suspend.
A USB device exits suspend mode when there is bus activity. A USB device may also request the host to
exit suspend mode or selective suspend by using electrical signaling to indicate remote wakeup. The
ability of a device to signal remote wakeup is optional. If a USB device is capable of remote wakeup
signaling, the device must support the ability of the host to enable and disable this capability. When thedevice is reset, remote wakeup signaling must be disabled.

9.1.2 Bus Enumeration
When a USB device is attached to or removed from the USB, the host uses a process known as bus
enumeration to identify and manage the device state changes necessary. When a USB device is attached toa powered port, the following actions are taken:
1.

The hub to which the USB device is now attached informs the host of the event via a reply on its statuschange pipe (refer to Section 11.13.3 for more information). At this point, the USB device is in thePowered state and the port to which it is attached is disabled.

2.

The host determines the exact nature of the change by querying the hub.

3.

Now that the host knows the port to which the new device has been attached, the host then waits for atleast 100 ms to allow completion of an insertion process and for power at the device to become stable.
The host then issues a port enable and reset command to that port. Refer to Section 7.1.7.1 and Figure
7-19 for sequence of events and timings of connection through device reset.

4.

The hub maintains the reset signal to that port for 10 ms (See Section 11.5.1.5). When the reset signal
is released, the port has been enabled. The USB device is now in the Default state and can draw nomore than 100mA from VBUS. All of its registers and state have been reset and it answers to thedefault address.

5.

The host assigns a unique address to the USB device, moving the device to the Address state.

6.

Before the USB device receives a unique address, its Default Control Pipe is still accessible via thedefault address. The host reads the device descriptor to determine what actual maximum data payload
size this USB device’s default pipe can use.

7.

The host reads the configuration information from the device by reading each configuration zero ton-1, where n is the number of configurations. This process may take several milliseconds to complete.

8.

Based on the configuration information and how the USB device will be used, the host assigns a
configuration value to the device. The device is now in the Configured state and all of the endpoints inthis configuration have taken on their described characteristics. The USB device may now draw theamount of VBUS power described in its descriptor for the selected configuration. From the device’s
point of view it is now ready for use.

When the USB device is removed, the hub again sends a notification to the host. Detaching a device
disables the port to which it had been attached. Upon receiving the detach notification, the host will update
its local topological information.

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9.2

Generic USB Device Operations
All USB devices support a common set of operations. This section describes those operations.

9.2.1 Dynamic Attachment and Removal
USB devices may be attached and removed at any time. The hub that provides the attachment point or port
is responsible for reporting any change in the state of the port.
The host enables the hub port where the device is attached upon detection of an attachment, which also has
the effect of resetting the device. A reset USB device has the following characteristics:
Responds to the default USB addressIs not configuredIs not initially suspended.

When a device is removed from a hub port, the hub disables the port where the device was attached andnotifies the host of the removal

9.2.2 Address Assignment
When a USB device is attached, the host is responsible for assigning a unique address to the device. Thisis done after the device has been reset by the host and the hub port where the device is attached has beenenabled.

9.2.3 Configuration
A USB device must be configured before its function(s) may be used. The host is responsible for
configuring a USB device. The host typically requests configuration information from the USB device todetermine the device’s capabilities.
As part of the configuration process, the host sets the device configuration and, where necessary, selects
the appropriate alternate settings for the interfaces.
Within a single configuration, a device may support multiple interfaces. An interface is a related set of
endpoints that present a single feature or function of the device to the host. The protocol used tocommunicate with this related set of endpoints and the purpose of each endpoint within the interface may
be specified as part of a device class or vendor-specific definition.
In addition, an interface within a configuration may have alternate settings that redefine the number orcharacteristics of the associated endpoints. If this is the case, the device shall support the GetInterface()and SetInterface() requests to report or select the current alternative setting for the specified interface.
Within each configuration, each interface descriptor contains fields that identify the interface number andthe alternate setting. Interfaces are numbered from zero to one less than the number of concurrent
interfaces supported by the configuration. Alternate settings range from zero to one less than the numberof alternate settings for a specific interface. The default setting when a device is initially configured isalternate setting zero.
In support of adaptive device drivers that are capable of managing a related group of USB devices, thedevice and interface descriptors contain Class, SubClass, and Protocol fields. These fields are used to
identify the function(s) provided by a USB device and the protocols used to communicate with thefunction(s) on the device. A class code is assigned to a group of related devices that has been characterized
as a part of a USB Class Specification. A class of devices may be further subdivided into subclasses andwithin a class or subclass a protocol code may define how the Host Software communicates with thedevice.

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Note: the assignment of class, subclass and protocol codes must be coordinated but is beyond the scope of
this specification.

9.2.4 Data Transfer
Data may be transferred between a USB device endpoint and the host in one of four ways. Refer to
Chapter 5 for the definition of the four types of transfers. An endpoint number may be used for different
types of data transfers in different alternate settings. However, once an alternate setting is selected(including the default setting of an interface), a USB device endpoint uses only one data transfer method
until a different alternate setting is selected.

9.2.5 Power Management
Power management on USB devices involves the issues described in the following sections.

9.2.5.1 Power BudgetingUSB bus power is a limited resource. During device enumeration, a host evaluates a device's power
requirements. If the power requirements of a particular configuration exceed the power available to thedevice, Host software shall not select that configuration.
USB devices shall limit the power they consume from VBUS to one unit load or less until configured.
Suspended devices, whether configured or not, shall limit their bus power consumption as defined in
Chapter 7. Depending on the power capabilities of the port to which the device is attached, a USB device
may be able to draw up to five unit loads from VBUS after configuration.

9.2.5.2 Remote Wakeup
Remote wakeup allows a suspended USB device to signal a host that may also be suspended. This notifies
the host that it should resume from its suspended mode, if necessary, and service the external event thattriggered the suspended USB device to signal the host. A USB device reports its ability to support remote
wakeup in a configuration descriptor. If a device supports remote wakeup, it must also allow the capability
to be enabled and disabled using the standard USB requests.
Remote wakeup is accomplished using electrical signaling described in Section 7.1.7.5.

9.2.6 Request Processing
With the exception of SetAddress() requests (see Section 9.4.6), a device may begin processing of a request
as soon as the device returns the ACK following the Setup. The device is expected to “complete”
processing of the request before it allows the Status stage to complete successfully. Some requests initiate
operations that take many milliseconds to complete. For requests such as this, the device class is required
to define a method other than Status stage completion to indicate that the operation has completed. For
example, a reset on a hub port takes at least 10 ms to complete. The SetPortFeature(PORT_RESET) (seeChapter 11) request “completes” when the reset on the port is initiated. Completion of the reset operation
is signaled when the port’s status change is set to indicate that the port is now enabled. This techniqueprevents the host from having to constantly poll for a completion when it is known that the request willtake a relatively long period of time.

9.2.6.1 Request Processing Timing
All devices are expected to handle requests in a timely manner. USB sets an upper limit of 5 seconds as
the upper limit for any command to be processed. This limit is not applicable in all instances. The
limitations are described in the following sections. It should be noted that the limitations given below areintended to encompass a wide range of implementations. If all devices in a USB system used the181Universal Serial Bus Specification Revision 1.1

maximum allotted time for request processing the user experience would suffer. For this reason,
implementations should strive to complete requests in times that are as short as possible.

9.2.6.2 Reset/Resume Recovery Time
After a port is reset or resumed, the USB System Software is expected to provide a “recovery” interval of
10 ms before the device attached to the port is expected to respond to data transfers. The device may
ignore any data transfers during the recovery interval.
After the end of the recovery interval (measured from the end of the reset or the end of the EOP at the endof the resume signaling) the device must accept data transfers at any time.

9.2.6.3 Set Address Processing
After the reset/resume recovery interval, if a device receives a SetAddress() request, the device must beable to complete processing of the request and be able to successfully complete the Status stage of therequest within 50 ms. In the case of the SetAddress() request, the Status stage successfully completes when
the devices sends the zero-length Status packet or when the device sees the ACK in response to the Status
stage data packet.
After successful completion of the Status stage, the device is allowed a SetAddress() recovery interval of 2
ms. At the end of this interval, the device must be able to accept Setup packets addressed to the new
address. Also, at the end of the recovery interval the device must not respond to tokens sent to the oldaddress (unless, of course, the old and new address is the same.)

9.2.6.4 Standard Device Requests
For standard device requests that require no Data stage, a device must be able to complete the request andbe able to successfully complete the Status stage of the request within 50 ms of receipt of the request. This
limitation applies to requests to the device, interface, or endpoint.
For standard device requests that require data stage transfer to the host, the device must be able to returnthe first data packet to the host within 500 ms of receipt of the request. For subsequent data packets, ifany, the device must be able to return them within 500 ms of successful completion of the transmission of
the previous packet. The device must then be able to successfully complete the status stage within 50 ms
after returning the last data packet.
For standard device requests that require a data stage transfer to the device, the 5-second limit applies.
This means that the device must be capable of accepting all data packets from the host and successfullycompleting the Status stage if the host provides the data at the maximum rate at which the device canaccept it. Delays between packets introduced by the host add to the time allowed for the device tocomplete the request.

9.2.6.5 Class-specific Requests
Unless specifically exempted in the class document, all class-specific requests must meet the timing
limitations for standard device requests. If a class document provides an exemption, the exemption may
only be specified on a request-by-request basis.
A class document may require that a device respond more quickly than is specified in this section. Faster
response may be required for standard and class-specific requests.

9.2.7 Request Error
When a request is received by a device that is not defined for the device, is inappropriate for the current
setting of the device, or has values that are not compatible with the request, then a Request Error exists.

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The device deals with the Request Error by returning a STALL PID in response to the next Data stagetransaction or in the Status stage of the message. It is preferred that the STALL PID be returned at the next
Data stage transaction, as this avoids unnecessary bus activity.

9.3

USB Device Requests
All USB devices respond to requests from the host on the device’s Default Control Pipe. These requests
are made using control transfers. The request and the request’s parameters are sent to the device in the
Setup packet. The host is responsible for establishing the values passed in the fields listed in Table 9-2.
Every Setup packet has eight bytes.
Table 9-2. Format of Setup Data

Offset

Field

Size

0

bmRequestType

1

Value
BitmapDescription
Characteristics of request:
D7:

Data transfer direction
0 = Host-to-device
1 = Device-to-hostD6...5:Type
0 = Standard
1 = Class
2 = Vendor
3 = Reserved

D4...0:

Recipient
0 = Device
1 = Interface
2 = Endpoint
3 = Other
4...31 = Reserved

1bRequest

1

Value

Specific request (refer to Table 9-3)

2

wValue2

Value

Word-sized field that varies according torequest4

wIndex2

Index or
OffsetWord-sized field that varies according torequest; typically used to pass an index oroffset6

wLength2

Count

Number of bytes to transfer if there is a
Data stage

9.3.1 bmRequestType
This bitmapped field identifies the characteristics of the specific request. In particular, this field identifies
the direction of data transfer in the second phase of the control transfer. The state of the Direction bit isignored if the wLength field is zero, signifying there is no Data stage.
The USB Specification defines a series of standard requests that all devices must support. These are
enumerated in Table 9-3. In addition, a device class may define additional requests. A device vendor may
also define requests supported by the device.

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Requests may be directed to the device, an interface on the device, or a specific endpoint on a device. This
field also specifies the intended recipient of the request. When an interface or endpoint is specified, thewIndex field identifies the interface or endpoint.

9.3.2 bRequest
This field specifies the particular request. The Type bits in the bmRequestType field modify the meaning of
this field. This specification defines values for the bRequest field only when the bits are reset to zero,
indicating a standard request (refer to Table 9-3).

9.3.3 wValue
The contents of this field vary according to the request. It is used to pass a parameter to the device,specific to the request.

9.3.4 wIndex
The contents of this field vary according to the request. It is used to pass a parameter to the device,specific to the request.
The wIndex field is often used in requests to specify an endpoint or an interface. Figure 9-2 shows theformat of wIndex when it is used to specify an endpoint.
D7

D6

Direction
D15

D5

D4

D3

Reserved (Reset to zero)
D14

D13

D2

D1

D0

Endpoint Number

D12

D11

D10

D9

D8

Reserved (Reset to zero)

Figure 9-2. wIndex Format when Specifying an Endpoint
The Direction bit is set to zero to indicate the OUT endpoint with the specified Endpoint Number and to
one to indicate the IN endpoint. In the case of a control pipe, the request should have the Direction bit set
to zero but the device may accept either value of the Direction bit.
Figure 9-3 shows the format of wIndex when it is used to specify an interface.
D7

D6

D5

D4

D3

D2

D1

D0

D10

D9

D8

Interface Number
D15

D14

D13

D12

D11

Reserved (Reset to zero)

Figure 9-3. wIndex Format when Specifying an Interface

9.3.5 wLength
This field specifies the length of the data transferred during the second phase of the control transfer. The
direction of data transfer (host-to-device or device-to-host) is indicated by the Direction bit of thebmRequestType field. If this field is zero, there is no data transfer phase.

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On an input request, a device must never return more data than is indicated by the wLength value; it may
return less. On an output request, wLength will always indicate the exact amount of data to be sent by thehost. Device behavior is undefined if the host should send more data than is specified in wLength.

9.4Standard Device Requests
This section describes the standard device requests defined for all USB devices. Table 9-3 outlines thestandard device requests, while Table 9-4 and Table 9-5 give the standard request codes and descriptor
types, respectively.
USB devices must respond to standard device requests, whether the device has been assigned a non-default
address or the device is currently configured.

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Table 9-3. Standard Device Requests
bmRequestTypebRequest

wValuewIndexwLengthData

00000000B
00000001B
00000010B

CLEAR_FEATURE

FeatureSelector

ZeroInterface
Endpoint

Zero

None

10000000B

GET_CONFIGURATION

Zero

Zero

One

Configuration
Value

10000000B

GET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Zero orLanguageID

Descriptor
LengthDescriptor

10000001B

GET_INTERFACE

Zero

Interface

One

Alternate
Interface

10000000B
10000001B
10000010B

GET_STATUS

Zero

Zero
Interface
Endpoint

Two

Device,Interface, orEndpoint
Status00000000B

SET_ADDRESS

DeviceAddress

Zero

Zero

None

00000000B

SET_CONFIGURATION

Configuration
Value

Zero

Zero

None

00000000B

SET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Zero orLanguageID

Descriptor
LengthDescriptor

00000000B
00000001B
00000010B

SET_FEATURE

FeatureSelector

ZeroInterface
Endpoint

Zero

None

00000001B

SET_INTERFACE

Alternate
Setting

Interface

Zero

None

10000010B

SYNCH_FRAME

Zero

Endpoint

Two

Frame Number

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Table 9-4. Standard Request CodesbRequest

Value

GET_STATUS

0

CLEAR_FEATURE

1

Reserved for future use

2

SET_FEATURE

3

Reserved for future use

4

SET_ADDRESS

5

GET_DESCRIPTOR

6

SET_DESCRIPTOR

7

GET_CONFIGURATION

8SET_CONFIGURATION

9GET_INTERFACE

10

SET_INTERFACE

11SYNCH_FRAME

12

Table 9-5. Descriptor Types
Descriptor Types

Value

DEVICE

1

CONFIGURATION2STRING

3

INTERFACE

4

ENDPOINT

5

Feature selectors are used when enabling or setting features, such as remote wakeup, specific to a device,interface, or endpoint. The values for the feature selectors are given in Table 9-6.

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Table 9-6. Standard Feature Selectors
Feature Selector

Recipient

Value

DEVICE_REMOTE_WAKEUP

Device1

ENDPOINT_HALTEndpoint

0

If an unsupported or invalid request is made to a USB device, the device responds by returning STALL inthe Data or Status stage of the request. If the device detects the error in the Setup stage, it is preferred thatthe device returns STALL at the earlier of the Data or Status stage. Receipt of an unsupported or invalid
request does NOT cause the optional Halt feature on the control pipe to be set. If for any reason, thedevice becomes unable to communicate via its Default Control Pipe due to an error condition, the device
must be reset to clear the condition and restart the Default Control Pipe.

9.4.1 Clear FeatureThis request is used to clear or disable a specific feature.
bmRequestTypebRequest

wValuewIndexwLengthData

00000000B
00000001B
00000010B

CLEAR_FEATURE

FeatureSelector

ZeroInterface
Endpoint

Zero

None

Feature selector values in wValue must be appropriate to the recipient. Only device feature selector values
may be used when the recipient is a device, only interface feature selector values may be used when therecipient is an interface, and only endpoint feature selector values may be used when the recipient is an
endpoint.
Refer to Table 9-6 for a definition of which feature selector values are defined for which recipients.
A ClearFeature() request that references a feature that cannot be cleared, that does not exist, or thatreferences an interface or endpoint that does not exist will cause the device to respond with a Request
Error.
If wLength is non-zero, then the device behavior is not specified.

188

Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:This request is valid when the device is in the Address state; references to interfaces
or to endpoints other than endpoint zero shall cause the device to respond with a
Request Error.

Configured state:This request is valid when the device is in the Configured state.

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9.4.2 Get Configuration
This request returns the current device configuration value.
bmRequestTypebRequest

wValuewIndexwLengthData

10000000B

GET_CONFIGURATION

Zero

Zero

One

Configuration
Value

If the returned value is zero, the device is not configured.
If wValue, wIndex, or wLength are not as specified above, then the device behavior is not specified.
Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

The value zero shall be returned.

Configured state:

The non-zero bConfigurationValue of the current configuration shall be returned.

9.4.3 Get Descriptor
This request returns the specified descriptor if the descriptor exists.
bmRequestTypebRequest

wValuewIndexwLengthData

10000000B

GET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Zero orLanguage ID(refer to
Section 9.6.5)

Descriptor
LengthDescriptor

The wValue field specifies the descriptor type in the high byte and the descriptor index in the low byte
(refer to Table 9-5). The wIndex field specifies the Language ID for string descriptors or is reset to zero forother descriptors. The wLength field specifies the number of bytes to return. If the descriptor is longer
than the wLength field, only the initial bytes of the descriptor are returned. If the descriptor is shorter than
the wLength field, the device indicates the end of the control transfer by sending a short packet whenfurther data is requested. A short packet is defined as a packet shorter than the maximum payload size or a
NULL data packet (refer to Chapter 5).
The standard request to a device supports three types of descriptors: DEVICE, CONFIGURATION, andSTRING. A request for a configuration descriptor returns the configuration descriptor, all interface
descriptors, and endpoint descriptors for all of the interfaces in a single request. The first interface
descriptor follows the configuration descriptor. The endpoint descriptors for the first interface follow the
first interface descriptor. If there are additional interfaces, their interface descriptor and endpoint
descriptors follow the first interface’s endpoint descriptors. Class-specific and/or vendor-specificdescriptors follow the standard descriptors they extend or modify.
All devices must provide a device descriptor and at least one configuration descriptor. If a device does notsupport a requested descriptor, it responds with a Request Error.
Default state:This is a valid request when the device is in the Default state.

Address state:This is a valid request when the device is in the Address state.
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Configured state:This is a valid request when the device is in the Configured state.

9.4.4 Get Interface
This request returns the selected alternate setting for the specified interface.
bmRequestTypebRequest

wValuewIndexwLengthData

10000001B

GET_INTERFACE

Zero

Interface

One

Alternate
Setting

Some USB devices have configurations with interfaces that have mutually exclusive settings. This request
allows the host to determine the currently selected alternative setting.
If wValue or wLength are not as specified above, then the device behavior is not specified.
If the interface specified does not exist, then the device responds with a Request Error.
Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

A Request Error response is given by the device.

Configured state:This is a valid request when the device is in the Configured state.

9.4.5 Get Status
This request returns status for the specified recipient.
bmRequestTypebRequest

wValuewIndexwLengthData

10000000B
10000001B
10000010B

GET_STATUS

Zero

Zero
Interface
Endpoint

Two

Device,Interface, orEndpoint
StatusThe Recipient bits of the bmRequestType field specify the desired recipient. The data returned is thecurrent status of the specified recipient.
If wValue or wLength are not as specified above, or if wIndex is non-zero for a device status request, thenthe behavior of the device is not specified.
If an interface or an endpoint is specified that does not exist then the device responds with a Request Error.

190

Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

If an interface or an endpoint other than endpoint zero is specified, then the device
responds with a Request Error.

Configured state:

If an interface or endpoint that does not exist is specified, then the device respondswith a Request Error.

Universal Serial Bus Specification Revision 1.1

A GetStatus() request to a device returns the information shown in Figure 9-4.
D7

D6

D5

D4

D3

D2

Reserved (Reset to zero)

D15

D14

D13

D12

D11

D10

D1

D0

RemoteWakeupSelf
Powered

D9

D8Reserved (Reset to zero)

Figure 9-4. Information Returned by a GetStatus() Request to a Device
The Self Powered field indicates whether the device is currently self-powered. If D0 is reset to zero, thedevice is bus-powered. If D0 is set to one, the device is self-powered. The Self Powered field may not bechanged by the SetFeature() or ClearFeature() requests.
The Remote Wakeup field indicates whether the device is currently enabled to request remote wakeup. The
default mode for devices that support remote wakeup is disabled. If D1 is reset to zero, the ability of thedevice to signal remote wakeup is disabled. If D1 is set to one, the ability of the device to signal remote
wakeup is enabled. The Remote Wakeup field can be modified by the SetFeature() and ClearFeature()requests using the DEVICE_REMOTE_WAKEUP feature selector. This field is reset to zero when thedevice is reset.
A GetStatus() request to an interface returns the information shown in Figure 9-5.
D7

D6

D5

D4

D3

D2

D1

D0

D10

D9

D8

Reserved (Reset to zero)
D15

D14

D13

D12

D11

Reserved (Reset to zero)

Figure 9-5. Information Returned by a GetStatus() Request to a Interface

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Universal Serial Bus Specification Revision 1.1

A GetStatus() request to an endpoint returns the information shown in Figure 9-6.
D7

D6

D5

D4

D3

D2

D1

Reserved (Reset to zero)
D15

D14

D13

D12

D0
Halt

D11

D10

D9

D8

Reserved (Reset to zero)

Figure 9-6. Information Returned by a GetStatus() Request to an Endpoint
The Halt feature is required to be implemented for all interrupt and bulk endpoint types. If the endpoint iscurrently halted, then the Halt feature is set to one. Otherwise, the Halt feature is reset to zero. The Haltfeature may optionally be set with the SetFeature(ENDPOINT_HALT) request. When set by the
SetFeature() request, the endpoint exhibits the same stall behavior as if the field had been set by a hardware
condition. If the condition causing a halt has been removed, clearing the Halt feature via a
ClearFeature(ENDPOINT_HALT) request results in the endpoint no longer returning a STALL. For
endpoints using data toggle, regardless of whether an endpoint has the Halt feature set, a
ClearFeature(ENDPOINT_HALT) request always results in the data toggle being reinitialized to DATA0.
The Halt feature is reset to zero after either a SetConfiguration() or SetInterface() request even if therequested configuration or interface is the same as the current configuration or interface.
It is neither required nor recommended that the Halt feature be implemented for the Default Control Pipe.
However, devices may set the Halt feature of the Default Control Pipe in order to reflect a functional error
condition. If the feature is set to one, the device will return STALL in the Data and Status stages of eachstandard request to the pipe except GetStatus(), SetFeature(), and ClearFeature() requests. The device need
not return STALL for class-specific and vendor-specific requests.

9.4.6 Set AddressThis request sets the device address for all future device accesses.
bmRequestTypebRequest

wValuewIndexwLengthData

00000000B

SET_ADDRESS

DeviceAddress

Zero

Zero

None

The wValue field specifies the device address to use for all subsequent accesses.
As noted elsewhere, requests actually may result in up to three stages. In the first stage, the Setup packet issent to the device. In the optional second stage, data is transferred between the host and the device. In the
final stage, status is transferred between the host and the device. The direction of data and status transfer
depends on whether the host is sending data to the device or the device is sending data to the host. The
Status stage transfer is always in the opposite direction of the Data stage. If there is no Data stage, theStatus stage is from the device to the host.
Stages after the initial Setup packet assume the same device address as the Setup packet. The USB device
does not change its device address until after the Status stage of this request is completed successfully.
Note that this is a difference between this request and all other requests. For all other requests, theoperation indicated must be completed before the Status stage.
If the specified device address is greater than 127, or if wIndex or wLength are non-zero, then the behaviorof the device is not specified.

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Universal Serial Bus Specification Revision 1.1

Device response to SetAddress() with a value of 0 is undefined.
Default state:

If the address specified is non-zero, then the device shall enter the Address state;otherwise, the device remains in the Default state (this is not an error condition).

Address state:

If the address specified is zero, then the device shall enter the Default state;otherwise, the device remains in the Address state but uses the newly-specified
address.

Configured state:

Device behavior when this request is received while the device is in the Configured
state is not specified.

9.4.7 Set Configuration
This request sets the device configuration.
bmRequestTypebRequest

wValuewIndexwLengthData

00000000B

SET_CONFIGURATION

Configuration Value

Zero

Zero

None

The lower byte of the wValue field specifies the desired configuration. This configuration value must be
zero or match a configuration value from a configuration descriptor. If the configuration value is zero, thedevice is placed in its Address state. The upper byte of the wValue field is reserved.
If wIndex, wLength, or the upper byte of wValue is non-zero, then the behavior of this request is not
specified.
Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

If the specified configuration value is zero, then the device remains in the Addressstate. If the specified configuration value matches the configuration value from a
configuration descriptor, then that configuration is selected and the device enters theConfigured state. Otherwise, the device responds with a Request Error.

Configured state:

If the specified configuration value is zero, then the device enters the Address state.
If the specified configuration value matches the configuration value from a
configuration descriptor, then that configuration is selected and the device remains in
the Configured state. Otherwise, the device responds with aRequest Error.

9.4.8 Set Descriptor
This request may be used to update existing descriptors or new descriptors may be added.
bmRequestTypebRequest

wValuewIndexwLengthData

00000000B

SET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Language ID(refer to
Section 9.6.5)
or zeroDescriptor
LengthDescriptor

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Universal Serial Bus Specification Revision 1.1

The wValue field specifies the descriptor type in the high byte and the descriptor index in the low byte
(refer to Table 9-5). The wIndex field specifies the Language ID for string descriptors or is reset to zero forother descriptors. The wLength field specifies the number of bytes to transfer from the host to the device.
If this request is not supported then the device will respond with a Request Error.
Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

If supported, this is a valid request when the device is in the Address state.

Configured state:

If supported, this is a valid request when the device is in the Configured state.

9.4.9 Set FeatureThis request is used to set or enable a specific feature.
bmRequestTypebRequest

wValuewIndexwLengthData

00000000B
00000001B
00000010B

SET_FEATURE

FeatureSelector

ZeroInterface
Endpoint

Zero

None

Feature selector values in wValue must be appropriate to the recipient. Only device feature selector values
may be used when the recipient is a device; only interface feature selector values may be used when therecipient is an interface, and only endpoint feature selector values may be used when the recipient is an
endpoint.
Refer to Table 9-6 for a definition of which feature selector values are defined for which recipients. A
SetFeature() request that references a feature that cannot be set or that does not exist causes a STALL to be
returned in the Status stage of the request.
If wLength is non-zero, then the behavior of the device is not specified.
If an endpoint or interface is specified that does not exist, then the device responds with a Request Error.

194

Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

If an interface or an endpoint other than endpoint zero is specified, then the device
responds with a Request Error.

Configured state:This is a valid request when the device is in the Configured state.

Universal Serial Bus Specification Revision 1.1

9.4.10 Set Interface
This request allows the host to select an alternate setting for the specified interface.
bmRequestTypebRequest

wValuewIndexwLengthData

00000001B

SET_INTERFACE

Alternative
Setting

Interface

Zero

None

Some USB devices have configurations with interfaces that have mutually exclusive settings. This request
allows the host to select the desired alternate setting. If a device only supports a default setting for the
specified interface, then a STALL may be returned in the Status stage of the request.
If the interface or the alternative setting does not exist, then the device responds with a Request Error. If
wLength is non-zero, then the behavior of the device is not specified.
Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

The device shall respond with a Request Error.

Configured state:This is a valid request when the device is in the Configured state.

9.4.11 Synch Frame
This request is used to set and then report an endpoint’s synchronization frame.
bmRequestTypebRequest

wValuewIndexwLengthData

10000010B

SYNCH_FRAME

Zero

Endpoint

Two

Frame
NumberWhen an endpoint supports isochronous transfers, the endpoint may also require per-frame transfers tovary in size according to a specific pattern. The host and the endpoint must agree on which frame therepeating pattern begins. The number of the frame in which the pattern began is returned to the host. This
frame number is the one conveyed to the endpoint by the last SOF prior to the first frame of the pattern.
Alternatively, the device may use this request to restart the pattern. In this case, the device would save theframe number in each SOF and return this value in the Data stage of this transfer and restart the pattern oneach IN of the Data stage.
This value is only used for isochronous data transfers using implicit pattern synchronization. If wValue is
non-zero or wLength is not two, then the behavior of the device is not specified.
If the specified endpoint does not support this request, then the device will respond with a Request Error.
Default state:

Device behavior when this request is received while the device is in the Default state
is not specified.

Address state:

The device shall respond with a Request Error.

Configured state:This is a valid request when the device is in the Configured state.

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Universal Serial Bus Specification Revision 1.1

9.5

Descriptors
USB devices report their attributes using descriptors. A descriptor is a data structure with a defined format.
Each descriptor begins with a byte-wide field that contains the total number of bytes in the descriptor
followed by a byte-wide field that identifies the descriptor type.
Using descriptors allows concise storage of the attributes of individual configurations because eachconfiguration may reuse descriptors or portions of descriptors from other configurations that have the samecharacteristics. In this manner, the descriptors resemble individual data records in a relational database.
Where appropriate, descriptors contain references to string descriptors that provide displayable information
describing a descriptor in human-readable form. The inclusion of string descriptors is optional. However,
the reference fields within descriptors are mandatory. If a device does not support string descriptors, stringreference fields must be reset to zero to indicate no string descriptor is available.
If a descriptor returns with a value in its length field that is less than defined by this specification, thedescriptor is invalid and should be rejected by the host. If the descriptor returns with a value in its length
field that is greater than defined by this specification, the extra bytes are ignored by the host, but the next
descriptor is located using the length returned rather than the length expected.
A device may return class- or vendor-specific descriptors in two ways.

9.6

1.

If the class or vendor specific descriptors use the same format as standard descriptors (e.g. start with alength byte and followed by a type byte), they may be returned interleaved with standard descriptors in
the configuration information returned by a GetDescriptor(Configuration) request. In this case, theclass or vendor-specific descriptors typically follow a related standard descriptor they modify orextend.

2.

If the class or vendor specifric descriptors are independentof configuration infomrationor use a nonstandard format, a GetDescriptor() request specifying the class or vendor specific descriptor type andindex may be used to retrieve the descriptor from the device. A class or vendor specification willdefine the appropriate way to retrieve these descriptors.

Standard USB Descriptor Definitions
The standard descriptors defined in this specification may only be modified or extended by revision of the
Universal Serial Bus Specification.
Note: An extension to the USB 1.0 standard endpoint descriptor has been published in Device Class
Specification for Audio Devices Revision 1.0. This is the only extension defined outside USB Specificationthat is allowed. Future revisions of the USB Specification that extend the standard endpoint descriptor will
do so as to not conflict with the extension defined in the Audio Device Class Specification Revision 1.0.

9.6.1 Device
A device descriptor describes general information about a USB device. It includes information that applies
globally to the device and all of the device’s configurations. A USB device has only one device descriptor.
All USB devices have a Default Control Pipe. The maximum packet size of a device’s Default Control
Pipe is described in the device descriptor. Endpoints specific to a configuration and its interface(s) aredescribed in the configuration descriptor. A configuration and its interface(s) do not include an endpoint
descriptor for the Default Control Pipe. Other than the maximum packet size, the characteristics of the
Default Control Pipe are defined by this specification and are the same for all USB devices.
The bNumConfigurations field identifies the number of configurations the device supports. Table 9-7
shows the standard device descriptor.

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Universal Serial Bus Specification Revision 1.1

Table 9-7. Standard Device Descriptor
Offset

Field

Size

Value

Description

0bLength1

NumberSize of this descriptor in bytes

1

bDescriptorType1

Constant

DEVICE Descriptor Type2bcdUSB

2

BCD

USB Specification Release Number in
Binary-Coded Decimal (i.e., 2.10 is 210H).
This field identifies the release of the USB
Specification with which the device and its
descriptors are compliant.

4

bDeviceClass

1

Class

Class code (assigned by the USB).
If this field is reset to zero, each interface
within a configuration specifies its own
class information and the various
interfaces operate independently.
If this field is set to a value between 1 andFEH, the device supports different classspecifications on different interfaces andthe interfaces may not operate
independently. This value identifies theclass definition used for the aggregateinterfaces. (For example, a CD-ROM
device with audio and digital datainterfaces that require transport control toeject CDs or start them spinning.)
If this field is set to FFH, the device classis vendor-specific.

5

bDeviceSubClass

1

SubClass

Subclass code (assigned by the USB).
These codes are qualified by the value ofthe bDeviceClass field.
If the bDeviceClass field is reset to zero,
this field must also be reset to zero.
If the bDeviceClass field is not set to FFH,
all values are reserved for assignment by
the USB.

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Universal Serial Bus Specification Revision 1.1

Table 9-7. Standard Device Descriptor (Continued)
Offset6

Field
bDeviceProtocol

Size
1

Value
ProtocolDescription
Protocol code (assigned by the USB).
These codes are qualified by the value ofthe bDeviceClass and thebDeviceSubClass fields. If a device
supports class-specific protocols on a
device basis as opposed to an interface
basis, this code identifies the protocols
that the device uses as defined by the
specification of the device class.
If this field is reset to zero, the device
does not use class-specific protocols on a
device basis. However, it may use classspecific protocols on an interface basis.
If this field is set to FFH, the device uses
a vendor-specific protocol on a device
basis.

198

7

bMaxPacketSize0

1

NumberMaximum packet size for endpoint zero
(only 8, 16, 32, or 64 are valid)8

idVendor

2

ID

Vendor ID (assigned by the USB)

10

idProduct

2

ID

Product ID (assigned by the
manufacturer)

12

bcdDevice

2

BCD

Device release number in binary-coded
decimal14

iManufacturer

1

Index

Index of string descriptor describing
manufacturer

15

iProduct

1

Index

Index of string descriptor describing
product

16

iSerialNumber

1

Index

Index of string descriptor describing thedevice’s serial number

17

bNumConfigurations

1

NumberNumber of possible configurations

Universal Serial Bus Specification Revision 1.1

9.6.2 Configuration
The configuration descriptor describes information about a specific device configuration. The descriptor
contains a bConfigurationValue field with a value that, when used as a parameter to the SetConfiguration()request, causes the device to assume the described configuration.
The descriptor describes the number of interfaces provided by the configuration. Each interface may
operate independently. For example, an ISDN device might be configured with two interfaces, eachproviding 64kB/s bi-directional channels that have separate data sources or sinks on the host. Anotherconfiguration might present the ISDN device as a single interface, bonding the two channels into one
128kB/s bi-directional channel.
When the host requests the configuration descriptor, all related interface and endpoint descriptors are
returned (refer to Section 9.4.2).
A USB device has one or more configuration descriptors. Each configuration has one or more interfaces
and each interface has zero or more endpoints. An endpoint is not shared among interfaces within a single
configuration unless the endpoint is used by alternate settings of the same interface. Endpoints may beshared among interfaces that are part of different configurations without this restriction.
Once configured, devices may support limited adjustments to the configuration. If a particular interface
has alternate settings, an alternate may be selected after configuration. Table 9-8 shows the standard
configuration descriptor.
Table 9-8. Standard Configuration Descriptor
Offset

Field

Size

Value

Description

0bLength1

NumberSize of this descriptor in bytes

1

bDescriptorType1

Constant

CONFIGURATION Descriptor Type2

wTotalLength2

NumberTotal length of data returned for thisconfiguration. Includes the combined lengthof all descriptors (configuration, interface,
endpoint, and class- or vendor-specific)
returned for this configuration.

4

bNumInterfaces

1

NumberNumber of interfaces supported by thisconfiguration

5bConfigurationValue

1

NumberValue to use as an argument to the
SetConfiguration() request to select thisconfiguration

6

iConfiguration

1

Index

Index of string descriptor describing thisconfiguration

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Universal Serial Bus Specification Revision 1.1

Table 9-8. Standard Configuration Descriptor (Continued)
Offset7

Field
bmAttributes

Size
1

Value

Description

BitmapConfiguration characteristics
D7:
D6:
D5:
D4...0:Reserved (set to one)Self-powered
Remote Wakeup
Reserved (reset to zero)

D7 is reserved and must be set to one forhistorical reasons.
A device configuration that uses power fromthe bus and a local source reports a non-zerovalue in MaxPower to indicate the amount ofbus power required and sets D6. The actual
power source at runtime may be determinedusing the GetStatus(DEVICE) request (seeSection 9.4.5).
If a device configuration supports remote
wakeup, D5 is set to one.
8

MaxPower

1

mA

Maximum power consumption of the USB
device from the bus in this specificconfiguration when the device is fully
operational. Expressed in 2mA units (i.e., 50= 100mA).
Note: a device configuration reports whether
the configuration is bus-powered or selfpowered. Device status reports whether thedevice is currently self-powered. If a device isdisconnected from its external power source,
it updates device status to indicate that it is nolonger self-powered.
A device may not increase its power draw
from the bus, when it loses its external powersource, beyond the amount reported by its
configuration.
If a device can continue to operate when
disconnected from its external power source,
it continues to do so. If the device cannot
continue to operate, it fails operations it canno longer support. The USB System Software
may determine the cause of the failure bychecking the status and noting the loss of thedevice’s power source.

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Universal Serial Bus Specification Revision 1.1

9.6.3 Interface
This descriptor describes a specific interface within a configuration. A configuration provides one or moreinterfaces, each with zero or more endpoint descriptors describing a unique set of endpoints within theconfiguration. When a configuration supports more than one interface, the endpoints for a particularinterface follow the interface descriptor in the data returned by the GetConfiguration() request. An
interface descriptor is always returned as part of a configuration descriptor. Interface descriptors cannot bedirectly accessed with a GetDescriptor() or SetDescriptor() request.
An interface may include alternate settings that allow the endpoints and/or their characteristics to be varied
after the device has been configured. The default setting for an interface is always alternate setting zero.
The SetInterface() request is used to select an alternate setting or to return to the default setting. The
GetInterface() request returns the selected alternate setting.
Alternate settings allow a portion of the device configuration to be varied while other interfaces remain in
operation. If a configuration has alternate settings for one or more of its interfaces, a separate interface
descriptor and its associated endpoints are included for each setting.
If a device configuration supported a single interface with two alternate settings, the configuration
descriptor would be followed by an interface descriptor with the bInterfaceNumber and bAlternateSetting
fields set to zero and then the endpoint descriptors for that setting, followed by another interface descriptor
and its associated endpoint descriptors. The second interface descriptor’s bInterfaceNumber field would
also be set to zero, but the bAlternateSetting field of the second interface descriptor would be set to one.
If an interface uses only endpoint zero, no endpoint descriptors follow the interface descriptor and theinterface identifies a request interface that uses the default pipe attached to endpoint zero. In this case, thebNumEndpoints field shall be set to zero.
An interface descriptor never includes endpoint zero in the number of endpoints. Table 9-9 shows thestandard interface descriptor.

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Universal Serial Bus Specification Revision 1.1

Table 9-9. Standard Interface Descriptor
Offset

Field

Size

Value

Description

0bLength1

NumberSize of this descriptor in bytes

1

bDescriptorType1

Constant

INTERFACE Descriptor Type

2

bInterfaceNumber

1

NumberNumber of interface. Zero-based value
identifying the index in the array of
concurrent interfaces supported by thisconfiguration.

3

bAlternateSetting

1

NumberValue used to select alternate setting forthe interface identified in the prior field

4

bNumEndpoints

1

NumberNumber of endpoints used by thisinterface (excluding endpoint zero). If thisvalue is zero, this interface only uses theDefault Control Pipe.

5

bInterfaceClass

1

Class

Class code (assigned by the USB).
A value of zero is reserved for future
standardization.
If this field is set to FFH, the interface
class is vendor-specific.
All other values are reserved for
assignment by the USB.

6bInterfaceSubClass

1

SubClass

Subclass code (assigned by the USB).
These codes are qualified by the value ofthe bInterfaceClass field.
If the bInterfaceClass field is reset tozero, this field must also be reset to zero.
If the bInterfaceClass field is not set to
FFH, all values are reserved for
assignment by the USB.

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Universal Serial Bus Specification Revision 1.1

Table 9-9. Standard Interface Descriptor (Continued)
Offset
7

Field

Size

bInterfaceProtocol

1

Value
ProtocolDescription
Protocol code (assigned by the USB).
These codes are qualified by the value ofthe bInterfaceClass and thebInterfaceSubClass fields. If an interface
supports class-specific requests, thiscode identifies the protocols that thedevice uses as defined by the
specification of the device class.
If this field is reset to zero, the device
does not use a class-specific protocol on
this interface.
If this field is set to FFH, the device uses
a vendor-specific protocol for thisinterface.

81

iInterface

Index

Index of string descriptor describing thisinterface

9.6.4 Endpoint
Each endpoint used for an interface has its own descriptor. This descriptor contains the information
required by the host to determine the bandwidth requirements of each endpoint. An endpoint descriptor isalways returned as part of the configuration information returned by a GetDescriptor(Configuration)
request. An endpoint descriptor cannot be directly accessed with a GetDescriptor() or SetDescriptor()
request. There is never an endpoint descriptor for endpoint zero. Table 9-10 shows the standard endpoint
descriptor.
Table 9-10. Standard Endpoint Descriptor
Offset

Field

Size

Value

Description

0bLength1

NumberSize of this descriptor in bytes

1

bDescriptorType1

Constant

ENDPOINT Descriptor Type2bEndpointAddress

1

Endpoint

The address of the endpoint on the USB
device described by this descriptor. The
address is encoded as follows:
Bit 3...0: The endpoint number
Bit 6...4: Reserved, reset to zeroBit 7:
Direction, ignored forcontrol endpoints
0 = OUT endpoint
1 = IN endpoint203

Universal Serial Bus Specification Revision 1.1

Table 9-10. Standard Endpoint Descriptor (Continued)
Offset3

Field
bmAttributes

Size
1

Value
BitmapDescription
This field describes the endpoint’s
attributes when it is configured using thebConfigurationValue.
Bit 1..0: Transfer Type00 = Control
01 = Isochronous
10 = Bulk
11 = Interrupt
All other bits are reserved.

4

wMaxPacketSize

2

NumberMaximum packet size this endpoint iscapable of sending or receiving when thisconfiguration is selected.
For isochronous endpoints, this value isused to reserve the bus time in theschedule, required for the per-frame data
payloads. The pipe may, on an ongoingbasis, actually use less bandwidth thanthat reserved. The device reports, if
necessary, the actual bandwidth used via
its normal, non-USB defined mechanisms.
For interrupt, bulk, and control endpoints,
smaller data payloads may be sent, butwill terminate the transfer and may or may
not require intervention to restart. Refer
to Chapter 5 for more information.

6

bInterval

1

NumberInterval for polling endpoint for datatransfers. Expressed in milliseconds.
This field is ignored for bulk and control
endpoints. For isochronous endpoints
this field must be set to 1. For interrupt
endpoints, this field may range from 1 to
255.

9.6.5 String
String descriptors are optional. As noted previously, if a device does not support string descriptors, allreferences to string descriptors within device, configuration, and interface descriptors must be reset to zero.
String descriptors use UNICODE encodings as defined by The Unicode Standard, Worldwide Character
Encoding, Version 1.0, Volumes 1 and 2, The Unicode Consortium, Addison-Wesley Publishing Company,
Reading, Massachusetts. The strings in a USB device may support multiple languages. When requesting astring descriptor, the requester specifies the desired language using a sixteen-bit language ID (LANGID)
defined by Microsoft for Windows as described in Developing International Software for Windows 95 andWindows NT, Nadine Kano, Microsoft Press, Redmond, Washington. String index zero for all languages
returns a string descriptor that contains an array of two-byte LANGID codes supported by the device.
Table 9-11 shows the LANGID code array. A USB device may omit all string descriptors. USB devices
that omit all string descriptors shall not return an array of LANGID codes.
204Universal Serial Bus Specification Revision 1.1

The array of LANGID codes is not NULL-terminated. The size of the array (in bytes) is computed bysubtracting two from the value of the first byte of the descriptor.
Table 9-11. Codes Representing Languages Supported by the Device
Offset

Field

Size

Value

Description

0bLength1

N+2

Size of this descriptor in bytes

1

bDescriptorType1

Constant

STRING Descriptor Type2wLANGID[0]2NumberLANGID code zero

...

...

...

...

...

N

wLANGID[x]2

NumberLANGID code x

The UNICODE string descriptor (shown in Table 9-12) is not NULL-terminated. The string length iscomputed by subtracting two from the value of the first byte of the descriptor.
Table 9-12. UNICODE String Descriptor
Offset

9.7

Field

Size

Value

Description

0bLength1

NumberSize of this descriptor in bytes

1

bDescriptorType1

Constant

STRING Descriptor Type2bStringN

NumberUNICODE encoded string

Device Class Definitions
All devices must support the requests and descriptor definitions described in this chapter. Most devices
provide additional requests and, possibly, descriptors for device-specific extensions. In addition, devices
may provide extended services that are common to a group of devices. In order to define a class ofdevices, the following information must be provided to completely define the appearance and behavior ofthe device class.

9.7.1 Descriptors
If the class requires any specific definition of the standard descriptors, the class definition must include
those requirements as part of the class definition. In addition, if the class defines a standard extended set ofdescriptors, they must also be fully defined in the class definition. Any extended descriptor definitions
should follow the approach used for standard descriptors; for example, all descriptors should begin with alength field.

9.7.2 Interface(s) and Endpoint Usage
When a class of devices is standardized, the interfaces used by the devices, including how endpoints areused, must be included in the device class definition. Devices may further extend a class definition with
proprietary features as long as they meet the base definition of the class.

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9.7.3 Requests
All of the requests specific to the class must be defined.

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Chapter 10
USB Host: Hardware and Software
The USB interconnect supports data traffic between a host and a USB device. This chapter describes thehost interfaces necessary to facilitate USB communication between a software client, resident on the host,and a function implemented on a device. The implementation described in this chapter is not required. This
implementation is provided as an example to illustrate the host system behavior expected by a USB device.
A host system may provide a different host software implementation as long as a USB device experiences
the shame host behavior.

10.1 Overview of the USB Host
10.1.1 Overview
The basic flow and interrelationships of the USB communications model are shown in Figure 10-1
Host

ClientUSB System

USB BusInterface

Interconnect

DeviceFunction

USB Device

USB BusInterface
Actual communications flow
Logical communications flow

Figure 10-1. Interlayer Communications Model
The host and the device are divided into the distinct layers depicted in Figure 10-1. Vertical arrows
indicate the actual communication on the host. The corresponding interfaces on the device are
implementation-specific. All communications between the host and device ultimately occur on thephysical USB wire. However, there are logical host-device interfaces between each horizontal layer.
These communications, between client software resident on the host and the function provided by thedevice, are typified by a contract based on the needs of the application currently using the device and thecapabilities provided by the device.
This client-function interaction creates the requirements for all of the underlying layers and their interfaces.

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This chapter describes this model from the point of view of the host and its layers. Figure 10-2 describes,
based on the overall view introduced in Chapter 5, the host’s view of its communication with the device.

Host

Interconnect

Clientmanages interfaces

Pipe Bundle
to an interface

Configuration

IRPs

Host
SoftwareUSB DriverDefault Pipe

HC Driverto Endpoint ZeroUSB Systemmanages pipes

HW-Defined

Host
Controller

HCDefined

SIE

USB Wire
USB BusInterface
Pipe: Represents connection
abstraction between two horizontal
layers
Optional
ComponentInterprocess Communication

Figure 10-2. Host Communications

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There is only one host for each USB. The major layers of a host consist of the following:USB bus interface



USB SystemClient.

The USB bus interface handles interactions for the electrical and protocol layers (refer to Chapter 7 andChapter 8). From the interconnect point of view, a similar USB bus interface is provided by both the USB
device and the host, as exemplified by the Serial Interface Engine (SIE). On the host, however, the USB
bus interface has additional responsibilities due to the unique role of the host on the USB and is
implemented as the Host Controller. The Host Controller has an integrated root hub providing attachment
points to the USB wire.
The USB System uses the Host Controller to manage data transfers between the host and USB devices.
The interface between the USB System and the Host Controller is dependent on the hardware definition of
the Host Controller. The USB System, in concert with the Host Controller, performs the translation
between the client’s view of data transfers and the USB transactions appearing on the interconnect. This
includes the addition of any USB feature support such as protocol wrappers. The USB System is also
responsible for managing USB resources, such as bandwidth and bus power, so that client access to the
USB is possible.
The USB System has three basic components:

Host Controller Driver

USB Driver

Host Software.

The Host Controller Driver (HCD) exists to more easily map the various Host Controller implementations
into the USB System, such that a client can interact with its device without knowing to which Host
Controller the device is connected. The USB Driver (USBD) provides the basic host interface (USBDI) for
clients to USB devices. The interface between the HCD and the USBD is known as the Host Controller
Driver Interface (HCDI). This interface is never available directly to clients and thus is not defined by the
USB Specification. A particular HCDI is, however, defined by each operating system that supports various
Host Controller implementations.
The USBD provides data transfer mechanisms in the form of I/O Request Packets (IRPs), which consist ofa request to transport data across a specific pipe. In addition to providing data transfer mechanisms, theUSBD is responsible for presenting to its clients an abstraction of a USB device that can be manipulated for
configuration and state management. As part of this abstraction, the USBD owns the default pipe (seeChapter 5 and Chapter 9) through which all USB devices are accessed for the purposes of standard USB
control. This default pipe represents a logical communication between the USBD and the abstraction of a
USB device as shown in Figure 10-2.
In some operating systems, additional non-USB System Software is available that provides configurationand loading mechanisms to device drivers. In such operating systems, the device driver shall use theprovided interfaces instead of directly accessing the USBDI mechanisms.
The client layer describes all the software entities that are responsible for directly interacting with USB
devices. When each device is attached to the system, these clients might interact directly with theperipheral hardware. The shared characteristics of the USB place USB System Software between the clientand its device; that is, a client cannot directly access the device’s hardware.

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Overall, the host layers provide the following capabilities:

Detecting the attachment and removal of USB devices

Managing USB standard control flow between the host and USB devices

Managing data flow between the host and USB devices

Collecting status and activity statistics

Controlling the electrical interface between the Host Controller and USB devices, including theprovision of a limited amount of power.

The following sections describe these responsibilities and the requirements placed on the USBDI in greater
detail. The actual interfaces used for a specific combination of host platform and operating system aredescribed in the appropriate operating system environment guide.
All hubs (see Chapter 11) report internal status changes and their port change status via the status change
pipe. This includes a notification of when a USB device is attached to or removed from one of their ports.
A USBD client generically known as the hub driver receives these notifications as owner of the hub’s
Status Change pipe. For device attachments, the hub driver then initiates the device configuration process.
In some systems, this hub driver is a part of the host software provided by the operating system for
managing devices.

10.1.2 Control Mechanisms
Control information may be passed between the host and a USB device using in-band or out-of-band
signaling. In-band signaling mixes control information with data in a pipe outside the awareness of thehost. Out-of-band signaling places control information in a separate pipe.
There is a message pipe called the default pipe for each attached USB device. This logical association
between a host and a USB device is used for USB standard control flow such as device enumeration andconfiguration. The default pipe provides a standard interface to all USB devices. The default pipe may
also be used for device-specific communications, as mediated by the USBD, which owns the default pipesof all of the USB devices.
A particular USB device may allow the use of additional message pipes to transfer device-specific control
information. These pipes use the same communications protocol as the default pipe, but the informationtransferred is specific to the USB device and is not standardized by the USB Specification.
The USBD supports the sharing of the default pipe, which it owns and uses, with its clients. It also
provides access to any other control pipes associated with the device.

10.1.3 Data Flow
The Host Controller is responsible for transferring streams of data between the host and USB devices.
These data transfers are treated as a continuous stream of bytes. The USB supports four basic types of datatransfers:
Control transfers



Isochronous transfersInterrupt transfers

Bulk transfers.

For additional information on transfer types, refer to Chapter 5.
Each device presents one or more interfaces that a client may use to communicate with the device. Eachinterface is composed of zero or more pipes that individually transfer data between the client and aparticular endpoint on the device. The USBD establishes interfaces and pipes at the explicit request of the
Host Software. The Host Controller provides service based on parameters provided by the Host Software
when the configuration request is made.
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A pipe has several characteristics based on the delivery requirements of the data to be transferred.
Examples of these characteristics include the following:

the rate at which data needs to be transferred

whether data is provided at a steady rate or sporadically

how long data may be delayed before delivery

whether the loss of data being transferred is catastrophic.

A USB device endpoint describes the characteristics required for a specific pipe. Endpoints are describedas part of a USB device’s characterization information. For additional details, refer to Chapter 9.

10.1.4 Collecting Status and Activity Statistics
As a common communicant for all control and data transfers between the host and USB devices, the USB
System and the Host Controller are well-positioned to track status and activity information. Suchinformation is provided upon request to the Host Software, allowing that software to manage status andactivity information. This specification does not identify any specific information that should be tracked or
require any particular format for reporting activity and status information.

10.1.5 Electrical Interface Considerations
The host provides power to USB devices attached to the root hub. The amount of power provided by a port
is specified in Chapter 7.

10.2 Host Controller Requirements
In all implementations, Host Controllers perform the same basic duties with regard to the USB and its
attached devices. These basic duties are described below.
The Host Controller has requirements from both the host and the USB. The following is a brief overview
of the functionality provided. Each capability is discussed in detail in subsequent sections.
State Handling

As a component of the host, the Host Controller reports and manages
its states.

Serializer/Deserializer

For data transmitted from the host, the Host Controller converts
protocol and data information from its native format to a bit stream
transmitted on the USB. For data being received into the host, thereverse operation is performed.

Frame Generation

The Host Controller produces SOF tokens at a period of 1ms.

Data Processing

The Host Controller processes requests for data transmission to andfrom the host.

Protocol Engine

The Host Controller supports the protocol specified by the USB.

Transmission Error
Handling

All Host Controllers exhibit the same behavior when detecting andreacting to the defined error categories.

Remote Wakeup

All host controlers must have the ability to place the bus into theSuspended state and to respond to bus wakeup events.

Root Hub

The root hub provides standard hub function to link the Host
Controller to one or more USB ports.

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Host System Interface

Provides a high-speed data path between the Host Controller and hostsystem.

The following sections present a more detailed discussion of the required capabilities of the Host
Controller.

10.2.1 State Handling
The Host Controller has a series of states that the USB System manages. Additionally, the Host Controller
provides the interface to the following two areas of USB-relevant state:State change propagation

Root hub.

The root hub presents to the hub driver the same standard states as other USB devices. The Host Controller
supports these states and their transitions for the hub. For detailed discussions of USB states, including
their interrelations and transitions, refer to Chapter 9.
The overall state of the Host Controller is inextricably linked with that of the root hub and of the overall
USB. Any Host Controller state changes that are visible to attached devices must be reflected in thecorresponding device state change information such that the resulting Host Controller and device states are
consistent.
USB devices request a wakeup through the use of resume signaling (refer to Chapter 7), devices to return totheir configured state. The Host Controller itself may cause a resume event through the same signaling
method. The Host Controller must notify the rest of the host of a resume event through a mechanism ormechanisms specific to that system’s implementation.

10.2.2 Serializer/Deserializer
The actual transmission of data across the physical USB takes places as a serial bit stream. A SerialInterface Engine (SIE), whether implemented as part of the host or a USB device, handles the serializationand deserialization of USB transmissions. On the host, this SIE is part of the Host Controller.

10.2.3 Frame Generation
It is the Host Controller’s responsibility to partition USB time into 1ms quantities called “frames.” Frames
are created by the Host Controller through issuing Start-of-Frame (SOF) tokens at 1.00ms intervals asshown in Figure 10-3. The SOF token is the first transmission in the frame period. After issuing a SOF
token, the Host Controller is free to transmit other transactions for the remainder of the frame period.
When the Host Controller is in its normal operating state, SOF tokens must be continuously generated at
the 1ms periodic rate, regardless of the other bus activity or lack thereof. If the Host Controller enters astate where it is not providing power on the bus , it must not generate SOFs. When the Host Controller is
not generating SOFs, it may enter a power-reduced state.

Frame N-1
SOF

Frame NSOF

EOF Interval (Frame N-1)

Frame N+1
SOF

EOF Interval (Frame N)

SOF
EOF Interval (Frame N+1)

Figure 10-3. Frame Creation
The SOF token holds the highest priority access to the bus. Babble circuitry in hubs electrically isolates
any active transmitters during the End-of-Frame (EOF) interval, providing an idle bus for the SOF
transmission.

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The Host Controller must allow the length of the USB frame to be adjusted by ±1 bit time (refer to Section
10.5.3.2.4). The Host Controller maintains the current frame number that may be read by the USB System.
The following apply to the current frame number:

Used to uniquely identify one frame from another

Incremented at the end of every frame period

Valid through the subsequent frame.

The host transmits the lower 11 bits of the current frame number in each SOF token transmission. Whenrequested from the Host Controller, the current frame number is the frame number in existence at the time
the request was fulfilled. The current frame number as returned by the host (Host Controller or HCD) is atleast 32 bits, although the Host Controller itself is not required to maintain more than 11 bits.
The Host Controller shall cease transmission during the EOF interval. When the EOF interval begins, anytransactions scheduled specifically for the frame that has just passed are retired. If the Host Controller is
executing a transaction at the time the EOF interval is encountered, the Host Controller terminates thetransaction.

10.2.4 Data Processing
The Host Controller is responsible for receiving data from the USB System and sending it to the USB andfor receiving data from the USB and sending it to the USB System. The particular format used for the data
communications between the USB System and the Host Controller is implementation specific, within therules for transfer behavior described in Chapter 5.

10.2.5 Protocol Engine
The Host Controller manages the USB protocol level interface. It inserts the appropriate protocol
information for outgoing transmissions. It also strips and interprets, as appropriate, the incoming protocol
information.

10.2.6 Transmission Error Handling
The Host Controller must be capable of detecting the following transmission error conditions, which aredefined from the host’s point of view:Timeout conditions after a host-transmitted token or packet. These errors occur when the addressed
endpoint is unresponsive or when the structure of the transmission is so badly damaged that thetargeted endpoint does not recognize it.

Data errors resulting in missing or invalid transmissions:

The Host Controller sends or receives a packet shorter than that required for the transmission; forexample, a transmission extending beyond EOF or a lack of resources available to the Host
Controller.



An invalid CRC field on a received data packet.

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Protocol errors:

An invalid handshake PID, such as a malformed or inappropriate handshakeA false EOP

A bit stuffing error.

For each bulk, command, and interrupt transaction, the host must maintain an error count tally. Errors
result from the conditions described above, not as a result of an endpoint NAKing a request. This value
reflects the number of times the transaction has encountered a transmission error. If the error count tallyfor a given transaction reaches three, the host retires the transfer. When a transfer is retired due toexcessive errors, the last error type will be indicated. Isochronous transactions are attempted only once,
regardless of outcome, and, therefore, no error count is maintained for this type.

10.2.7 Remote Wakeup
If USB System wishes to place the bus in the Suspended state, it commands the Host Controller to stop all
bus traffic, including SOFs. This causes all USB devices to enter the Suspended state. In this state, the
USB System may enable the Host Controller to respond to bus wakeup events. This allows the Host
Controller to respond to bus wakeup signaling to restart the host system.

10.2.8 Root Hub
The root hub provides the connection between the Host Controller and one or more USB ports. The root
hub provides the same functionality as other hubs (See Chapter 11), except that the hardware and softwareinterface between the root hub and the Host Controller is defined by the specific hardware implementation.

10.2.8.1 Port Resets
Section 7.1.7.3 describes the requirements of a hub to ensure all upstream resume attempts are
overpowered with a long reset downstream. Root hubs may provide an aggregate reset period of at least
50ms. If the reset duration is controlled in hardware and the hardware timer is <50ms, the USB System canissue several consecutive resets to accumulate a sufficiently long reset to the device.

10.2.9 Host System Interface
The Host Controller provides a high-speed bus-mastering interface to and from main system memory. The
physical transfer between memory and the USB wire is performed automatically by the Host Controller.
When data buffers need to be filled or emptied, the Host Controller informs the USB System.

10.3 Overview of Software Mechanisms
The HCD and the USBD present software interfaces based on different levels of abstraction. They are,however, expected to operate together in a specified manner to satisfy the overall requirements of the USB
System (see Figure 10-2). The requirements for the USB System are expressed primarily as requirements
for the USBDI. The division of duties between the USBD and the HCD is not defined. However, the one
requirement of the HCDI that must be met is that it supports, in the specified operating system context,
multiple Host Controller implementations.
The HCD provides an abstraction of the Host Controller and an abstraction of the Host Controller’s view ofdata transfer across the USB. The USBD provides an abstraction of the USB device and of the datatransfers between the client of the USBD and the function on the USB device. Overall, the USB Systemacts as a facilitator for transmitting data between the client and the function and as a control point for theUSB-specific interfaces of the USB device. As part of facilitating data transfer, the USB System provides
buffer management capabilities and allows the synchronization of the data transmittal to the needs of theclient and the function.

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The specific requirements for the USBDI are described later in this chapter. The exact functions that fulfill
these requirements are described in the relevant operating system environment guide for the HCDI and the
USBDI. The procedures involved in accomplishing data transfers via the USBDI are described in thefollowing sections.

10.3.1 Device Configuration
Different operating system environments perform device configuration using different software
components and different sequences of events. The USB System does not assume a specific operating
system method. However, there are some basic requirements that must be fulfilled by any USB System
implementation. In some operating systems existing host software provides these requirements. In others,
the USB System provides the capabilities.
The USB System assumes a specialized client of the USBD, called a hub driver, that acts as aclearinghouse for the addition and removal of devices from a particular hub. Once the hub driver receives
such notifications, it will employ additional host software and other USBD clients, in an operating system
specific manner, to recognize and configure the device. This model, shown in Figure 10-4, is the basis of
the following discussion.

DeviceDriverHost Software
Configuration
Support

Hub
DriverOptional
Component

Configuration
Control

USBD

Optional
Configuration
Control

HCD

Figure 10-4. Configuration Interactions
When a device is attached, the hub driver receives a notification from the hub detecting the change. The
hub driver, using the information provided by the hub, requests a device identifier from the USBD. The
USBD in turn sets up the default pipe for that device and returns a device identifier to the hub driver.

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The device is now ready to be configured for use. For each device, there are three configurations that must
be complete before that device is ready for use:
1.

Device Configuration: This includes setting up all of the device’s USB parameters and allocating all
USB host resources that are visible to the device. This is accomplished by setting the configuration
value on the device. A limited set of configuration changes, such as alternate settings, is allowed
without totally reconfiguring the device. Once the device is configured, it is, from its point of view,
ready for use.

2.

USB Configuration: In order to actually create a USBD pipe ready for use by a client, additional USB
information, not visible to the device, must be specified by the client. This information, known as the
Policy for the pipe, describes how the client will use the pipe. This includes such items as the
maximum amount of data the client will transfer with one IRP, the maximum service interval the client
will use, the client’s notification identification, and so on.

3.

Function Configuration: Once configuration types 1 and 2 have been accomplished, the pipe iscompletely ready for use from the USB’s point of view. However, additional vendor- or class-specificsetup may be required before the client can actually use the pipe. This configuration is a private matter
between the device and the client and is not standardized by the USBD.

The following paragraphs describe the device and USB configuration requirements.
The responsible configuring software performs the actual device configuration. Depending on theparticular operating system implementation, the software responsible for configuration can include thefollowing:The hub driver

Other host software

A device driver.

The configuring software first reads the device descriptor, then requests the description for each possible
configuration. It may use the information provided to load a particular client, such as a device driver,which initially interacts with the device. The configuring software, perhaps with input from that device
driver, chooses a configuration for the device. Setting the device configuration sets up all of the endpoints
on the device and returns a collection of interfaces to be used for data transfer by USBD clients. Eachinterface is a collection of pipes owned by a single client.
This initial configuration uses the default settings for interfaces and the default bandwidth for each
endpoint. A USBD implementation may additionally allow the client to specify alternate interfaces whenselecting the initial configuration. The USB System will verify that the resources required for the supportof the endpoint are available and, if so, will allocate the bandwidth required. Refer to Section 10.3.2 for a
discussion of resource management.
The device is now configured, but the created pipes are not yet ready for use. The USB configuration isaccomplished when the client initializes each pipe by setting a Policy to specify how it will interact withthe pipe. Among the information specified is the client’s maximum service interval and notification
information. Among the actions taken by the USB System, as a result of setting the Policy, is determining
the amount of buffer working space required beyond the data buffer space provided by the client. The sizeof the buffers required is based upon the usage chosen by the client and upon the per-transfer needs of the
USB System.
The client receives notifications when IRPs complete, successfully or due to errors. The client may also
wake up independently of USB notification to check the status of pending IRPs.
The client may also choose to make configuration modifications, such as enabling an alternate setting for
an interface or changing the bandwidth allocated to a particular pipe. In order to perform these changes,
the interface or pipe, respectively, must be idle.

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10.3.2 Resource Management
Whenever a pipe is setup by the USBD for a given endpoint, the USB System must determine if it can
support the pipe. The USB System makes this determination based on the requirements stated in theendpoint descriptor. One of the endpoint requirements, which must be supported in order to create a pipe
for an endpoint, is the bandwidth necessary for that endpoint’s transfers. There are two stages to check foravailable bandwidth. First the maximum execution time for a transaction is calculated. Then, the frame
schedule is consulted to determine if the indicated transaction will fit.
The allocation of the guaranteed bandwidth for isochronous and interrupt pipes, and the determination ofwhether a particular control or bulk transaction will fit into a given frame, can be determined by a software
heuristic in the USB System. If the actual transaction execution time in the Host Controller exceeds theheuristically determined value, the Host Controller is responsible for ensuring that frame integrity is
maintained (refer to Section 10.2.3). The following discussion describes the requirements for the USB
System heuristic.
In order to determine if bandwidth can be allocated, or if a transaction can be fit into a particular frame, themaximum transaction execution time must be calculated. The calculation of the maximum transaction
execution time requires that the following information be provided. (Note that an agent other than theclient may provide some of this information.)

Number of data bytes (wMaxPacketSize) to be transmitted.

Transfer type.



Depth in the topology. If less precision is allowed, the maximum topology depth may be assumed.

This calculation must include the bit transmission time, the signal propagation delay through the topology,
and any implementation-specific delays, such as preparation or recovery time required by the Host
Controller itself. Refer to Chapter 5 for examples of formulas that can be used for such calculations.

10.3.3 Data Transfers
The basis for all client-function communication is the interface: a bundle of related pipes associated with aparticular USB device.
Exactly one client on the host manages a given interface. The client initializes each pipe of an interface bysetting the Policy for that pipe. This includes the maximum amount of data to be transmitted per IRP and
the maximum service interval for the pipe. A service interval is stated in milliseconds and describes theinterval over which an IRP’s data will be transmitted for an isochronous pipe. It describes the pollinginterval for an interrupt pipe. The client is notified when a specified request is completed. The client
manages the size of each IRP such that its duty cycle and latency constraints are maintained. Additional
Policy information includes the notification information for the client.
The client provides the buffer space required to hold the transmitted data. The USB System uses the Policy
to determine the additional working space it will require.
The client views its data as a contiguous serial stream, which it manages in a similar manner to thosestreams provided over other types of bus technologies. Internally, the USB System may, depending on its
own Policy and any Host Controller constraints, break the client request down into smaller requests to besent across the USB. However, two requirements must be met whenever the USB System chooses toundertake such division:The division of the data stream into smaller chunks is not visible to the client.

USB samples are not split across bus transactions.

When a client wishes to transfer data, it will send an IRP to the USBD. Depending on the direction of datatransfer, a full or empty data buffer will be provided. When the request is complete (successfully or due toan error condition), the IRP and its status is returned to the client. Where relevant, this status is also
provided on a per-transaction basis.

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10.3.4 Common Data Definitions
In order to allow the client to receive request results as directly as possible from its device, it is desirable tominimize the amount of processing and copying required between the device and the client. To facilitate
this, some control aspects of the IRP are standardized such that different layers in the stack may directly
use the information provided by the client. The particular format for this data is dependent on theactualization of the USBDI in the operating system. Some data elements may in fact not be directly visible
to the client at all, but are generated as a result of the client request.
The following data elements define the relevant information for a request:

Identification of the pipe associated with the request. Identifying this pipe also describes informationsuch as transfer type for this request.

Notification identification for the particular client.

Location and length of data buffer that is to be transmitted or received.

Completion status for the request. Both the summary status and, as required, detailed per-transaction
status must be provided.



Location and length of working space. This is implementation-ependent.

The actual mechanisms used to communicate requests to the USBD are operating system-pecific.
However, beyond the requirements stated above for what request-related information must be available,
there are also requirements on how requests will be processed. The basic requirements are described in
Chapter 5. Additionally, the USBD provides a mechanism to designate a group of isochronous IRPs forwhich the transmission of the first transaction of each IRP will occur in the same frame. The USBD also
provides a mechanism for designating an uninterruptable set of vendor- or class-specific requests to adefault pipe. No other requests to that default pipe, including standard, class, or vendor request may beinserted in the execution flow for such an uninterruptable set. If any request in this set fails, the entire set isretired.

10.4 Host Controller Driver
The Host Controller Driver (HCD) is an abstraction of Host Controller hardware and the Host Controller’sview of data transmission over the USB. The HCDI meets the following requirements:
Provides an abstraction of the Host Controller hardware.

Provides an abstraction for data transfers by the Host Controller across the USB interconnect.

Provides an abstraction for the allocation (and de-allocation) of Host Controller resources, to support
guaranteed service to USB devices.



Presents the root hub and its behavior according to the hub class definition. This includes supporting
the root hub such that the hub driver interacts with the root hub exactly as it would for any hub. In
particular, even though a root hub can be implemented in a combination of hardware and software, theroot hub responds initially to the default device address (from a client perspective), returns descriptor
information, supports having its device address set, and supports the other hub class requests.
However, bus transactions may or may not need to be generated to accomplish this behavior given theclose integration possible between the Host Controller and the root hub.

The HCD provides a software interface (HCDI) that implements the required abstractions. The function of
the HCD is to provide an abstraction, which hides the details of the Host Controller hardware. Below the
Host Controller hardware is the physical USB and all the attached USB devices.
The HCD is the lowest tier in the USB software stack. The HCD has only one client: the Universal Serial
Bus Driver (USBD). The USBD maps requests from many clients to the appropriate HCD. A given HCD
may manage many Host Controllers.
The HCDI is not directly accessible from a client. Therefore, the specific interface requirements for the
HCDI are not discussed here.

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10.5 Universal Serial Bus Driver
The USBD provides a collection of mechanisms that operating system components, typically device
drivers, use to access USB devices. The only access to a USB device is that provided by the USBD. The
USBD implementations are operating system-specific. The mechanisms provided by the USBD are
implemented using as appropriate and augmenting as necessary the mechanisms provided by the operating
system environment in which the USB runs. The following discussion centers on the basic capabilities
required for all USBD implementations. For specifics of the USBD operation within a specificenvironment, see the relevant operating system environment guide for the USBD. A single instance of the
USBD directs accesses to one or more HCDs that in turn connect to one or more Host Controllers. If
allowed, how USBD instancing is managed is dependent upon the operating system environment.
However, from the client’s point of view, the USBD with which the client communicates manages all of
the attached USB devices.

10.5.1 USBD Overview
Clients of USBD direct commands to devices or move streams of data to or from pipes. The USBD
presents two groups of software mechanisms to clients: command mechanisms and pipe mechanisms.
Command mechanisms allow clients to configure and control USBD operation as well as to configure andgenerically control a USB device. In particular, command mechanisms provide all access to the device’s
default pipe.
Pipe mechanisms allow a USBD client to manage device specific data and control transfers. Pipemechanisms do not allow a client to directly address the device’s default pipe.

Pipe Interfaces

Power Control

Bus and Device
Management

Device Data
Access(Hub)Interrupt Transfer

MessageandStreamPipe
AccessConfiguration
Management

Figure 10-5 presents an overview of the USBD structure.

Command Interfaces
Services

Universal Serial Bus Driver

Host
Controller
DriverUSB Host
Controller

Host
Controller
DriverUSB Host
Controller

Figure 10-5. Universal Serial Bus Driver Structure

10.5.1.1 USBD Initialization
Specific USBD initialization is operating system-dependent. When a particular USB managed by USBD is
initialized, the management information for that USB is also created. Part of this management informationis the default address device and its default pipe.

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When a device is attached to a USB, it responds to a special address known as the default address (refer to
Chapter 9) until its unique address is assigned by the bus enumerator. In order for the USB System tointeract with the new device, the default device address and the device’s default pipe must be available tothe hub driver whenr a device is attached. During device initialization, the default address is changed to aunique address.

10.5.1.2 USBD Pipe Usage
Pipes are the method by which a device endpoint is associated with a Host Software entity. Pipes are
owned by exactly one such entity on the host. Although the basic concept of a pipe is the same no matter
who the owner, some distinction of capabilities provided to the USBD client occurs between two groups ofpipes:


Default pipes, which are owned and managed by the USBDAll other pipes, which are owned and managed by clients of the USBD.

Default pipes are never directly accessed by clients, although they are often used to fulfill some part ofclient requests relayed via command mechanisms.

10.5.1.2.1 Default Pipes
The USBD is responsible for allocating and managing appropriate buffering to support transfers on thedefault pipe that are not directly visible to the client such as setting a device address. For those transfersthat are directly visible to the client, such as sending vendor and class commands or reading a device
descriptor, the client must provide the required buffering.

10.5.1.2.2 Client Pipes
Any pipe not owned and managed by the USBD can be owned and managed by a USBD client. From the
USBD viewpoint, a single client owns the pipe. In fact, a cooperative group of clients can manage the pipe,
provided they behave as a single coordinated entity when using the pipe.
The client is responsible for providing the amount of buffering it needs to service the data transfer rate of
the pipe within a service interval attainable by the client. Additional buffering requirements for working
space are specified by the USB System.

10.5.1.3 USBD Service Capabilities
The USBD provides services in the following categories:

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Configuration via command mechanisms

Transfer services via both command and pipe mechanisms

Event notification

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10.5.2 USBD Command Mechanism Requirements
USBD command mechanisms allow a client generic access to a USB device. Generally, these commands
allow the client to make read or write accesses to one of potentially several device data and control spaces.
The client provides as little as a device identifier and the relevant data or empty buffer pointer.
USBD command transfers do not require that the USB device be configured. Many of the device
configuration facilities provided by the USBD are command transfers.
Following are the specific requirements on the command mechanisms provided.

10.5.2.1 Interface State Control
USBD clients must be able to set a specified interface to any settable pipe state. Setting an interface state
results in all of the pipes in that interface moving to that state. Additionally, all of the pipes in an interface
may be reset or aborted.

10.5.2.2 Pipe State Control
USBD pipe state has two components:

Host status

Reflected endpoint status.

Whenever the pipe status is reported, the value for both components will be identified. The pipe statusreflected from the endpoint is the result of the endpoint being in a particular state. The USBD client
manages the pipe state as reported by the USBD. For any pipe state reflected from the endpoint, the client
must also interact with the endpoint to change the state.
A USBD pipe is in exactly one of the following states:
Active: The pipe’s Policy has been set and the pipe is able to transmit data. The client can query as towhether any IRPs are outstanding for a particular pipe. Pipes for which there are no outstanding IRPs
are still considered to be in the Active state as long as they are able to accept new IRPs.



Halted: An error has occurred on the pipe. This state may also be a reflection of the corresponding
Halted endpoint on the device.

A pipe and endpoint are considered active when the device is configured and the pipe and/or endpoint is
not stalled. Clients may manipulate pipe state in the following ways:

Aborting a Pipe: All of the IRPs scheduled for a pipe are retired immediately and returned to the clientwith a status indicating they have been aborted. Neither the host state nor the reflected endpoint stateof the pipe is affected.

Resetting a Pipe: The pipe’s IRPs are aborted. The host state is moved to Active. If the reflected
endpoint state needs to be changed, that must be commanded explicitly by the USBD client.



Clearing a Halted pipe: The pipe's state is cleared from Halted to Active.

Halting a pipe: The pipe's state is set to Halted.

10.5.2.3 Getting Descriptors
The USBDI must provide a mechanism to retrieve standard device, configuration and string descriptors, aswell as any class- or vendor-specific descriptors.

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10.5.2.4 Getting Current Configuration Settings
The USBDI must provide a facility to return, for any specified device, the current configuration descriptor.
If the device is not configured, no configuration descriptor is returned. This action is equivalent toreturning the configuration descriptor for the current configuration by requesting the specific configuration
descriptor. It does not, however, require the client to know the identifier for the current configuration.
This will return all of the configuration information, including the following:All of the configuration descriptor information as stored on the device, including all of the alternate
settings for all of the interfaces

Indicators for which of the alternate settings for interfaces are activePipe handles for endpoints in the active alternate settings for interfacesActual wMaxPacketSize values for endpoints in the active alternate settings for interfaces.

Additionally, for any specified pipe, the USBDI must provide a facility to return the wMaxPacketSize thatis currently being used by the pipe.

10.5.2.5 Adding Devices
The USBDI must provide a mechanism for the hub driver to inform USBD of the addition of a new device
to a specified USB and to retrieve the USBD ID of the new USB device. The USBD tasks include
assigning the device address and preparing the device’s default pipe for use.

10.5.2.6 Removing Devices
The USBDI must provide a facility for the hub driver to inform the USBD that a specific device has been
removed.

10.5.2.7 Managing Status
The USBDI must provide a mechanism for obtaining and clearing device-based status, on a device,interface, or pipe basis.

10.5.2.8 Sending Class Commands
This USBDI mechanism is used by a client, typically a class-specific or adaptive driver, to send one ormore class-specific commands to a device.

10.5.2.9 Sending Vendor Commands
This USBDI mechanism is used by a client to send one or more vendor-specific commands to a device.

10.5.2.10 Establishing Alternate Settings
The USBDI must provide a mechanism to change the alternate setting for a specified interface. As a result,the pipe handles for the previous setting are released and new pipe handles for the interface are returned.
For this request to succeed the interface must be idle; i.e., no data buffers may be queued for any pipes in
the interface.

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10.5.2.11 Establishing a Configuration
Configuring software requests a configuration by passing a buffer containing the configuration information
to the USBD. The USBD requests resources for the endpoints in the configuration, and if all resource
requests succeed, the USBD sets the device configuration and returns interface handles with corresponding
pipe handles for all of the active endpoints. The default values are used for all alternate settings forinterfaces.
Note: the interface implementing the configuration may require specific alternate settings to be identified.

10.5.2.12 Setting Descriptors
For devices supporting this behavior, the USBDI allows existing descriptors to be updated or new
descriptors to be added.

10.5.3 USBD Pipe Mechanisms
This part of the USBDI offers clients the highest-speed, lowest overhead data transfer services possible.
Higher performance is achieved by shifting some pipe management responsibilities from the USBD to theclient. As a result, the pipe mechanisms are implemented at a more primitive level than the data transfer
services provided by the USBD command mechanisms. Pipe mechanisms do not allow access to a device’s
default pipe.
USBD pipe transfers are available only after both the device and USB configuration have completed
successfully. At the time the device is configured, the USBD requests the resources required to support all
device pipes in the configuration. Clients are allowed to modify the configuration, constrained by whether
the specified interface or pipe is idle.
Clients provide full buffers to outgoing pipes and retrieve transfer status information following thecompletion of a request. The transfer status returned for an outgoing pipe allows the client to determine thesuccess or failure of the transfer.
Clients provide empty buffers to incoming pipes and retrieve the filled buffers and transfer status
information from incoming pipes following the completion of a request. The transfer status returned for an
incoming pipe allows a client to determine the amount and the quality of the data received.

10.5.3.1 Supported Pipe Types
The four types of pipes supported, based on the four transfer types, are described in the following sections.

10.5.3.1.1 Isochronous Data Transfers
Each buffer queued for an isochronous pipe is required to be viewable as a stream of samples. As with all
pipe transfers, the client establishes a Policy for using this isochronous pipe, including the relevant service
interval for this client. Lost or missing bytes, which are detected on input, and transmission problems,
which are noted on output, are indicated to the client.
The client queues a first buffer, starting the pipe streaming service. To maintain the continuous streamingtransfer model used in all isochronous transfers, the client queues an additional buffer before the current
buffer is retired.
The USBD is required to be able to provide a sample stream view of the client’s data stream. In otherwords, using the client’s specified method of synchronization, the precise packetization of the data ishidden from the client. Additionally, a given transaction is always contained completely within some client
data buffer.
For an output pipe, the client provides a buffer of data. The USBD allocates the data across the frames forthe service period using the client’s chosen method of synchronization.

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For an input pipe, the client must provide an empty buffer large enough to hold the maximum number ofbytes the client’s device will deliver in the service period. Where missing or invalid bytes are indicated, theUSBD may leave the space that the bytes would have occupied in place in the buffer and identify the error.
One of the consequences of using no synchronization method is that this reserved space is assumed to be
the maximum packet size. The buffer-retired notification occurs when the IRP completes. Note that the
input buffer need not be full when returned to the client.
The USBD may optionally provide additional views of isochronous data streams. The USBD is also
required to be able to provide a packet stream view of the client’s data stream.

10.5.3.1.2 Interrupt Transfers
The Interrupt out transfer originates in the client of the USBD and is delivered to the USB device. The
Interrupt in transfer originates in a USB device and is delivered to a client of the USBD. The USB Systemguarantees that the transfers meet the maximum latency specified by the USB endpoint descriptor.
The client queues a buffer large enough to hold the interrupt transfer data (typically a single USBtransaction). When all of the data is transferred, or if the error threshold is exceeded, the IRP is returned tothe client.

10.5.3.1.3 Bulk Transfers
Bulk transfers may originate either from the device or the client. No periodicity or guaranteed latency is
assumed. When all of the data is transferred, or if the error threshold is exceeded, the IRP is returned to theclient.

10.5.3.1.4 Control Transfers
All message pipes transfer data in both directions. In all cases, the client outputs a setup stage to the device
endpoint. The optional data stage may be either input or output and the final status is always logicallypresented to the host. For details of the defined message protocol, refer to Chapter 8.
The client prepares a buffer specifying the command phase and any optional data or empty buffer space.
The client receives a buffer-retired notification when all phases of the control transfer are complete, or anerror notification, if the transfer is aborted due to transmission error.

10.5.3.2 USBD Pipe Mechanism Requirements
The following pipe mechanisms are provided.

10.5.3.2.1 Aborting IRPs
The USBDI must allow IRPs for a particular pipe to be aborted.

10.5.3.2.2 Managing Pipe Policy
The USBDI must allow a client to set and clear the Policy for an individual pipe or for an entire interface.
Any IRPs made by the client prior to successfully setting a Policy are rejected by the USBD.

10.5.3.2.3 Queuing IRPs
The USBDI must allow clients to queue IRPs for a given pipe. When IRPs are returned to the client, therequest status is also returned. A mechanism is provided by the USBD to identify a group of isochronous
IRPs whose first transactions will all occur in the same frame.

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10.5.3.2.4 Being a Master Client
The Master Client is allowed to adjust the number of bit times in a frame. This mechanism is used tosynchronize the USB to a device, such as an ISDN port. A client requesting master status identifies itself
with an interface handle for the device from which it is mastering.
The USBDI must allow a client to request becoming a Master Client for a given USB and to release thiscapability when it is no longer required. The USB will grant Master Client Status only to a single client.
Attempts by other clients to become the Master Client are ignored until the current Master Client
relinquishes control. The Master Client may explicitly release master status, or the client’s master statuswill be automatically released when the referenced device is reset or detached.

10.5.4 Managing the USB via the USBD Mechanisms
Using the provided USBD mechanisms, the following general capabilities are supported by any USB
System.

10.5.4.1 Configuration Services
Configuration services operate on a per-device basis. The configuring software tells the USBD when to
perform device configuration. A hub driver has a special role in device management and provides at least
the following capabilities:

Device attach/detach recognition, driven by an interrupt pipe owned by the hub driver

Device reset, accomplished by the hub driver by resetting the hub port upstream of the deviceTells the USBD to perform device address assignmentPower control.

The USBDI additionally provides the following configuration facilities, which may be used by the hub
driver or other configuring software available on the host:Device identification and access to configuration information (via access to descriptors on the device)Device configuration via command mechanisms.

When the hub driver informs the USBD of a device attachment, the USBD establishes the default pipe forthe new device.

10.5.4.1.1 Configuration Management
Configuration management services are provided primarily as a set of specific interface commands thatgenerate USB transactions on the default pipe. The notable exception is the use of an additional interrupt
pipe that delivers hub status directly to the hub driver.
Every hub initiates an interrupt transfer when there is a change in the state of one of the hub ports.
Generally, the port state change will be the connection or removal of a downstream USB device. (Refer to
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10.5.4.1.2 Initial Device Configuration
The device configuration process begins when a hub reports, via its status change pipe, the connection of a
new USB device.
Configuration management services allow configuring software to select a USB device configuration fromthe set of configurations listed in the device. The USBD verifies that adequate power is available and thedata transfer rates given for all endpoints in the configuration do not exceed the capabilities of the USB
with the current schedule before setting the device configuration.

10.5.4.1.3 Modifying a Device Configuration
Configuration management services allow configuring software to replace a USB device configuration withanother configuration from the set of configurations listed in the device. The operation succeeds if
adequate power is available and the data transfer rates given for all endpoints in the new configuration fit
within the capabilities of the USB with the current schedule. If the new configuration is rejected, theprevious configuration remains.
Configuration management services allow configuring software to return a USB device to a Not
Configured state.

10.5.4.1.4 Device Removal
Error recovery and/or device removal processing begins when a hub reports via its status change pipe that
the USB device has been removed.

10.5.4.2 Bus and Device Management
Bus and Device Management services allow a client to become the Master Client on a USB, and as the
Master Client, to adjust the number of bit times in a frame on that bus. A Master Client may add or
subtract one bit time to the current USB frame. Adjusting SOFs more frequently than once every 6ms has
undefined results.

10.5.4.3 Power Control
There are two cooperating levels of power management for the USB: bus and device level management.
This specification provides mechanisms for managing power on the USB bus. Device classes may define
class-specific power control capabilities.
All USB devices must support the Suspended state (refer to Chapter 9). The device is placed into theSuspended state via control of the hub port to which the device is attached. Normal device operation ceases
in the Suspend State, however, if the device is capable of wakeup signaling and the device is enabled forremote wakeup it may generate resume signaling in response to external events.
The power management system may transition a device to the Suspended state or power-off the device inorder to control and conserve power. The USB provides neither requirements nor commands for the device
state to be saved and restored across these transitions. Device classes may define class-specific device statesave-and-restore capabilities.
The USB System coordinates the interaction between device power states and the Suspended state.

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10.5.4.4 Event Notifications
USBD clients receive several kinds of event notifications through a number of sources:

Completion of an action initiated by a client.

Interrupt transfers over stream pipes can deliver notice of device events directly to USBD clients. For
example, hubs use an interrupt pipe to deliver events corresponding to changes in hub status.

Event data can be embedded by devices in streams.

Standard device interface commands, device class commands, vendor-specific commands, and evengeneral control transfers over message pipes can all be used to poll devices for event conditions.

10.5.4.5 Status Reporting and Error Recovery Services
The command and pipe mechanisms both provide status reporting on individual requests as they are
invoked and completed.
Additionally, USB device status is available to USBD clients using the command mechanisms.
The USBD provides clients with pipe error recovery mechanisms by allowing pipes to be reset or aborted.

10.5.4.6 Managing Remote Wakeup Devices
The USB System can minimize the resume power consumption of a suspended USB tree. This isaccomplished by explicitly enabling devices capable of resume signaling and controlling propagation of
resume signaling via selectively suspending and/or disabling hub ports between the device and the nearest
self-powered, awake hub.
In some error-recovery scenarios, the USB System will need to re-enumerate sub-trees. The sub-tree may
be partially or completely suspended. During error-recovery, the USB System must avoid contention
between a device issuing resume signaling and simultaneously driving reset down the port. Avoidance isaccomplished via management of the devices’ remote wakeup feature and the hubs’ port features. The
rules are as follows:Issue a SetDeviceFeature(DEVICE_REMOTE_WAKEUP) request to the leaf device, only just prior to
selectively suspending any port between where the device is connected and the root port (via a
SetPortFeature(PORT_SUSPEND) request).



Do not reset a suspended port that has had a device enabled for remote wakeup without first enablingthat port.

10.5.5 Passing USB Preboot Control to the Operating System
A single software driver owns the Host Controller. If the host system implements USB services before theoperating system loads, the Host Controller must provide a mechanism that disables access by the preboot
software and allows the operating system to gain control. Preboot USB configuration is not passed to theoperating system. Once the operating system gains control it is responsible to fully configure the bus. If
the operating system provides a mechanism to pass control back to the preboot environment, the bus will bein an unknown state. The preboot software should treat this event as a powerup.

10.6 Operating System Environment Guides
As noted previously, the actual interfaces between the USB System and host software are specific to thehost platform and operating system. A companion specification is required for each combination of
platform and operating system with USB support. These specifications describe the specific interfaces used
to integrate the USB into the host. Each operating system provider for the USB System identifies acompatible Universal USB Specification revision.

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Chapter 11
Hub Specification
This chapter describes the architectural requirements for the USB hub. It contains a description of the two
principal sub-blocks: the Hub Repeater and the Hub Controller. The chapter also describes the hub's
operation for error recovery, reset, and suspend/resume. The second half of the chapter defines hub request
behavior and hub descriptors.
The hub specification supplies sufficient additional information to permit an implementer to design a hubthat conforms to the USB specification.

11.1 Overview
Hubs provide the electrical interface between USB devices and the host. Hubs are directly responsible forsupporting many of the attributes that make USB user friendly and hide its complexity from the user.
Listed below are the major aspects of USB functionality that hubs must support:Connectivity behavior

Power management

Device connect/disconnect detection

Bus fault detection and recovery

Full- and low-speed device support.

A hub consists of two components: the Hub Repeater and the Hub Controller. The Hub Repeater is
responsible for connectivity setup and tear-down. It also supports exception handling, such as bus fault
detection and recovery and connect/disconnect detect. The Hub Controller provides the mechanism forhost-to-hub communication. Hub-specific status and control commands permit the host to configure a hub
and to monitor and control its individual downstream ports.

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11.1.1 Hub Architecture
Figure 11-1 shows a hub and the locations of its upstream and downstream ports. A hub consists of a Hub
Repeater section and a Hub Controller section. The Hub Repeater is responsible for managing connectivity
on a per-packet basis, while the Hub Controller provides status and control and permits host access to thehub.
Port 0
Upstream Port State Machine

Upstream Port

Hub
Repeater

Hub
State
MachineHub
Controller

Downstream Port
State Machine(s)...
Port 1

Port 2

Port N

Downstream Ports

Figure 11-1. Hub Architecture

11.1.2 Hub Connectivity
Hubs display differing connectivity behavior, depending on whether they are propagating packet traffic orresume signaling, or are in the Idle state.

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11.1.2.1 Packet Signaling Connectivity
The Hub Repeater contains one port that must always connect in the upstream direction (referred to as the
upstream port) and one or more downstream ports. Upstream connectivity is defined as being towards thehost, and downstream connectivity is defined as being towards a device. Figure 11-2 shows the packet
signaling connectivity behavior for hubs in the upstream and downstream directions. A hub also has an
Idle state, during which the hub makes no connectivity. When in the Idle state, all of the hub’s ports are in
the receive mode waiting for the start of the next packet.
UpstreamPort

Downstream
Ports

Downstream
Connectivity

Upstream
Connectivity

Idle(No Connectivity)

Enabled Port
Port not Enabled

Figure 11-2. Hub Signaling Connectivity
If a downstream hub port is enabled (i.e., in a state where it can propagate signaling through the hub) andthe hub detects a Start-of-Packet (SOP) on that port, connectivity is established in an upstream direction to
the upstream port of that hub, but not to any other downstream ports. This means that when a device or a
hub transmits a packet upstream, only those hubs in line between the transmitting device and the host will
see the packet. Refer to Section 11.8.3 for optional behavior when a hub detects simultaneous upstream
signaling on more than one port.
In the downstream direction, hubs operate in a broadcast mode. When a hub detects an SOP on its
upstream port, it establishes connectivity to all enabled downstream ports. If a port is not enabled, it does
not propagate packet signaling downstream.

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11.1.2.2 Resume Connectivity
Hubs exhibit different connectivity behaviors for upstream- and downstream-directed resume signaling. A
hub that is suspended reflects resume signaling from its upstream port to all of its enabled downstream
ports. Figure 11-3 illustrates hub upstream and downstream resume connectivity.

Upstream
Port

Upstream
Port
Enabled Port
Disabled or
Suspended
Port
Enabled or
Suspended
Port

Downstream
Ports

Source of resume
signaling

Downstream Connectivity

Upstream Connectivity

Figure 11-3. Resume Connectivity
If a hub is suspended and detects resume signaling from a selectively suspended or an enabled downstream
port, the hub reflects that signaling upstream and to all of its enabled downstream ports, including the portthat initiated the resume sequence. Resume signaling is not reflected to disabled or suspended ports. A
detailed discussion of resume connectivity appears in Section 11.9.

11.1.2.3 Hub Fault Recovery Mechanisms
Hubs are the essential USB component for establishing connectivity between the host and other devices. It
is vital that any connectivity faults, especially those that might result in a deadlock, be detected andprevented from occurring. Hubs need to handle connectivity faults only when they are in the repeater
mode.
Hubs must also be able to detect and recover from lost or corrupted packets that are addressed to the HubController. Because the Hub Controller is, in fact, another USB device, it must adhere to the same timeout
rules as other USB devices, as described in Chapter 8.

11.2 Hub Frame Timer
Each hub has a frame timer whose timing is derived from the hub’s local clock and is synchronized to thehost frame period by the host-generated Start-of-Frame (SOF). The frame timer provides timing referencesthat are used to allow the hub to detect a babbling device and prevent the hub from being disabled by the
upstream hub. The hub frame timer must track the host frame period and be capable of remaining
synchronized with the host even if two consecutive SOF tokens are missed by the hub.

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The frame timer must lock to the host’s frame timing for worst case tolerances and offsets between the hostand hub. The offsets have to accommodate the hub oscillator tolerance ( 500ppm) and accuracy( 2500ppm) as well as the host’s allowed frame tolerance of  500ppm. The range of the hub frame timer
is:12,000 * 1(hub accuracy + hub tolerance + host tolerance)
The host tolerance is allowed to be 500ppm, meaning that a frame time is between 0.9995ms and
1.0005ms, absolute. If the hub’s oscillator is at the limits of its accuracy and tolerance, it can be running at
between 11,964,000Hz and 12,036,000Hz. If the host is generating an SOF every 1.0005ms and the hub isrunning at 12,036,000Hz, then the hub’s frame timer will count 12,042 times between each SOF. If thehost is generating an SOF every 0.9995ms and the hub is running at 11,964,000Hz, then the hub’s frame
timer will count 11,958 times between each SOF. If the hub accuracy and tolerance are both zero, the hub
frame timer range is 6 bit times.

11.2.1 Frame Timer Synchronization
A hub’s frame timer is clocked by the hub’s clock source and is synchronized to SOF packets that arederived from the host’s frame timer. After a reset or resume, the hub’s frame timer is not synchronized.
Whenever the hub receives two consecutive SOF packets, its frame timer should be synchronized.
Synchronized is synonymous with lock(ed). A example for a method of constructing a timer that properly
synchronizes is as follows.
The hub maintains three timer values: frame timer (down counter), current frame (up counter), and next
frame (register). After a reset or resume, a flag is set to indicate that the frame timer is not synchronized.
When the first SOF token is detected, the current frame timer resets and starts counting once per hub bit
time. On the next SOF, if the timer has not rolled over, the value in the current frame timer is loaded into
the next frame register and into the frame timer. The current frame timer is reset to zero and continues tocount and the flag is set to indicate that the frame timer is locked. If the current frame timer has rolled over(exceeded 12,043 – a test at 16,383 is adequate), then an SOF was missed and the frame timer and next
frame values are not loaded and the flag indicating that the timer is not synchronized remains set.
Whenever the frame timer counts down to zero, the current value of the next frame register is loaded into
the frame timer. When an SOF is detected, and the current frame timer has not rolled over, the value of thecurrent frame timer is loaded into the frame timer and the next frame registers. The current frame timer isthen reset to zero and continues to count. If the current frame timer has rolled over, then the value in thenext frame register is loaded into the frame timer. This process can cause the frame timer to be updated
twice in a single frame: once when the frame timer reaches zero and once when the SOF is detected.
The synchronization circuit described above depends on successfully decoding an SOF packet identifier(PID). This means that the frame timer will be synchronized to a time that is at least 16 bit times into theframe. Each implementation will take some time to react to the SOF decode and set the appropriate
timer/counter values. (This reaction time is implementation-dependent but is assumed to be less than four
full-speed bit times.) Subsequent sections describe the actions that are controlled by the frame timer.
These actions are defined at the EOF1, EOF2, and EOF points, which should nominally be the same points
in time throughout the bus. EOF1 and EOF2 are defined in later sections. These sections assume that the
hub’s frame timer will count to zero at the end of the frame (EOF). The circuitry described above will
have the frame timer counting to zero 16-20 bit times after the start of a frame (or end of previous frame).
The timings and bit offsets in the later sections should be advanced to account for this offset (add 16-20 bit
times to the EOF1 and EOF2 points.)
The frame timer provides a indication to the hub Repeater state machine to indicate that the frame timer has
synchronized to SOF and that the frame timer is capable of generating the EOF1 and EOF2 timing points.
This signal is important after a global resume because of the possibility that a device may have been
detached and a different speed device attached while the host was generating a long resume (several
seconds) and the disconnect cannot be detected. A different speed device will bias D+ and D- to appear

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like a K on the hub which would then be treated as an SOP and, unless inhibited, this SOP would propagate
though the resumed hubs. Since the hubs would not have seen any SOF’s at this point, the hubs would not
be synchronized and, thus, unable to generate the EOF1 and EOF2 timing points. The only recovery fromthis would be for the host to reset and re-enumerate the section of the bus containing the changed device.
This scenario is prevented by inhibiting any downstream port from establishing connectivity until the hub
is locked after a resume.

11.2.2 EOF1 and EOF2 Timing Points
The EOF1 and EOF2 are timing points that are derived from the hub’s frame timer. These timing points
are used to ensure that devices and hubs do not interfere with the proper transmission of the SOF packet
from the host. These timing points have meaning only when the frame timer has been synchronized to the
SOF.
The host and hub frame markers, while all synchronized to the host’s SOF, are subject to certain skews that
dictate the placement of the EOF points. Figure 11-4 illustrates critical End-of-Frame (EOF) timing points.
Table 11-1 summarizes the host and hub EOF timing points.
EOF1

EOF2

SOF

Bit times

50

40

30
EOF1 range

20

10

0

EOF2 range

Figure 11-4. EOF Timing Points
At the EOF2 point, any port that has upstream connectivity will be disabled as a babbler. Hubs prevent
becoming disabled by sending an End-of-Packet (EOP) to the upstream hub before that hub reaches its
EOF2 point (i.e., at EOF1).
Note: a hub is permitted to send the EOP if upstream connectivity is not established at EOF1 time. A hub
must send the EOP if connectivity is established from any downstream port at the EOF1 point.
The EOF2 point is defined to occur at least one bit time before the first bit of the SYNC for an SOP. The
period allowed for an EOP is four full-speed bit times (the upstream port on a hub is always full-speed.)
Although the hub is synchronized to the SOF, timing skew can accumulate between the host and a hub or
between hubs. This timing skew represents the difference between different frame timers on different hubs
and the host. The total accumulated skew can be as large as 9 bit times. This is composed of 1 bit times
per frame of quantization error and 1 bit per frame of wander. The quantization error occurs when the
hub times the interval between SOFs and arrives at a value that is off by a fraction of a bit time but, due toquantization, is rounded to a full bit. Frame wander occurs when the host's frame timer is adjusted by the
USB System Software so that the value sampled by the hub in a previous frame differs from the frameinterval being used by the host. These values accumulate over multiple frames because SOF packets canbe lost and the hub cannot resynchronize its frame timer. This specification allows for the loss of twoconsecutive SOFs. During this interval the quantization error accumulates to 3 bit times and the wander
accumulates to 1  2  3 = 6 for a total of 9 bit times of accumulated skew in three frames. This skew
timing affects the placement of the EOF1 and EOF2 points as follows.
Note: although the USB System Software is not allowed to cause the frame interval to change more than
one bit time every six frames, the hub skew timing assumes that the frame interval can change one bit time
per frame. This cannot be reduced because it would create interoperability problems with hubs designed toprevious versions of this specification.
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A hub must reach its EOF2 point one bit time before the end of the frame. In order to ensure this, a 9-bit
time guard-band must be added so that the EOF2 point is set to occur when the hub's local frame timer
reaches 10. A hub must complete its EOP before the hub to which it is attached reaches its EOF2 point. A
hub may reach its EOF2 point nine bit times before bit time 10 (at bit time 19 before the SOF). To ensurethat the EOP is completed by bit time 19, it must start before bit time 23. To ensure that the hub starts at
bit time 23 with respect to another hub, a hub must set its EOF1 point nine bit times ahead of bit time 23(at bit time 32). If a hub sets its timer to generate an EOP at bit time 32, that EOP may start as much as 9
bit times early (at bit time 41).
Table 11-1. Hub and Host EOF Timing Points

Description

Nominal
Number of
Bits fromStart of SOF

Notes

EOF1

32

End-of-Frame point #1

EOF2

10

End-of-Frame point #2

11.3 Host Behavior at End-of-Frame
It is the responsibility of the USB host controller (the host) to not provoke a response from a device if the
response would cause the device to be sending a packet at the EOF2 point. Furthermore, because a hub
will terminate an upstream directed packet when the hub reaches its EOF1 point, the host should not start atransaction if a response from the device (data or handshake) would be pending or in process when a hub
reaches its EOF1 point. The implications of these limitations are described in the following sections.
In defining the timing points below, the last bit interval in a frame is designated as bit time zero. Bit times
in a frame that occur before the last have values that increase the further they are from bit time zero (earlier
bit times have higher numbers). These bit time designations are used for convenience only and are notintended to imply a particular implementation. The only requirement of an implementation is that therelative bit time values be preserved.

11.3.1 Latest Host Packet
Hubs are allowed to send an EOP on their upstream ports at the EOF1 point if there is no downstreamdirected traffic in progress at that time. To prevent potential contention, the host is not allowed to start apacket if connectivity will not be established on all connections before a hub reaches its EOF1 point. Thismeans that the host must not start a packet after bit time 42.
Note: although there is as much as a six-bit time delay between the time the host starts a packet and allconnections are established, this time need not be added to the packet start time as this phase delay exists
for the SOF packet as well, causing all hub frame timers to be phase delayed with respect to the host by the
propagation delay. There is only one bit time of phase delay between any two adjacent hubs and this has
been accounted for in the skew calculations.

11.3.2 Packet Nullification
If a device is sending a packet (data or handshake) when a hub in the device’s upstream path reaches its
EOF1 point, the hub will send a full-speed EOP. Any packet that is truncated by a hub must be discarded.
A host implementation may discard any packet that is being received at bit time 41. Alternatively, a hostimplementation may attempt to maximize bus utilization by accepting a packet if the packet is predicted tostart at or before bit time 41.

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11.3.3 Transaction Completion Prediction
A device can send two types of packets: data and handshake. A handshake packet is always exactly 16 bittimes long (sync byte plus PID byte.) The time from the end of a packet from the host until the first bit of
the handshake must be seen at the host is 17 bit times. This gives a total allocation of 35 bit times from theend of data packet from the root (start of EOP) until it is predicted that the handshake will be completed(start of EOP) from the device. Therefore, if the host is sending a data packet for which the device canreturn a handshake (anything other than an isochronous packet), then if the host completes the data packet
and starts sending EOP before bit time 76, then the host can predict that the device will complete thehandshake and start the EOP for the handshake on or before bit time 41. For a low-speed device, the 36 bittimes from start of EOP from root to start of EOP from the device are low-speed bit times, which convert 1
to eight into full-speed bit times. Therefore, if the host completes the low-speed data packet by bit time329, then the low-speed device can be predicted to complete the handshake before bit time 41
Note: if the host cannot accept a full-speed EOP as a valid end of a low-speed packet, then the low-speed
EOP will need to complete before bit time 41, which will add 13 full-speed bit times to the low-speed
handshake time.
As the host approaches the end of the frame, it must ensure that it does not require a device to send a
handshake if that handshake can’t be completed before bit time 41. The host expects to receive ahandshake after any valid, non-isochronous data packet. Therefore, if the host is sending a nonisochronous data packet when it reaches bit time 76 (329 for low-speed), then the host should start anabnormal termination sequence to ensure that the device will not try to respond. This abnormal termination
sequence consists of 7 consecutive bits of 1 followed by an EOP. The abnormal termination sequence issent at the speed of the current packet.
If the host is preparing to send an IN token, it may not send the token if the predicted packet from thedevice would not complete by bit time 41. The maximum valid length of the response from the device is
known by the host and should be used in the prediction calculation. For a full-speed packet, the maximum
interval between the start of the IN token and the end of a data packet is:token_length + (packet_length + header + CRC) * 7/6 + 18
Where token_length is 34 bit times, packet_length is the maximum number of data bits in the packet,
header is eight bits of sync and eight bits of PID, and CRC is 16 bits. The 7/6 multiplier accounts for theabsolute worst case bit-stuff on the packet and the 18 extra bits allow for worst case turn-around delay. For
a low-speed device, the same calculation applies but the result must be multiplied by 8 to convert to fullspeed bit times and an additional 20 full-speed bit times must be added to account for the low-speed prefix.
This gives the maximum number of bit times between the start of the IN token and the end of the data
packet, so the token cannot be sent if this number of bit times does not exist before the earliest EOF1 point(bit time 41). (E.g., take the results of the above calculation and add 41. If the number of bits left in theframe is less than this value, the token may not be sent.)
The host is allowed to use a more conservative algorithm than the one given above for deciding whether or
not to start a transaction. The calculation might also include the time required for the host to send thehandshake when one is required, as there is no benefit in starting a transfer if the handshake cannot becompleted.

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11.4 Internal Port
The internal port is the connection between the Hub Controller and the Hub Repeater. Besides conveying
the serial data to/from the Hub Controller, the internal port is the source of certain resume signals. Figure11-5 illustrates the internal port state machine; Table 11-2 defines the internal port signals and events.

!Rx_Suspend

Inactive! = Logical NOT

Rx_Suspend

Suspend Delay
EOI

Fsus
Resume_Event

GResumeFigure 11-5. Internal Port State Machine

Table 11-2. Internal Port Signal/Event Definitions
Signal/Event Name

Event/Signal
SourceDescription

EOI

Internal

End of timed interval

Rx_Suspend

Receiver

Receiver is in the Suspend state

Resume_Event

Hub Controller

A resume condition exists in the Hub Controller

11.4.1 Inactive
This state is entered whenever the Receiver is not in the Suspend state.

11.4.2 Suspend Delay
This state is entered from the Inactive state when the Receiver transitions to the Suspend state.
This is a timed state with a 2ms interval.

11.4.3 Full Suspend (Fsus)
This state is entered when the Suspend Delay interval expires.

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11.4.4 Generate Resume (GResume)
This state is entered from the Fsus state when a resume condition exists in the Hub Controller. A resume
condition exists if the C_PORT_SUSPEND bit is set in any port or if the hub is enabled as a wakeup
source and any bit is set in a Port Change field or the Hub Change field (as described in Table 11-14 andTable 11-10, respectively).
In this state, the internal port generates signaling to emulate an SOP_FD to the Hub Repeater.

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11.5 Downstream Ports
The following sections provide a functional description of a state machine that exhibits the correct behavior
for a downstream port on a hub.
Figure 11-6 is an illustration of the downstream port state machine. The events and signals are defined inTable 11-3. Each of the states is described in Section 11.5.1. In the diagram below, some of the entry
conditions into states are shown without origin. These conditions have multiple origin states and theindividual transitions lines are not shown so that the diagram can be simplified. The description of theentered state indicates from which states the transition is applicable.
# = Logical OR

Configuration = 0

Port Outputs in States& = Logical AND
Not
Configured

ClearPortFeature(PORT_POWER) #SetConfiguration(non-zero) #Power_Source_Off #Over-current!

= Logical NOT

The hub is not configured.

SetConfiguration(non-zero)
Powered_off: Port (or gang) requires
explicit request to transition.

Powered-off
SetPortFeature(PORT_POWER)

Disconnect_Detect

Disconnected: Port does not propagate
any traffic in either direction. The port isin HiZ. Port is timing length of J/K (2.5s
to 2ms).

Disconnected
EOI

SetPortFeature(PORT_DISABLE)
Disabled

Disabled: Port cannot propagate any
traffic. The port is in HiZ.

ResettingResetting: Drive SE0 through the port for
10ms.

SetPortFeature(PORT_RESET)EOI
Port_Error

Rx_Suspend & (SE0 # K)
EnabledLS & SOF

Enabled: Port can propagate both
upstream and downstream traffic.

Rptr_Enter_WFEOPFU# Rx_Resume
Transmit: Port propagates downstream
directed traffic.

Transmit
SetPortFeature(PORT_SUSPEND)

Rx_Suspend & (SE0 # K)

Rptr_Exit_WFEOPFU

Suspended: No traffic is propagateddownstream or upstream.

Suspended(!Rx_Suspend & K) #ClearPortFeature(PORT_SUSPEND)
Resuming: Drive 'K' for 20ms.

Resuming
EOI

SendEOP

Restart_S

EOI

SendEOP: Low-speed EOP is sent (2 lowspeed bit times of SE0 followed by 1 lowspeed bit time of 'J').

K

Restart_S and Restart_E: Port enters oneof these states to wait for clocks to restart.
Delay interval is implementationdependent but cannot be more than 10ms.

EOI
K
Restart_E
EOI

Figure 11-6. Downstream Hub Port State Machine239

Universal Serial Bus Specification Revision 1.1

Table 11-3. Downstream Hub Port Signal/Event Definitions
Signal/Event Name

Event/Signal
SourceDescription

Power_source_off

Implementationdependent

Power to the port not available due to over-current or
termination of source power (e.g., external power removed)

Over-current

Hub Controller

Over-current condition exists on the hub or the port

EOI

Internal

End of a timed interval or sequence

SE0

Internal

SE0 received on port

Disconnect_Detect

InternalLong SE0 detected on port (See Section 11.5.2)

LS

Hub Controller

Low-speed device attached to this port

SOF

Hub Controller

SOF token received

JInternal‘J’ received on port

K

Internal‘K’ received on port

Rx_Resume

Receiver

Upstream Receiver in Resume state

Rx_Suspend

Receiver

Upstream Receiver in Suspend state

Rptr_Exit_WFEOPFU

Hub Repeater

Hub Repeater exits the WFEOPFU state

Rptr_Enter_WFEOPFU

Hub Repeater

Hub Repeater enters the WFEOPFU state

Port_Error

Internal

Error condition detected (see Section 11.8.1)

Configuration = 0

Hub Controller

Hub controller's configuration value is zero

11.5.1 Downstream Port State Descriptions
11.5.1.1 Not Configured
A port transitions to and remains in this state whenever the value of the hub configuration is zero. While
the port is in this state, the hub will drive an SE0 on the port (this behavior is optional on root hubs). No
other active signaling takes place on the port when it is in this state.

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11.5.1.2 Powered-off
This state is supported for all hubs.
A port transitions to this state in any of the following situations:

From any state except Not Configured when the hub receives a ClearPortFeature(PORT_POWER)
request for this port

From any state when the hub receives a SetConfiguration() request with a configuration value otherthan zeroFrom any state except Not Configured when power is lost to the port or an over-current condition
exists.

A port will enter this state due to an over-current condition on another port if that over-current condition
may have caused the power supplied to this port to drop below specified limits for port power (see Section
7.2.1.2.1 and Section 7.2.4.1).
If a hub was configured while the hub was self-powered, then if external power is lost the hub must place
all ports in the Powered-off state. If the hub is configured while bus powered, then the hub need notchange port status if the hub switched to externally applied power. However, if external power issubsequently lost, the hub must place ports in the Powered-off state.
In this state, the port's differential and single-ended transmitters and receivers are disabled.
Control of power to the port is covered in Section 11.11.

11.5.1.3 Disconnected
A port transitions to this state in any of the following situations: from the Powered-off state when the hub receives a SetPortFeature(PORT_POWER) request
 from any state except the Not Configured and Powered-off states when the port's disconnect timer timesout from the Restart_S or Restart_E state at the end of the restart interval.

In the Disconnected state, the port's differential transmitter and receiver are disabled and only connection
detection is possible.
This is a timed state. While in this state, the timer is reset as long as the port’s signal lines are in the SE0
state. If another signaling state is detected, the timer starts. Unless the hub is suspended with clocks
stopped, this timer's duration is 2.5s to 2ms.
If the hub is suspended with its remote wakeup feature enabled then on a transition from the SE0 state on a
Disconnected port the hub will start its clocks and time this event. The hub must be able to start its clocks
and time this event within 12ms of the transition. If a hub does not have its remote wakeup featureenabled, then transitions on a port that is in the Disconnected state are ignored until the hub is resumed.

11.5.1.4 Disabled
A port transitions to this state in any of the following situations:

From the Disconnected state when the timer expires indicating a connection is detected on the port

From any but the Powered-off, Disconnected, or SenseSE0 states on receipt of aClearPortFeature(PORT_ENABLE) request

From the Enabled state when an error condition is detected on the port

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A port in the Disabled state will not propagate signaling in either the upstream or the downstream direction.
While in this state, the duration of any SE0 received on the port is timed.

11.5.1.5 Resetting
Unless it is in the Powered-off or Disconnected states, a port transitions to the Resetting state upon receipt
of a SetPortFeature(PORT_RESET) request. The hub drives SE0 on the port during this timed interval.
The duration of the Resetting state is nominally 10ms to 20ms (10ms is preferred).

11.5.1.6 Enabled
A port transitions to this state in any of the following situations:

At the end of the Resetting stateFrom the Transmit state when the Hub Repeater exits the WFEOPFU stateFrom the Suspended state if the upstream Receiver is in the Suspend state when a 'K' is detected on theportAt the end of the SendEOP state.

While in this state, the output of the port's differential receiver is available to the Hub Repeater so that 'J'to-'K' transitions can establish upstream connectivity.

11.5.1.7 Transmit
For full- and low-speed ports this state is entered in either of the following situations:from the Enabled state if the upstream Receiver is in the Resume state

immediately from the Restart_S or Restart_E state if a 'K' is detected on the port.

For a full-speed port, this state is entered from the Enabled state on the transition of the Hub Repeater tothe WFEOPFU state. While in this state, the port will transmit the data that is received on the upstream
port.
For a low-speed port, this state is entered from the Enabled state if a full-speed PRE PID is received on the
upstream port. While in this state, the port will retransmit the data that is received on the upstream port
(after proper inversion).

11.5.1.8 Suspended
A port enters the Suspended state from the Enabled state when it receives a
SetPortFeature(PORT_SUSPEND) request. While a port is in the Suspended state, the port's differential
transmitter is disabled.
An implementation is allowed to have a SE0 ‘noise’ filter for a port that is in the suspended state. Thisfilter can time the length of SE0 and, if the length of the SE0 is shorter than 2.5s, the port may remain inthis state. However, this filter may not be used if the hub is suspended and the clocks are stopped. Rather,
if the hub is suspended with its clocks stopped, a transition to SE0 on a suspended port must cause the port
to immediately transition to the Restart_S state. This is to insure that the attached device is not reset andplaced at the default address without having the hub disable the port.

11.5.1.9 Resuming
A port enters this state from the Suspended state in either of the following situations:242

If a 'K' is detected on the port and the Receiver is not in the Suspend stateUniversal Serial Bus Specification Revision 1.1

When a ClearPortFeature(PORT_SUSPEND) request is received.

This is a timed state with a nominal duration of 20ms (the interval may be longer under the conditions
described in the note below). While in this state, the hub drives a 'K' on the port.
Note: a single timer is allowed to be used to time both the Resetting interval and the Resuming interval
and that timer may be shared among multiple ports. When shared, the timer is reset when a port enters theResuming state or the Resetting state. If shared, it may not be shared among more than ten ports as the
cumulative delay could exceed the amount of time required to replace a device and a disconnect could be
missed.

11.5.1.10 SendEOPThis state is entered from the Resuming state if the 20ms timer expires. It is also entered from the Enabledstate when an SOF (or other FS token) is received and a low-speed devices is attached to this port. In thisstate, the hub will send a low-speed EOP (two low-speed bits times of SE0 followed by one low-speed bittimes of J). At the end of the EOP, the state ends.
Since the transmitted EOP should be of fixed length, the SendEOP timer, if shared, should not be reset. If
the hub implementation shares the SendEOP timing circuits between ports, then the Resuming state should
not end until an SOF (or other FS token) has been received (see Section 11.8.4.1 for Keep-alive generation
rules).

11.5.1.11 Restart_S/Restart_E
A port enters the Restart_S state from the Suspended state or enters the Restart_E state from the Enabledstate when an SE0 or 'K' is seen at the port and the Receiver is in the Suspended state.
These states are needed to ensure that a transient SE0, which may be seen at the start of resume signaling,
does not cause the port to be disabled.
In these states, the port continuously monitors the bus state and exits to the Transmit state immediately on
seeing the K state. In this case, the port completes its transition to the Transmit state within 100s after
entering the Restart_S or Restart_E state. If the bus state is not 'K', the port transitions to the Disconnectedstate. This transition should happen within 10ms of entering the Restart_S or Restart_E state.

11.5.2 Disconnect Detect Timer
Each port is required to have a disconnect timer. This timer is used to constantly monitor the ports singleended receivers to detect a disconnect event. The reason for constant monitoring is that a noise event on thebus can cause the attached device to detect a reset condition on the bus after 2.5s of SE0 on the bus. If the
hub does not place the port in the disconnect state before the device resets, then the device can be at in the
Default Address state with the port enabled. This can cause systems errors that are very difficult to isolate
and correct.
This timer should be reset whenever the D+ and D- lines on the port are not in the SE0 state or when theport is not in the Enabled, Suspended, or Disabled states. This timer may have a timeout that is a short as
1.994s (2.0s - 3000ppm) but should not be longer than 2.508s (+3000 ppm). When this timer expires, it
generates the Disconnect_Detect signal to the port state machine.

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11.6 Upstream PortThe upstream port has four components: transmitter, transmitter state machine, receiver and receiver statemachine. The transmitter and its state machine are the Transmitter, while the receiver and its state machine
are the Receiver. Both the transmitter and receiver have differential and single-ended components. The
differential transmitter and receiver can send/receive 'J' or 'K' to/from the bus while the single-ended
components are used to send/receive SE0, suspend, and resume signaling. In this section, when it isnecessary to differentiate the signals sent/received by the differential component of the transmitter/receiverfrom those of the single-ended components, DJ and DK will be used to denote the differential signal and
SJ, SK and SE0 will be used for the single-ended signals.
It is assumed that the differential transmitter and receiver are turned off during suspend to minimize power
consumption. The single-ended components are left on at all times, as they will take minimal power.

11.6.1 Receiver
The receiver state machine is responsible for monitoring the signaling state of the upstream connection to
detect long-term signaling events such as bus reset, resume, and suspend. Figure 11-7 illustrates the statetransition diagram. Table 11-4 defines the signals and events referenced in Figure 11-7.

Tx_active
J

State Machine Exports:
Rx_Bus_Reset(Bus_Reset)
Rx_Suspend(Suspend)
Rx_Resume(Resume)

ReceivingJ
EOI
SuspendK
ReceivingK
Tx_resume # K
EOI# = Logical OR

ResumeSE0
ReceivingSE0
EOI
POR

Bus_Reset
Figure 11-7. Upstream Port Receiver State Machine244

Universal Serial Bus Specification Revision 1.1

Table 11-4. Upstream Hub Port Receiver Signal/Event Definitions
Signal/Event
Name

Event/Signal
SourceDescription

Tx_active

Transmitter

Transmitter in the Active state

J

Internal

Receiving a 'J' (IDLE) on the upstream port

EOI

Internal

End of timed interval

KInternal

Receiving a 'K' on the upstream port

Tx_resume

Transmitter

Transmitter is in the Sresume state

SE0

Internal

Receiving an SE0 on the upstream port

POR

Implementationdependent

Power_On_Reset

11.6.1.1 ReceivingJ
This state is entered from any state except the Suspend state if the receiver detects an SJ (or Idle) conditionon the bus or while the Transmitter is in the Active state.
This is a timed state with an interval of 3ms. The timer is reset each time this state is entered.
The timer only advances if the Transmitter is in the Inactive state.

11.6.1.2 Suspend
This state is entered if the 3ms timer expires in the ReceivingJ state. When the Receiver enters this state,
the Hub Controller starts a 2ms timer. If that timer expires while the Receiver is still in this state, then the
Hub Controller is suspended. When the Hub Controller is suspended, it may generate resume signaling.

11.6.1.3 ReceivingKThis state is entered from any state except the Resume state when the receiver detects an SK condition on
the bus and the Hub Repeater is in the WFSOP or WFSOPFU state. This is a timed state with a duration of
2.5s to 100s. The timer is reset each time this state starts.

11.6.1.4 Resume
This state is entered from the ReceivingK state when the timer expires.
This state is also entered from the Suspend state while the Transmitter is in the Sresume state or if there is atransition to the K state on the upstream port.
If the hub enters this state when its timing reference is not available, the hub may remain in this state until
the hub’s timing reference becomes stable. If this state is being held pending stabilization of the hub’s
clock, the Receiver should provide a K to the repeater for propagation to the downstream ports. Whenclocks are stable, the Receiver should repeat the incoming signals.
Note: constraints on hub behavior after reset require that the hub be able to start clocks and get them stablein less than 10ms.

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11.6.1.5 ReceivingSE0
This state is entered from any state except Bus_Reset when the receiver detects an SE0 condition and the
Hub Repeater is in the WFSOP or WFSOPFU state. This is a timed state. The minimum interval for thisstate is 2.5s. The maximum depends on the hub but this interval must timeout early enough such that if
the width of the SE0 on the upstream port is only 10ms, the Receiver will enter the Bus_Reset state with
sufficient time remaining in the 10ms interval for the hub to complete its reset processing. Furthermore, if
the hub is suspended when the Receiver enters this state, the hub must be able to start its clocks, time thisinterval, and complete its reset processing within 10ms. It is preferred that this interval be as long as
possible given the constraints listed here. This will provide for the maximum immunity to noise on the
upstream port and reduce the probability that the device will reset in the presence of noise before theupstream hub disables the port.
The timer is reset each time this state starts.

11.6.1.6 Bus_ResetThis state is entered from the ReceivingSE0 state when the timer expires. As long as the port continues toreceive SE0, the Receiver will remain in this state.
This state is also entered while power-on-reset (POR) is being generated by the hub’s local circuitry. The
state machine cannot exit this state while POR is active.

11.6.2 TransmitterThis state machine is used to monitor the upstream port while the Hub Repeater has connectivity in the
upstream direction. The purpose of this monitoring activity is to prevent propagation of erroneous
indications in the upstream direction. In particular, this machine prevents babble and disconnect events on
the downstream ports of this hub from propagating and causing this hub to be disabled or disconnected by
the hub to which it is attached. Figure 11-8 is the transmitter state transition diagram. Table 11-5 defines
the signals and events referenced in Figure 11-8.

Rx_Bus_Reset

State Machine Exports:

Inactive
Rptr_WFEOP &!Rx_Suspend

EOF1

Tx_Active(Active)
Tx_Resume(Sresume)

ActiveSE0sent

EOF1

RepeatingSE0

K# = Logical OR

EOI # J
SendJ

EOI

& = Logical AND! = Logical NOT

EOIGEOPTU
Rx_Suspend &Rptr_WFEOP

SresumeEOI

Figure 11-8. Upstream Hub Port Transmitter State Machine246

Universal Serial Bus Specification Revision 1.1

Table 11-5. Upstream Hub Port Transmit Signal/Event Definitions
Signal/Event
Name

Event/Signal
SourceDescription

Rx_Bus_Reset

Receiver

Receiver is in the Bus_Reset state

EOF1

Frame Timer

Hub frame time has reached the EOF1 point or is between
EOF1 and the end of the frame

JInternal

Transmitter transitions to sending a 'J' and transmits a 'J'

Rptr_WFEOP

Hub Repeater

Hub Repeater is in the WFOEP state

KInternal

Transmitter transmits a 'K'

SE0sent

Internal

At least one bit time of SE0 has been sent through thetransmitter

Rx_Suspend

Receiver

Receiver is in Suspend state

EOI

Internal

End of timed interval

11.6.2.1 Inactive
This state is entered at the end of the SendJ state or while the Receiver is in the Bus_Reset state. This stateis also entered at the end of the Sresume state. While the transmitter is in this state, both the differential
and single-ended transmit circuits are disabled and placed in their high-impedance state.

11.6.2.2 Active
This state is entered from the Inactive state when the Hub Repeater transitions to the WFEOP state. This
state is entered from the RepeatingSE0 state if the first transition after the SE0 is not to the J state. In thisstate, the data from a downstream port is repeated and transmitted on the upstream port.

11.6.2.3 RepeatingSE0
The port enters this state from the Active state when one bit time of SE0 has been sent on the upstream
port. While in this state, the transmitter is still active and downstream signaling is repeated on the port.
This is a timed state with a duration of 23 full-speed bit times.

11.6.2.4 SendJ
The port enters this state from the RepeatingSE0 state if either the bit timer reaches 23 or the repeatedsignaling changes from SE0 to 'J'. This state is also entered at the end of the GEOPTU state. This state
lasts for one full-speed bit time. During this state, the hub drives an SJ on the port.

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11.6.2.5 Generate End of Packet Towards Upstream Port (GEOPTU)
The port enters this state from the Active or RepeatingSEO state if the frame timer reaches the EOF1 point.
In this state, the port transmits SE0 for two full-speed bit times.

11.6.2.6 Send Resume (Sresume)
The port enters this state from the Inactive state if the Receiver is in the Suspend state and the Hub
Repeater transitions to the WFEOP state. This indicates that a downstream device (or the port to the HubController) has generated resume signaling, causing upstream connectivity to be established.
On entering this state, the hub will restart clocks if they had been turned off during the Suspend state.
While in this state, the Transmitter will drive a 'K' on the upstream port. While the Transmitter is in thisstate, the Receiver is held in the Resume state. While in the Resume state, all downstream ports that are in
the Enable state are placed in the Transmit state and the resume on this port is transmitted to those
downstream ports.
The port stays in this state for at least 1ms but for no more than 15ms.

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11.7 Hub Repeater
The Hub Repeater provides the following functions:

Sets up and tears down connectivity on packet boundaries

Ensures orderly entry into and out of the Suspend state, including proper handling of remote wakeups

The state machine in Figure 11-9 shows the states and transitions needed to implement the Hub Repeater.
Table 11-6 defines the Hub Repeater signals and events. The following sections describe the states and thetransitions.
Several of the state transitions below will occur when an EOP is detected. When such a transition isindicated, the transition does not occur until after the hub has repeated the SE0-to-'J' transition and has
driven 'J' for at least one bit time (bit time is determined by the speed of the port.)
Some of the transitions are triggered by an SOP. Transitions of this type occur as soon as the hub detects
the 'J'-to-'K' transition, ensuring that the initial edge of the SYNC field is preserved.
Rx_Bus_Reset

State Machine Exports:

WFSOPFU
UEOP & !Lock

SOP_FU

Rptr_WFEOP(WFEOP)
Rptr_WFSOPFU(WFSOPFU)
Rptr_Enter_WFEOPFU
Rptr_Exit_WFEOPFU

Rx_Resume
WFEOPFU
UEOP & Lock

SOP_FU

Rx_Suspend# = Logical OR& = Logical AND! = Logical NOT

WFSOP

EOF1
SOP_FD

DEOP

WFEOP

EOF2

Figure 11-9. Hub Repeater State Machine249

Universal Serial Bus Specification Revision 1.1

Table 11-6. Hub Repeater Signal/Event Definitions
Signal/Event
Name

Event/Signal
SourceDescription

Rx_Bus_Reset

Receiver

Receiver is in the Bus_Reset state

UEOP

Internal

EOP received from the upstream port

DEOP

Internal

Generated when the Transmitter enters the SendJ state

EOF1

Frame Timer

Frame timer is at the EOF1 point or between EOF1 and End-ofFrame

EOF2

Frame Timer

Frame timer is at the EOF2 point or between EOF2 and End-ofFrame

Lock

Frame Timer

Frame timer is locked

Rx_Suspend

Receiver

Receiver is in the Suspend state

Rx_Resume

Receiver

Receiver is in the Resume state

SOP_FDInternal

SOP received from downstream port or Hub Controller.
Generated on the transition from the Idle to K state on a port.

SOP_FUInternal

SOP received from upstream port. Generated on the transition
from the Idle to K state on the upstream port.

11.7.1 Wait for Start of Packet from Upstream Port (WFSOPFU)
This state is entered in either of the following situations:

From any other state when the upstream Receiver is in the Bus_Reset stateFrom the WFSOP state if the frame timer is at or has passed the EOF1 point

From the WFEOP state at the EOF2 point.



From the WFEOPFU if the frame timer is not synchronized (locked) when an EOP is received on the
upstream port.

In this state, the hub is waiting for an SOP on the upstream port and transitions on downstream ports are
ignored by the Hub Repeater. While the Hub Repeater is in this state, connectivity is not established.
This state is used during the End-of-Frame (past the EOF1 point) to ensure that the hub will be able toreceive the SOF when it is sent by the host.

11.7.2 Wait for End of Packet from Upstream Port (WFEOPFU)
The hub enters this state if the hub is in the WFSOP or WFSOPFU state and an SOP is detected on the
upstream port. The hub also enters this state from the WFSOP, WFSOPFU, or WFEOP states when the
Receiver enters the Resume state.
While in this state, connectivity is established from the upstream port to all enabled downstream ports.
Downstream ports that are in the Enabled state are placed in the Transmit state on the transition to thisstate.

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11.7.3 Wait for Start of Packet (WFSOP)This state is entered in any of the following situations:

From the WFEOPstate when an EOP is detected from the downstream port

From the WFEOPFU state if the frame timer is synchronized (locked) when an EOP is received fromupstreamFrom the WFSOPFU or WFEOPFU states when the upstream Receiver transitions to the Suspend
state.

A hub in this state is waiting for an SOP on the upstream port or any downstream port that is in the
Enabled state. While the Hub Repeater is in this state, connectivity is not established.

11.7.4 Wait for End of Packet (WFEOP)
This state is entered from the WFSOP state when an SOP is received from a downstream port in the
Enabled state.
In this state, the hub has connectivity established in the upstream direction and the signaling received on an
enabled downstream port is repeated and driven on the upstream port. The upstream Transmitter is placedin the Active state on the transition to this state.
If the Hub Repeater is in this state when the EOF2 point is reached, the downstream port for whichconnectivity is established is disabled as a babble port.
Note: the Transmitter will send an EOP at EOF1 but the Hub Repeater stays in this state until the device
sends an EOP or the EOF2 point is reached.

11.8 Bus State Evaluation
A hub is required to evaluate the state of the connection on a port in order to make appropriate port statetransitions. This section describes the appropriate times and means for several of these evaluations.

11.8.1 Port Error
A Port Error can occurs on a port that is in the Enabled state. A Port Error condition exists when::The hub is in the WFEOP state with connectivity established upstream from the port when the frame
timer reaches the EOF2 point.

At the EOF2 point the Hub Repeater is in the WFSOPFU state and there is other than an Idle/J state on
the port.

If upstream-directed connectivity is established when the frame timer reaches the EOF1 point, the upstream
Transmitter will generate a full-speed EOP to prevent the hub from being disabled by the upstream hub.
The connected port is then disabled if it has not ended the packet and returned to the Idle state before theframe timer reaches the EOF2 point.

11.8.2 Speed Detection
The speed of an attached device is determined by the placement of a pull-up resistor on the device (seeSection 7.1.5). When a device is attached, the hub is expected to detect the speed of the device by sensing
the Bus Idle state. Due to connect and start-up transients, the hub may not be able to reliably determine thespeed of the device until the transients have ended. The USB System Software is required to "debounce"
the connection and provide a delay between the time a connection is detected and the device is used (seeSection 7.1.7.1). At the end of the debounce interval, the device is expected to have placed its upstream

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port in the Idle state and be able to react to reset signaling. The USB System Software must send a
SetPortFeature(PORT_RESET) request to the port to enable the port and make the attached device readyfor use. This provides a convenient time for the hub to evaluate the speed of the device attached to theport. Speed detection can be done at the beginning of the port reset as the port leaves the Disabled state orat the end of the port reset between the end of the Resetting state and the start of the Enabled state.
If an implementation chooses to do speed evaluation on entry to the Resetting state from the Disabled state,
it will set the PORT_LOW_SPEED status according to the condition of the D+ and D- lines at that time.
(Note: if both D+ and D- are high at this time, the hub may stay in the Disabled state and set the
C_PORT_ENABLE bit to indicate that the hub could not determine the speed of the device. Otherwise the
hub should assume that the device is low-speed.) This determines the speed of the device and the Idle/J
state for the port. The hub will then drive an SE0 for the duration of the Resetting state timer. At the endof the Resetting state, the hub will drive the lines to the J state that is appropriate for the speed of theattached device and transition to the Enabled state.
Note: because the SendEOP state also exits to the Enabled state, an implementation might exit the
Resetting state to the SendEOP state without driving the 'J' and then let the SendEOP circuit complete theoperation.
If an implementation chooses to do speed evaluation on exit from the Resetting state, then it will need an
additional state called the Speed_eval state. At the end of the Resetting state, the hub will float the D+ andD- lines and allow the lines to settle to the Idle state appropriate for the attached device. At the end of the
Speed_eval state, the hub will set the PORT_LOW_SPEED status as appropriate. The Speed_eval statemust last for at least 2.5s but no longer than 1ms. It is possible that the port will detect a disconnect
condition during the speed evaluation. If so, the port transitions to the Disconnected state and will notenter the Enabled state.

11.8.3 Collision
If the Hub Repeater is in the WFEOP state and an SOP is detected on another enabled port, a Collisioncondition exists. There are two allowed behaviors for the hub in this instance.
The first, and preferred, behavior is to ‘garble’ the message so that the host can detect the problem. The
hub garbles the message by transmitting a 'K' on the upstream port. This 'K' should persist until packet
traffic from all downstream ports ends. The hub should use the last EOP to terminate the garbled packet.
babble detection is enabled during this garbled message.
A second behavior is to block the second packet and, when the first message ends, return the hub to the
WFSOPFU or WFSOP state as appropriate. If the second stream is still active, the hub may reestablish
connectivity upstream. This method is not preferred, as it does not convey the problem to the host.
Additionally, if the second stream causes the hub to reestablish upstream connectivity as the host is trying
to establish downstream connectivity, additional packets can be lost and the host cannot properly associate
the problem.

11.8.4 Full- versus Low-speed Behavior
The upstream connection of a hub must always be a full-speed connection. All downstream ports of a hubthat are attached to USB connectors must be able to support both full-speed and low-speed devices. When
low-speed data is sent or received through a hub's upstream connection, the signaling is full-speed eventhough the bit times are low-speed.
Full-speed signaling must not be transmitted to low-speed ports.
If a port is detected to be attached to a low-speed device, the hub port’s output buffers are configured tooperate at the slow slew rate (75-300ns), and the port will not propagate downstream-directed packets
unless they are prefaced with a PRE PID. When a hub receives a PRE PID, it must enable the drivers on
the enabled, low-speed ports within four bit times of receiving the last bit of the PID.

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Note: when the driver is turned on, the upstream port will be in the 'J' state and the downstream ports
should be driven to the same state.
Low-speed data follows the PID and is propagated to both low and full-speed devices. Hubs continue topropagate downstream signaling to all enabled ports until a downstream EOP is detected, at which time all
output drivers are turned off.
Full-speed devices will not misinterpret low-speed traffic because no low-speed data pattern can generate avalid full-speed PID.
When a low-speed device transmits, it does not preface its data packet with a PRE PID. Hubs willpropagate upstream-directed packets of any speed using full-speed signaling polarity and edge rates.
For both upstream and downstream low-speed data, the hub is responsible for inverting the polarity of thedata before transmitting to/from a low-speed port.
Although a low-speed device will send a low-speed EOP to properly terminate a packet, a hub may
truncate a low-speed packet at the EOF1 point with a full-speed EOP. Thus, hubs must always be able totear down connectivity in response to a full-speed EOP regardless of the data rate of the packet.
Because of the slow transitions on low-speed ports, when the D+ and D- signal lines are switching between
the 'J' and 'K', they may both be below 2.0V for a period of time that is longer than a full-speed bit time. A
hub must ensure that these slow transitions do not result in termination of connectivity and must not resultin an SE0 being sent upstream.

11.8.4.1 Low-speed Keep-alive
All hub ports to which low-speed devices are connected must generate a low-speed keep-alive strobe,
generated at the beginning of the frame, which consists of a valid low-speed EOP (described in Section
7.1.13.2). The strobe must be generated at least once in each frame in which an SOF is received from thehost. This strobe is used to prevent low-speed devices from suspending if there is no other low-speed traffic
on the bus. The hub can generate the keep-alive on any valid full-speed token packet. The following rules
for generation of a low-speed keep-alive must be adhered to:

A keep-alive must minimally be derived from each SOF. It is recommended that a keep-alive be
generated on any valid full-speed token.

The keep-alive must start by the eighth bit after the PID of the full-speed token.

11.9 Suspend and Resume
Hubs must support suspend and resume both as a USB device and in terms of propagating suspend andresume signaling. Hubs support both global and selective suspend and resume. Global and selective
suspend are defined in Section 7.1.7.4. Global suspend/resume refers to the entire bus being suspended orresumed without affecting any hub’s downstream port states; selective suspend/resume refers to adownstream port of a hub being suspended or resumed without affecting the hub state. Global
suspend/resume is implemented through the root port(s) at the host. Selective suspend/resume isimplemented via requests to a hub. Device-initiated resume is called remote-wakeup (see Section 7.1.7.5).
Figure 11-10 shows the timing relationships for an example remote-wakeup sequence. This example
illustrates a device initiating resume signaling through a suspended hub (‘B’) to an awake hub (‘A’). Hub‘A’ in this example times and completes the resume sequence and is the "Controlling Hub". The timings
and events are defined in Section 7.1.7.5.

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Bus driving
Bus Idle ordriven at other end

Everything
below Hub ‘A’in Suspendstate

Hub ‘A’(Controlling Hub)Controlling Hubsuspended DS
PortControlling Hub
sends EOP ending
resumeControlling Hub Drives Resume (DS)20ms (nominal)

Idle (‘J’)

Resume (‘K’)

Controlling Hub Reflects Resume(DS) 100s

Hub
UpstreamPort

Hub ‘B’
Enabled DS

Idle (‘J’)

Idle (‘J’)

Resume (‘K’)

Hub Ports
Hub ‘B’ Drives Resume (US and DS)[1ms, 15ms]
Function
Hub Port
DeviceDeviceHub ‘B’ Reflects Resume (US and DS)
100s

DeviceRemoteWakeupIdle (‘J’)

Resume (‘K’)

Idle (‘J’)

t0

t1

t2

t3

Device Drives Resume[1ms, 15ms]

t4

t5

Figure 11-10. Example Remote-Wakeup Resume Signaling
Here is an explanation of what happens at each tn:
t0 Suspended device initiates remote-wakeup by driving a 'K' on the data lines.
t1 Suspended hub ‘B’ detects the ‘K’ on its downstream port, wakes up enough within 100s to reflect
the resume upstream and down through all enabled ports.
t2 Hub ‘A’ is not suspended (implication is that the port at which ‘B’ is attached is selectively
suspended), detects the ‘K’ on the selectively suspended port where ‘B’ is attached, and reflects theresume signal back to ‘B’ within 100s.
t3 Device ceases driving ‘K’ upstream.
t4 Hub ‘B’ ceases driving ‘K’ upstream and down all enabled ports and begins repeating upstream
signaling to all enabled downstream ports.
t5 Hub ‘A’ completes resume sequence, after appropriate timing interval, by driving a low-speed EOP
downstream.
The hub reflection time is much smaller than the minimum duration a USB device will drive resume
upstream. This relationship guarantees that resume will be propagated upstream and downstream without
any gaps.

11.10 Hub Reset Behavior
The following sections describe hub reset behavior and its interactions with resume, attach detect, andpower-on.

11.10.1 Hub Receiving Reset on Upstream PortReset signaling to a hub is defined only in the downstream direction, which is at the hub's upstream port.
A hub may start its reset sequence if it detects 2.5s or more of continuous SE0 signaling and must

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complete its reset sequence by the end of the reset signaling.
Note: the 2.5s lower limit is set by a need to prevent low-speed EOP strobes from being interpreted as
reset.
A suspended hub must interpret the start of reset as a wakeup event; it must be awake and have completed
its reset sequence by the end of reset signaling.
After completion of the reset sequence, a hub is in the following state:

Hub Controller default address is 0

Hub status change bits are set to zero

Hub Repeater is in the WFSOPFU stateTransmitter is in the Inactive stateDownstream ports are in the Not Configured state and SE0 driven on all downstream ports.

11.11 Hub Port Power Control
Self-powered hubs may have power switches that control delivery of power downstream ports but it is not
required. Bus-powered hubs are required to have power switches. A hub with power switches can switch
power to all ports as a group/gang, to each port individually, or have an arbitrary number of gangs of one
or more ports .
A hub indicates whether or not it supports power switching by the setting of the Logical Power Switching
Mode field in wHubCharacteristics.If a hub supports per-port power switching, then the power to a port isturned on when a SetPortFeature(PORT_POWER) request is received for the port. Port power is turned off
when the port is in the Powered-off or Not Configured states. If a hub supports ganged power switching,then the power to all ports in a gang is turned on when any port in a gang receives a
SetPortFeature(PORT_POWER) request. The power to a gang is not turned off unless all ports in a gang
are in the Powered-off or Not Configured states. Note, the power to a port is not turned on by a
SetPortFeature(PORT_POWER) if both C_HUB_LOCAL_POWER and Local Power Status (inwHubStatus) are set to 1B at the time when the request is executed and the PORT_POWER feature would
be turned on.
Although a self-powered hub is not required to implement power switching, the hub must support thePowered-off state for all ports. Additionally, the hub must implement the PortPwrCtrlMask (all bits set to1b) even though the hub has no power switches that can be controlled by the USB System Software.
Note: to ensure compatibility with previous versions of USB software, hubs must implement the LogicalPower Switching Mode field in wHubCharacteristics. This is because some versions of SW will not use
the SetPortFeature() request if the hub indicates in wHubCharacteristics that the port does not support port
power switching. Otherwise, the Logical Power Switching Mode field in wHubCharacteristics would havebecome redundant as of this version of the specification.
The setting of the Logical Power Switching Mode for hubs with no power switches should reflect themanner in which over-current is reported. For example, if the hub reports over-current conditions on a perport basis, then the Logical Power Switching Mode should be set to indicate that power switching iscontrolled on a per-port basis.
For a hub with no power switches, bPwrOn2PwrGood should be set to zero.

11.11.1 Multiple Gangs
A hub may implement any number of power and/or over-current gangs. A hub that implements more than
one over-current and/or power switching gang must set both the Logical Power Switching Mode and the255Universal Serial Bus Specification Revision 1.1

Over-current Reporting Mode to indicate that power switching and over-current reporting are on a per port
basis (these fields are in wHubCharacteristics.) Also, all bits in PortPwrCtrlMask must be set to 1b.
When an over-current condition occurs on an over-current protection device, the over-current is signaled
on all ports that are protected by that device. When the over-current is signaled, all the ports in the groupare placed in the Powered-off state, and the C_PORT_OVER-CURRENT field is set to 1B on all the ports.
When port status is read from any port in the group, the PORT_OVER-CURRENT field will be set to 1b aslong as the over-current condition exists. The C_PORT_OVER-CURRENT field must be cleared in eachport individually.
When multiple ports share a power switch, setting PORT_POWER on any port in the group will cause thepower to all ports in the group to turn on. It will not, however, cause the other ports in that group to leave
the Powered-off state. When all the ports in a group are in the Powered-off state or the hub is notconfigured, the power to the ports is turned off.
If a hub implements both power switching and over-current, it is not necessary for the over-current groups
to be the same as the power switching groups.
If an over-current condition occurs and power switches are present, then all power switches associated withan over-current protection circuit must be turned off. If multiple over-current protection devices areassociate with a single power switch then that switch will be turned off when any of the over-current
protection circuits indicates an over-current condition.

11.12 Hub I/O Buffer Requirements
All hub ports must be able to detect and generate all the bus signaling states described in Table 7-1. Thisrequires that hub be able to independently drive and monitor the D+ and D- outputs on each of its ports.
Each hub port must have single-ended receivers and transmitters on the D+ and D- lines as well as adifferential receiver and transmitter. Details on voltage levels and drive requirements appear in Chapter 7.

11.12.1 Pull-up and Pull-down Resistors
Hubs, and the devices to which they connect, use a combination of pull-up and pull-down resistors tocontrol D+ and D- in the absence of their being actively driven. These resistors establish voltage levels
used to signal connect and disconnect and maintain the data lines at their idle values when not being
actively driven. Each hub downstream port requires a pull-down resistor (Rpd) on each data line; the hub
upstream port requires a pull-up resistor (Rpu) on its D+ line. Values for Rpu and Rpd appear in Chapter 7.

11.12.2 Edge Rate Control
Downstream hub ports must support transmission and reception of both low-speed and full-speed edge
rates. The respective signaling specifications are given in Chapter 7. Edge rate on a downstream port must
be selectable, based upon whether a downstream device was detected as being full-speed or low-speed.
The hub upstream port always uses full-speed signaling, and its output buffers always operate with fullspeed edge rates and signal polarities.

11.13 Hub Controller
The Hub Controller is logically organized as shown in Figure 11-11.

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Universal Serial Bus Specification Revision 1.1

UPSTREAM CONNECTION

Status Change
Endpoint

ENDPOINT 0:
Configuration
Information

Port NPort 1

Port 2

Port 3Figure 11-11. Example Hub Controller Organization

11.13.1 Endpoint Organization
The Hub Class defines one additional endpoint beyond Default Control Pipe, which is required for all
devices: the Status Change endpoint. The host system receives port and hub status change notifications
through the Status Change endpoint. The Status Change endpoint is an interrupt endpoint. If no hub or
port status change bits are set, then the hub returns an NAK when the Status Change endpoint is polled.
When a status change bit is set, the hub responds with data, as shown in Section 11.13.4, indicating theentity (hub or port) with a change bit set. The USB System Software can use this data to determine which
status registers to access in order to determine the exact cause of the status change interrupt.

11.13.2 Hub Information Architecture and Operation
Figure 11-12 shows how status, status change, and control information relate to device states. Hubdescriptors and Hub/Port Status and Control are accessible through the Default Control Pipe. The Hub
descriptors may be read at any time. When a hub detects a change on a port or when the hub changes its
own state, the Status Change endpoint transfers data to the host in the form specified in Section 11.13.4.
Hub or port status change bits can be set because of hardware or software events. When set, these bits
remain set until cleared directly by the USB System Software through a ClearPortFeature() request or by a
hub reset. While a change bit is set, the hub continues to report a status change when polled until allchange bits have been cleared by the USB System Software.

257

Status Information(static)stu
ta e slS g
Al h a nC

Host Software (e.g., Hub
Driver)Universal Serial Bus Specification Revision 1.1

Change Information(due to hardware
events)

Hard

ware

Eve

nts

Hardware Events

Change Device
State

Device Control
Control Information(change device state)

Software Device
Control

Figure 11-12. Relationship of Status, Status Change, and Control Information to Device States
The USB System Software uses the interrupt pipe associated with the Status Change endpoint to detectchanges in hub and port status.

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Universal Serial Bus Specification Revision 1.1

11.13.3 Port Change Information Processing
Hubs report a port's status through port commands on a per-port basis. The USB System Software
acknowledges a port change by clearing the change state corresponding to the status change reported by thehub. The acknowledgment clears the change state for that port so future data transfers to the Status Change
endpoint do not report the previous event. This allows the process to repeat for further changes (see Figure11-13.)
Begin

System Software requests Interrupt Pipe notification for Status Change Information

Hub NAKs
status change
IN token

No

Change Data
Available ?
YesInterrupt Pipe returns Hub and Port Status Change Bitmap

Interrupt Pipe notification retired

System Software reads Hub or Port status (for affected ports)

Yes
Any Changed
State?

 Accumulate change information
 System Software clears

corresponding change state

No
System Software processes accumulated change information

Re-initialize Interrupt Pipe for Status Change endpoint

Return to
beginning

Figure 11-13. Port Status Handling Method

11.13.4 Hub and Port Status Change Bitmap
The Hub and Port Status Change Bitmap, shown in Figure 11-14, indicates whether the hub or a port has
experienced a status change. This bitmap also indicates which port(s) have had a change in status. The
hub returns this value on the Status Change endpoint. Hubs report this value in byte-increments. That is, if
a hub has six ports, it returns a byte quantity and reports a zero in the invalid port number field locations.
the USB System Software is aware of the number of ports on a hub (this is reported in the hub descriptor)

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and decodes the Hub and Port Status Change Bitmap accordingly. The hub reports any changes in hub
status in bit zero of the Hub and Port Status Change Bitmap.
The Hub and Port Status Change Bitmap size varies from a minimum size of one byte. Hubs report only as
many bits as there are ports on the hub, subject to the byte-granularity requirement (i.e., round up to thenearest byte).

N

2

1

0

Port N change detected

Port 2 change detected
Port 1 change detected
Hub change detected

Figure 11-14. Hub and Port Status Change Bitmap
Any time the Status Change endpoint is polled by the host controller and any of the Status Changed bits are
non-zero, the Hub and Port Status Change Bitmap is returned. Figure 11-15 shows an example creation
mechanism for hub and port change bits.
Per-Port Logic

Ex

Logical OR

ChangeInformation

pleChangeDetect Logic

am

Port NHub and Port Status Change Bitmap

N
Figure 11-15. Example Hub and Port Change Bit Sampling

11.13.5 Over-current Reporting and Recovery
USB devices must be designed to meet applicable safety standards. Usually, this will mean that a selfpowered hub implement current limiting on its downstream ports. If an over-current condition occurs, itcauses a status and state change in one or more ports. This change is reported to the USB System Software
so that it can take corrective action.

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A hub may be designed to report over-current as either a port or a hub event. The hub descriptor field
wHubCharacteristics is used to indicate the reporting capabilities of a particular hub (see Section 11.15.2).
The over-current status bit in the hub or port status field indicates the state of the over-current detection
when the status is returned. The over-current status change bit in the Hub or Port Change field indicates if
the over-current status has changed.
When a hub experiences an over-current condition, it must place all affected ports in the Powered-off state.
If a hub has per-port power switching and per-port current limiting, an over-current on one port may stillcause the power on another port to fall below specified minimums. In this case, the affected port is placedin the Powered-off state and C_PORT_OVER_CURRENT is set for the port, but
PORT_OVER_CURRENT is not set. If the hub has over-current detection on a hub basis, then an overcurrent condition on the hub will cause all ports to enter the Powered-off state. However, in this case,neither C_PORT_OVER_CURRENT nor PORT_OVER_CURRENT is set for the affected ports.
Host recovery actions for an over-current event should include the following:1.

Host gets change notification from hub with over-current event.

2.

Host extracts appropriate hub or port change information (depending on the information in thechange bitmap).

3.

Host waits for over-current status bit to be cleared to 0.

4.

Host cycles power on to all of the necessary ports (e.g., issues a SetPortFeature(PORT_POWER)
request for each port).

5.

Host re-enumerates all affected ports.

11.14 Hub Configuration
Hubs are configured through the standard USB device configuration commands. A hub that is notconfigured behaves like any other device that is not configured with respect to power requirements andaddressing. If a hub implements power switching, no power is provided to the downstream ports while the
hub is not configured. Configuring a hub enables the Status Change endpoint. The USB System Software
may then issue commands to the hub to switch port power on and off at appropriate times.
The USB System Software examines hub descriptor information to determine the hub’s characteristics. By
examining the hub’s characteristics, the USB System Software ensures that illegal power topologies are notallowed by not powering on the hub’s ports if doing so would violate the USB power topology. The device
status and configuration information can be used to determine whether the hub should be used as a bus or
self-powered device. Table 11-7 summarizes the information and how it can be used to determine thecurrent power requirements of the hub.
Table 11-7. Hub Power Operating Mode Summary
Configuration Descriptor
MaxPower

bmAttributes(Self Powered)

Hub
Device Status(Self Power)

0

0

N/A

N/A
This is an illegal set of information.

0

1

0

N/A
A device which is only self-powered, but does
not have local power cannot connect to the Bus
and communicate.

Explanation

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Table 11-7. Hub Power Operating Mode Summary (Continued)
Configuration Descriptor

Hub
Device Status(Self Power)

Explanation

MaxPower

bmAttributes(Self Powered)

0

1

1

Self-powered only hub and local power supply isgood. Hub status also indicates local power
good, see Section 11.16.2.5. Hub functionality isvalid anywhere depth restriction is not violated.

0

0

N/A

Bus-powered only hub. Downstream ports may
not be powered unless allowed in current
topology. Hub device status reporting Self
Powered is meaningless in combination of azeroed bmAttributes.Self-Powered.

0

1

0

This hub is capable of both self- and buspowered operating modes. It is currently only
available as a bus-powered hub.

0

1

1

This hub is capable of both self- and buspowered operating modes. It is currently
available as a self-powered hub.

A self-powered hub has a local power supply, but may optionally draw one unit load from its upstream
connection. This allows the interface to function when local power is not available (see Section 7.2.1.2).
When local power is removed (either a hub-wide over-current condition or local supply is off), a hub of
this type remains in the Configured state but transitions all ports (whether removable or non-removable) tothe Powered-off state. While local power is off, all port status and change information read as zero and all
SetPortFeature() requests are ignored (request is treated as a no-operation). The hub will use the Status
Change endpoint to notify the USB System Software of the hub event (see Section 11.16.2.5 for details on
hub status).
The MaxPower field in the configuration descriptor is used to report to the system the maximum power the
hub will draw from VBUS when the configuration is selected. For bus-powered hubs, the reported value
must not include the power for any of external downstream ports. The external devices attaching to the hub
will report their individual power requirements.
A compound device may power both the hub electronics and the permanently attached devices from VBUS.
The entire load may be reported in the hubs' configuration descriptor with the permanently attacheddevices each reporting self-powered, with zero MaxPower in their respective configuration descriptors.

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11.15 Descriptors
Hub descriptors are derived from the general USB device framework. Hub descriptors define a hub device
and the ports on that hub. The host accesses hub descriptors through the hub’s default pipe.
The USB specification (refer to Chapter 9) defines the following descriptors:


DeviceConfiguration

InterfaceEndpointString (optional).

The hub class defines additional descriptors (refer to Section 11.15.2). In addition, vendor-specificdescriptors are allowed in the USB device framework. Hubs support standard USB device commands as
defined in Chapter 9.

11.15.1 Standard Descriptors
The hub class pre-defines certain fields in standard USB descriptors. Other fields are either
implementation-dependent or not applicable to this class.
Note: for the descriptors and fields shown below, the bits in a field are organized in a little-endian fashion;that is, bit location 0 is the least significant bit and bit location 7 is the most significant bit of a byte value.
Device Descriptor
bDeviceClass=

HUB_CLASSCODE (09H)

bDeviceSubClass= 0

Interface Descriptor
bNumEndpoints= 1bInterfaceClass= HUB_CLASSCODE (09H)

bInterfaceSubClass = 0
bInterfaceProtocol= 0

Configuration Descriptor
MaxPower= The maximum amount of bus power the hub will consume in thisconfiguration

Endpoint Descriptor (for Status Change Endpoint)bEndpointAddress= Implementation-dependent; Bit 7: Direction = In(1)wMaxPacketSize= Implementation-dependent

bmAttributes= Transfer Type = Interrupt (00000111B )bInterval= FFH (Maximum allowable interval)

The hub class driver retrieves a device configuration from the USB System Software using the
GetDescriptor() device request. The only endpoint descriptor that is returned by the GetDescriptor()
request is the Status Change endpoint descriptor.

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11.15.2 Class-specific Descriptors
11.15.2.1 Hub Descriptor
Table 11-8 outlines the various fields contained by the hub descriptor.
Table 11-8. Hub Descriptor
Offset

Field

Size

Description0bDescLength1

Number of bytes in this descriptor, including this byte

1bDescriptorType

1

Descriptor Type, value: 29H for hub descriptor

2

bNbrPorts

1

Number of downstream ports that this hub supports

3

wHubCharacteristics

D1...D0: Logical Power Switching Mode
00: Ganged power switching (all ports’ power atonce)
01: Individual port power switching
1X: Reserved. Used only on 1.0 compliant hubsthat implement no power switching.
D2:
0:
1:

Identifies a Compound Device
Hub is not part of a compound device
Hub is part of a compound device

D4...D3: Over-current Protection Mode
00: Global Over-current Protection. The hub
reports over-current as a summation of all
ports’ current draw, without a breakdown of
individual port over-current status.
01: Individual Port Over-current Protection. The
hub reports over-current on a per-port basis.
Each port has an over-current indicator.
1X: No Over-current Protection. This option isallowed only for bus-powered hubs that do not
implement over-current protection.
D15...D5:

264

Reserved

5

bPwrOn2PwrGood

1

Time (in 2ms intervals) from the time the power-on
sequence begins on a port until power is good on that
port. The USB System Software uses this value todetermine how long to wait before accessing a
powered-on port.

6

bHubContrCurrent

1

Maximum current requirements of the Hub Controller
electronics in mA.

Universal Serial Bus Specification Revision 1.1

Table 11-8. Hub Descriptor (Continued)
Offset7

Field
DeviceRemovable

Size
Variable,dependingonnumber ofports onhub

Description
Indicates if a port has a removable device attached.
This field is reported on byte-granularity. Within abyte, if no port exists for a given location, the field
representing the port characteristics returns 0.
Bit value definition:
0B - Device is removable
1B - Device is non-removable
This is a bitmap corresponding to the individual portson the hub:
Bit 0: Reserved for future use
Bit 1: Port 1
Bit 2: Port 2....
Bit n: Port n (implementation-dependent, up to a
maximum of 255 ports).

Variable

PortPwrCtrlMask

Variable,dependingonnumber ofports onhub

This field exists for reasons of compatibility with
software written for 1.0 compliant devices. All bits inthis field should be set to 1B. This field has one bit foreach port on the hub with additional pad bits, if
necessary, to make the number of bits in the field an
integer multiple of 8.

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11.16 Requests
11.16.1 Standard Requests
Hubs have tighter constraints on request processing timing than specified in Section 9.2.6 for standard
devices because they are crucial to the 'time to availability' of all devices attached to USB. The worst caserequest timing requirements are listed below (apply to both Standard and Hub Class requests):.
1.

Completion time for requests with no data stage:

50 ms

2.

Completion times for standard requests with data stage(s)Time from setup packet to first data stage:
Time between each subsequent data stage:
Time between last data stage and status stage:

50 ms
50 ms
50 ms

As hubs play such a crucial role in bus enumeration, it is recommended that hubs average response times
be less than 5ms for all requests.
Table 11-9 outlines the various standard device requests.
Table 11-9. Hub Responses to Standard Device Requests
bRequest

Hub Response

CLEAR_FEATURE

Standard

GET_CONFIGURATION

Standard

GET_DESCRIPTOR

Standard

GET_INTERFACE

Undefined. Hubs are allowed to support only one interface

GET_STATUS

Standard

SET_ADDRESSStandard

SET_CONFIGURATION

Standard

SET_DESCRIPTOR

Optional

SET_FEATURE

Standard

SET_INTERFACE

Undefined. Hubs are allowed to support only one interface

SYNCH_FRAME

Undefined. Hubs are not allowed to have isochronous
endpoints

Optional requests that are not implemented shall return a STALL in the Data stage or Status stage of therequest.

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11.16.2 Class-specific Requests
The hub class defines requests to which hubs respond, as outlined in Table 11-10. Table 11-11 defines the
hub class request codes. All requests in the table below except for GetBusState() and SetHubDescriptor()
are mandatory.
Table 11-10. Hub Class Requests
RequestbmRequestTypebRequest

wValuewIndexwLength

Data

ClearHubFeature

00100000B

CLEAR_ FEATURE

FeatureSelector

Zero

Zero

None

ClearPortFeature

00100011B

CLEAR_ FEATURE

FeatureSelector

Port

Zero

None

GetBusState

10100011B

GET_ STATE

Zero

Port

One

Per-Port
Bus State

GetHubDescriptor

10100000B

GET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Zero orLanguageID

Descriptor
LengthDescriptor

GetHubStatus

10100000B

GET_ STATUS

Zero

Zero

Four

Hub
StatusandChangeIndicators

GetPortStatus

10100011B

GET_ STATUS

Zero

Port

Four

Port
StatusandChangeIndicators

SetHubDescriptor

00100000B

SET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Zero orLanguageID

Descriptor
LengthDescriptor

SetHubFeature

00100000B

SET_ FEATURE

FeatureSelector

Zero

Zero

None

SetPortFeature

00100011B

SET_ FEATURE

FeatureSelector

Port

Zero

None

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Table 11-11. Hub Class Request CodesbRequest

Value

GET_ STATUS

0

CLEAR_ FEATURE

1

GET_STATE2

SET_ FEATURE

3

Reserved for future use

4-5

GET_DESCRIPTOR

6

SET_DESCRIPTOR

7

Table 11-12 gives the valid feature selectors for the hub class. See Section 11.16.2.5 and Section 11.16.2.6 fora description of the features.
Table 11-12. Hub Class Feature Selectors

268

RecipientValue

C_HUB_LOCAL_POWER

Hub

0

C_HUB_OVER_CURRENT

Hub1

PORT_CONNECTION

Port

0

PORT_ENABLE

Port

1

PORT_SUSPEND

Port

2

PORT_OVER_CURRENT

Port

3

PORT_RESET

Port

4

PORT_POWER

Port

8

PORT_LOW_SPEED

Port

9

C_PORT_CONNECTION

Port

16

C_PORT_ENABLE

Port

17

C_PORT_SUSPEND

Port

18

C_PORT_OVER_CURRENT

Port

19

C_PORT_RESET

Port

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Universal Serial Bus Specification Revision 1.1

11.16.2.1 Clear Hub FeatureThis request resets a value reported in the hub status.
bmRequestTypebRequest

wValuewIndexwLengthData

00100000B

CLEAR_ FEATURE

FeatureSelector

Zero

Zero

None

Clearing a feature disables that feature; refer to Table 11-12 for the feature selector definitions that apply tothe hub as a recipient. If the feature selector is associated with a change indicator, clearing that indicator
acknowledges the change. This request format is used to clear either the C_HUB_LOCAL_POWER or
C_HUB_OVER_CURRENT features.
It is a Request Error if wValue is not a feature selector listed in Table 11-12 or if wIndex or wLength are notas specified above.
If the hub is not configured, the hub's response to this request is undefined.

11.16.2.2 Clear Port FeatureThis request resets a value reported in the port status.
bmRequestType

Brequest

wValuewIndexwLengthData

00100011B

CLEAR_ FEATURE

FeatureSelector

Port

Zero

None

The port number must be a valid port number for that hub, greater than zero.
Clearing a feature disables that feature or starts a process associated with the feature; refer to Table 11-12
for the feature selector definitions. If the feature selector is associated with a change indicator, clearingthat indicator acknowledges the change. This request format is used to clear the following features:

PORT_ENABLE

PORT_SUSPEND

PORT_POWER

C_PORT_CONNECTION

C_PORT_RESETC_PORT_ENABLE



C_PORT_SUSPEND



C_PORT_OVER_CURRENT.

Clearing the PORT_SUSPEND feature causes a host-initiated resume on the specified port. If the port is
not in the Suspended state, the hub should treat this request as a functional no-operation.
Clearing the PORT_ENABLE feature causes the port to beplaced in the Disabled state. If the port is in thePowered-off state, the hub should treat this request as a functional no-operation.
Clearing the PORT_POWER feature causes the port to be placed in the Powered-off state and may, subject
to the constraints due to the hub’s method of power switching, result in power being removed from theport. Refer to Section 11.11 on rules for how this request is used with ports that are gang-powered.
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It is a Request Error if wValue is not a feature selector listed in Table 11-12, if wIndex specifies a port that
doesn't exist, or if wLength is not as specified above. It is not an error for this request to try to clear afeature that is already cleared (hub should treat as a function no-operation).
If the hub is not configured, the hub's response to this request is undefined.

11.16.2.3 Get Bus State
This is an optional per-port diagnostic request that returns the bus state value, as sampled at the last EOF2
point.
bmRequestTypebRequest

wValuewIndexwLengthData

10100011B

GET_ STATE

Zero

Port

One

Per-Port Bus
State

The port number must be a valid port number for that hub, greater than zero. If an invalid port number is
specified or if wValue or wLength are not as specified above, then the hub shall return a STALL in the Data
stage of the request (aborting the Status stage).
Hubs may implement an optional diagnostic aid to facilitate system debug. Hubs implement this aid
through this optional request. This diagnostic feature provides a glimpse of the USB bus state as sampled
at the last EOF2 sample point.
Hubs that implement this diagnostic feature should store the bus state at each EOF2 state in preparation fora potential request in the following USB frame.
The data returned is bitmapped in the following manner:
Bit 0:The value of the D- signal

Bit 1:The value of the D+ signal

Bits 2-7: Reserved for future use and are reset to zero.
The hub must be able to return the bus state in the Data stage transaction within the frame in which therequest was received. If the hub does not receive ACK for the data packet, the device is not required toreturn the same data packet if the host continues with the Data stage. Rather, the hub will always return thebus state at the immediately prior EOF2 sample point along with the DATA0 PID.
Hubs that do not implement this request shall return a STALL in the Data stage of the request (aborting the
Status stage).
If the hub is not configured, the hub's response to this request is undefined.

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11.16.2.4 Get Hub Descriptor
This request returns the hub descriptor.
bmRequestTypebRequest

wValuewIndexwLengthData

10100000B

GET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Zero

Descriptor
LengthDescriptor

The GetDescriptor() request for the hub class descriptor follows the same usage model as that of thestandard GetDescriptor() request (refer to Chapter 9). The standard hub descriptor is denoted by using thevalue bDescriptorType defined in Section 11.15.2.1. All hubs are required to implement one hub
descriptor, with descriptor index zero.
If wLength is larger than the actual length of the descriptor, then only the actual length is returned. If
wLength is less than the actual length of the descriptor, then only the first wLength bytes of the descriptor
are returned; this is not considered an error even if wLength is zero.
It is a Request Error if wValue or wIndex are other than as specified above.
If the hub is not configured, the hub's response to this request is undefined.

11.16.2.5 Get Hub Status
This request returns the current hub status and the states that have changed since the previous
acknowledgment.
bmRequestTypebRequest

wValuewIndexwLengthData

10100000B

GET_ STATUS

Zero

Zero

Four

Hub Status
and Change
Indicators

The first word of data contains wHubStatus (refer to Table 11-13). The second word of data contains
wHubChange (refer to Table 11-14).
It is a Request Error if wValue, wIndex, or wLength are other than as specified above.
If the hub is not configured, the hub's response to this request is undefined.

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Table 11-13. Hub Status Field, wHubStatus
Bit
0

Description
Local Power Source: This is the source of the local power supply.
This field indicates whether hub power (for other than the SIE) is being provided by an external source orfrom the USB. . This field allows the USB System Software to determine the amount of power available froma hub to downstream devices.
0 = Local power supply good
1 = Local power supply lost (inactive)

1Over-current Indicator:
If the hub supports over-current reporting on a hub basis, this field indicates that the sum of all the ports’current has exceeded the specified maximum and all ports have been places in the Powered-off state. If the
hub reports over-current on a per-port basis or has no over-current detection capabilities, this field is always
zero. For more details on over-current protection, see Section 7.2.1.2.1.
0 = No over-current condition currently exists
1 = A hub over-current condition exists

2-15

Reserved
These bits return 0 when read.

There are no defined feature selector values for these status bits and they can neither be set nor cleared by
the USB System Software.
Table 11-14. Hub Change Field, wHubChange
Bit0

Description
Local Power Status Change: (C_HUB_LOCAL_POWER) This field indicates that a change has
occurred in the hub’s Local Power Source field in wHubStatus.
This field is initialized to zero when the hub receives a bus reset.
0 = No change has occurred to Local Power Status
1 = Local Power Status has changed

1Over-Current Indicator Change: (C_HUB_OVER_CURRENT) This field indicates if a change has
occurred in the Over-Current field in wHubStatus.
This field is initialized to zero when the hub receives a bus reset.
0 = No change has occurred to the Over-Current Indicator
1 = Over-Current Indicator has changed

2-15

Reserved
These bits return 0 when read.

Hubs may allow setting of these change bits with SetHubFeature() requests in order to support diagnostics.
If the hub does not support setting of these bits, it should either treat the SetHubFeature() request as aRequest Error or as a functional no-operation. When set, these bits may be cleared by a ClearHubFeature()request. A request to set a feature that is already set or to clear a feature that is already clear has no effectand the hub will not fail the request.

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11.16.2.6 Get Port Status
This request returns the current port status and the current value of the port status change bits.
bmRequestType

BRequest

wValuewIndexwLengthData

10100011B

GET_STATUS

Zero

Port

Four

Port Status
and Change
Indicators

The port number must be a valid port number for that hub, greater than zero.
The first word of data contains wPortStatus (refer to Table 11-15). The second word of data contains
wPortChange (refer to Table 11-14).
The bit locations in the wPortStatus and wPortChange fields correspond in a one-to-one fashion whereapplicable.
It is a Request Error if wValue or wLength are other than as specified above or if wIndex specifies a portthat does not exist.
If the hub is not configured, the behavior of the hub in response to this request is undefined.

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11.16.2.6.1 Port Status Bits
Table 11-15. Port Status Field, wPortStatus
Bit
0

Description
Current Connect Status: (PORT_CONNECTION) This field reflects whether or not a device is currently
connected to this port.
0 = No device is present
1 = A device is present on this port

1

Port Enabled/Disabled: (PORT_ENABLE) Ports can be enabled by the USB System Software only. Ports
can be disabled by either a fault condition (disconnect event or other fault condition) or by the USB System
Software.
0 = Port is disabled
1 = Port is enabled2

3

Suspend: (PORT_SUSPEND) This field indicates whether or not the device on this port is suspended.
Setting this field causes the device to suspend by not propagating bus traffic downstreamThis field my be resetby a request or by resume signaling from the device attached to the port.
0 = Not suspended
1 = Suspended or resuming
Over-current Indicator: (PORT_OVER_CURRENT)
If the hub reports over-current conditions on a per-port basis, this field will indicate that that the current drain on
the port exceeds the specified maximum.For more details, see Section 7.2.1.2.1.
0 = All no over-current condition exists on this port
1 = An over-current condition exists on this port.

4

Reset: (PORT_RESET) This field is set when the host wishes to reset the attached device. It remains set
until the reset signaling is turned off by the hub.
0 = Reset signaling not asserted
1 = Reset signaling asserted

5-7Reserved
These bits return 0 when read.

8

Port Power: (PORT_POWER) This field reflects a port’s logical, power control state. Because hubs canimplement different methods of port power switching, this field may or may not represent whether power isapplied to the port. The device descriptor reports the type of power switching implemented by the hub.
0 = This port is in the Powered-off state
1 = This port is not in the Powered-off state

9

Low Speed Device Attached: (PORT_LOW_SPEED) This is relevant only if a device is attached.
0 = Full-speed device attached to this port
1 = Low-speed device attached to this port

10-15

Reserved
These bits return 0 when read.

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11.16.2.6.1.1 PORT_CONNECTION
When the Port Power bit is one, this bit indicates whether or not a device is attached. This field reads as
one when a device is attached; it reads as zero when no device is attached. This bit is reset to zero when
the port is in the Powered-off state or the Disconnected states. It is set to one when the port is in thePowered state, a device attach is detected (see Section 7.1.7.1) and the port transitions from the
Disconnected state to the Disabled state.
SetPortFeature(PORT_CONNECTION) and ClearPortFeature(PORT_CONNECTION) requests shall not
be used by the USB System Software and must be treated as no-operation requests by hubs.

11.16.2.6.1.2 PORT_ENABLE
This bit is set when the port is allowed to send or receive packet data or resume signaling.
This bit may be set only as a result of a SetPortFeature(PORT_RESET) request. When the hub exits theResetting state or, if present, the Speed_eval state, this bit is set and bus traffic may be transmitted to theport. This bit may be cleared as the result of any of the following:The port being in the Powered-off stateReceipt of a ClearPortFeature(PORT_ENABLE) request

Port_Error detection

Disconnect detectionWhen the port enters the Resetting state as a result of receiving the SetPortFeature(PORT_RESET)
request.

The hub response to a SetPortFeature(PORT_ENABLE) request is not specified. The preferred behavior isthat the hub respond with a Request Error. This may not be used by the USB System Software. The
ClearPortFeature(PORT_ENABLE) request is supported as specified in Section 11.5.1.4.

11.16.2.6.1.3 PORT_SUSPEND
This bit is set to one when the port is selectively suspended by the USB System Software. While this bit isset, the hub does not propagate downstream-directed traffic to this port, but the hub will respond to resume
signaling from the port. This bit can be set only if the port’s PORT_ENABLE bit is set and the hubreceives a SetPortFeature(PORT_SUSPEND) request. This bit is cleared to zero on the transition from the
SendEOP state to the Enabled state, or on the transition from the Restart_S state to the Transmit state, or on
any event that causes the PORT_ENABLE bit to be cleared while the PORT_SUSPEND bit is set.
The SetPortFeature(PORT_SUSPEND) request may be issued by the USB System Software at any time but
will have an effect only as specified in Section 11.5.

11.16.2.6.1.4 PORT_OVER-CURRENT
This bit is set to one while an over-current condition exists on the port. This bit is cleared when an overcurrent condition does not exist on the port.
If the voltage on this port is affected by an over-current condition on another port then this bit is set andremains set until the over-current condition on the affecting port is removed. When the over-current
condition on the affecting port is removed, this bit is reset to zero if an over-current condition does notexist on this port.
Over-current protection is required on self-powered hubs (it is optional on bus-powered hubs) as outlined
in Section 7.2.1.2.1.

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The SetPortFeature(PORT_OVER_CURRENT) and ClearPortFeature(PORT_OVER_CURRENT)
requests shall not be used by the USB System Software and may be treated as no-operation requests by
hubs.

11.16.2.6.1.5 PORT_RESETThis bit is set while the port is in the Resetting state. A SetPortFeature(PORT_RESET) request will initiate
the Resetting state if the conditions in Section 11.5.1.5 are met. This bit is set to zero while the port is in
the Powered-off state.
The ClearPortFeature(PORT_RESET) request shall not be used by the USB System Software and may betreated as a no-operation request by hubs.

11.16.2.6.1.6 PORT_POWER
This bit reflects the current power state of a port. This bit is implemented on all ports whether or not actual
port power switching devices are present.
While this bit is zero, the port is in the Powered-off state. Similarly, anything that causes this port to go to
the Power-off state will cause this bit to be set to zero.
A SetPortFeature(PORT_POWER) will set this bit to one unless both C_HUB_LOCAL_POWER and
Local Power Status (in wHubStatus) are set to one in which case the request is treated as a functional nooperation.
This bit may be cleared under the following curcumstances: Hub receives a ClearPortFeature(PORT_POWER).
 An over-current condition exists on the port.
 An over-current condition on another port causes the power on this port to be shut off.

The SetPortFeature(PORT_POWER) and ClearPortFeature(PORT_POWER) requests may be issued by the
USB System Software whenever the port is not in the Not Configured state, but will have an effect only as
specified in Section 11.11.

11.16.2.6.1.7 PORT_LOW_SPEED
This bit has meaning only when the PORT_ENABLE bit is set. This bit is set to one if the attached device
is low-speed.
The SetPortFeature(PORT_LOW_SPEED) and ClearPortFeature(PORT_LOW_SPEED) requests shall not
be used by the USB System Software and may be treated as no-operation requests by hubs.

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11.16.2.6.2 Port Status Change Bits
Port status change bits are used to indicate changes in port status bits that are not the direct result of
requests. Port status change bits can be cleared with a ClearPortFeature() request or by a hub reset. Hubs
may allow setting of the status change bits with a SetPortFeature() request for diagnostic purposes. If a
hub does not support setting of the status change bits, it may either treat the request as a Request Error or as
a functional no-operation. Table 11-16 describes the various bits in the wPortChange field.
Table 11-16. Port Change Field, wPortChange
Bit

Description

0

Connect Status Change: (C_PORT_CONNECTION) Indicates a change has occurred in the port’s Current
Connect Status. The hub device sets this field as described in Section 11.16.2.6.2.1.
0 = No change has occurred to Current Connect status
1 = Current Connect status has changed

1

Port Enable/Disable Change: (C_PORT_ENABLE) This field is set to one when a port is disabled becauseof a Port_Error condition (see Section 11.8.1).

2

Suspend Change: (C_PORT_SUSPEND) This field indicates a change in the host-visible suspend state of
the attached device. It indicates the device has transitioned out of the Suspend stateThis field is set only whenthe entire resume process has completed. That is, the hub has ceased signaling resume on this port.
0 = No change
1 = Resume complete

3Over-Current Indicator Change: (C_PORT_OVER_CURRENT) This field applies only to hubs that reportover-current conditions on a per-port basis (as reported in the hub descriptor).
0 = No change has occurred to Over-Current Indicator
1 = Over-Current Indicator has changed
If the hub does not report over-current on a per-port basis, then this field is always zero.

4

Reset Change: (C_PORT_RESET) This field is set when reset processing on this port is complete.
0 = No change
1 = Reset complete

5-15

Reserved
These bits return 0 when read.

11.16.2.6.2.1 C_PORT_CONNECTION
This bit is set when the PORT_CONNECTION bit changes because of an attach or detach detect event (seeSection 7.1.7.1). This bit will be cleared to zero by a ClearPortFeature(C_PORT_CONNECTION) requestor while the port is in the Powered-off state.

11.16.2.6.2.2 C_PORT_ENABLE.
This bit is set when the PORT_ENABLE bit changes from one to zero as a result of a Port Error condition(see Section 11.8.1.). This bit is not set on any other changes to PORT_ENABLE.
This bit may be set if, on a SetPortFeature(PORT_RESET) the port stays in the Disabled state because aninvalid idle state exists on the bus (see Section 11.8.2).
This bit will be cleared by a ClearPortFeature(C_PORT_ENABLE) request or while the port is in thePowered-off state.

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Universal Serial Bus Specification Revision 1.1

11.16.2.6.2.3 C_PORT_SUSPEND
This bit is set on the following transitions:on transition from the Resuming state to the SendEOP stateon transition from the Restart_S state to the Transmit state.

This bit will be cleared by a ClearPortFeature(C_PORT_SUSPEND) request, or while the port is in thePowered-off state.

11.16.2.6.2.4 C_PORT_OVER-CURRENT
This bit is set when the PORT_OVER_CURRENT bit changes from zero to one or from one to zero. Thisbit is also set if the port is placed in the Powered-off state due to an over-current condition on another port.
This bit will be cleared when the port is in the Not Configured state or by a
ClearPortFeature(C_PORT_OVER-CURRENT) request.

11.16.2.6.2.5 C_PORT_RESETThis bit is set when the port transitions from the Resetting state (or, if present, the Speed_eval state) to the
Enabled state.
This bit will be cleared by a ClearPortFeature(C_PORT_RESET) request, or while the port is in thePowered-off state.

11.16.2.7 Set Hub Descriptor
This request overwrites the hub descriptor.
bmRequestTypebRequest

wValuewIndexwLengthData

00100000B

SET_DESCRIPTOR

Descriptor
Type andDescriptor
Index

Zero

Descriptor
LengthDescriptor

The SetDescriptor request for the hub class descriptor follows the same usage model as that of the standard
SetDescriptor request (refer to Chapter 9). The standard hub descriptor is denoted by using the valuebDescriptorType defined in Section 11.15.2.1. All hubs are required to implement one hub descriptor,
with descriptor index zero.
This request is optional. This request writes data to a class-specific descriptor. The host provides the datathat is to be transferred to the hub during the data transfer phase of the control transaction. This request
writes the entire hub descriptor at once.
Hubs must buffer all the bytes received from this request to ensure that the entire descriptor has beensuccessfully transmitted from the host. Upon successful completion of the bus transfer, the hub updates thecontents of the specified descriptor.
It is a Request Error if wIndex is not zero or if wLength does not match the amount of data sent by the host.
Hubs that do not support this request respond with a STALL during the Data stage of the request.
If the hub is not configured, the hub's response to this request is undefined.

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Universal Serial Bus Specification Revision 1.1

11.16.2.8 Set Hub FeatureThis request sets a value reported in the hub status.
bmRequestTypebRequest

wValuewIndexwLengthData

00100000B

SET_ FEATURE

FeatureSelector

Zero

Zero

None

Setting a feature enables that feature; refer to Table 11-12for the feature selector definitions that apply tothe hub as recipient. Change indicators may not be acknowledged using this request.
It is a Request Error if wValue is not a feature selector listed in Table 11-12 or if wIndex or wLength are notas specified above.
If the hub is not configured, the hub's response to this request is undefined.

11.16.2.9 Set Port FeatureThis request sets a value reported in the port status.
bmRequestTypebRequest

wValuewIndexwLengthData

00100011B

SET_ FEATURE

FeatureSelector

Port

Zero

None

The port number must be a valid port number for that hub, greater than zero.
Setting a feature enables that feature or starts a process associated with that feature; see Table 11-12 for thefeature selector definitions that apply to a port as a recipient. Change indicators may not be acknowledgedusing this request. Features that can be set with this request are:


PORT_RESETPORT_SUSPEND

PORT_POWER

C_PORT_CONNECTION*



C_PORT_RESET*C_PORT_ENABLE*

C_PORT_SUSPEND*



C_PORT_OVER_CURRENT*.

*denotes features that are not required to be set by this request.
Setting the PORT_SUSPEND feature causes bus traffic to cease on that port and, consequently, the device
to suspend. Setting the reset feature PORT_RESET causes the hub to signal reset on that port. When thereset signaling is complete, the hub sets the C_PORT_RESET change indicator and immediately enables
the port. Refer to Section 11.10 for a complete discussion of host-initiated reset behavior. Also see
Section 11.16.2.6.1 for further details.
The hub must meet the following requirements:

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If the port is in the Powered-off state, the hub must treat a SetPortFeature(PORT_RESET) request as afunctional no-operation.



If the port is not in the Enabled or Transmitting state, the hub must treat a
SetPortFeature(PORT_SUSPEND) request as a functional no-operation.



If the port is not in the Powered-off state, the hub must treat a SetPortFeature(PORT_POWER) request
as a functional no-operation.

It is a Request Error if wValue is not a feature selector listed in Table 11-12, if wIndex specifies a port thatdoen't exist, or if wLength is not as specified above.
If the hub is not configured, the hub's response to this request is undefined.

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Universal Serial Bus Specification Revision 1.1

Index
4X over-sampling state machine DPLLs, 7.1.15

A
abnormal termination sequences, 11.3.3
aborting/retiring transfers
aborting control transfers, 5.5.5
client role in, 10.5.2.2
conditions for, 5.3.2
message pipes and, 5.3.2.2
packet size and, 5.5.3
USBDI role, 10.5.3.2.1
access frequency of control pipes, 5.5.4
Acknowledge packet. See ACKs
ACKs, 8.3.1 Table 8-1
in bulk transfers, 8.5.1
in control transfers, 8.5.2, 8.5.2.1
corrupted ACK handshake, 8.5.2.3, 8.6.4
in data toggle, 8.6, 8.6.1, 8.6.2
defined, 2.0 glossaryfunction response to OUT transactions, 8.4.5.3
host response to IN transactions, 8.4.5.2
overview, 8.4.4
in request processing, 9.2.6
A connectors. See Series "A" and "B" connectors
active devices, defined, 2.0 glossary
active pipes, 10.5.2.2
Active state (transmitter state machine), 11.6.2,
11.6.2.2
adaptive endpoints
connection requirements, 5.10.4.3
feedback for isochronous transfers, 5.10.4.2
overview, 5.10.4.1.3
adding devices. See dynamic insertion and removal
Address device statebus enumeration process, 9.1.2
overview, 9.1.1.4
standard device requests, 9.4.1:9.4.11
visible device state table, 9.1.1 Table 9-1
addresses
Address device state, 9.1.1 Table 9-1, 9.1.1.4,
9.1.2, 9.4.1:9.4.11
aliasing, 8.3.2
assignment
after dynamic insertion or removal, 4.6.3
bus enumeration, 2.0 glossary, 4.6.3, 9.1.2
device initialization, 10.5.1.1
operations overview, 9.2.2
re-enumerating sub-trees, 10.5.4.6
staged power switching in functions and, 7.2.1.4
time limits for completing, 9.2.6.3
USB System Software role, 4.9
endpoint addresses, 5.3.1, 9.6.4
SetAddress() request, 9.4.6
address fields
address field (ADDR), 8.3.2.1, 8.3.5.1, 8.4.1
endpoint field (ENDP), 8.3.2.2, 8.3.5.1, 8.4.1
packet address fields, 8.3.2:8.3.2.2

ADDR field
overview, 8.3.2.1
token CRCs, 8.3.5.1
in token packets, 8.4.1
aging, data-rate inaccuracies and, 7.1.11
aliasing addresses, 8.3.2
allocating bit times in handshake packets, 11.3.3
allocating buffers. See buffers
allocating USB bandwidthtransfer management, 5.9.1:5.9.1.5
USB System role, 10.3.2
alternate settings for interfaces
configuration requirements, 10.3.1
GetInterface() request, 9.4.4
in interface descriptors, 9.6.3
SetInterface() request, 9.4.10
USBDI mechanisms, 10.5.2.10
USB support for, 9.2.3
American National Standard/Electronic Industries
Association, 6.7.1
American Standard Test Materials, 6.7.1
ANSI/EIA-364-C (12/94), 6.7.1
applications
in source-to-sink connectivity, 5.10.4.3
USB suitability for, 3.3
application space taxonomy, 3.2
architectural overview of USB
architectural extensions, 4.10
bus protocol, 4.4
bus topology, 4.1.1
data flow types, 4.7, 4.7:4.7.5
hub architecture, 4.8.2.1, 11.1.1, 11.13.2
mechanical and electrical specifications, 4.2:4.2.2,
6.1
physical interface, 4.2, 4.2:4.2.2
power, 4.3, 4.3:4.3.2
robustness and error handling, 4.5, 4.5:4.5.2
system configuration, 4.6, 4.6:4.6.3
USB devices, 4.1.1.2, 4.8, 4.8:4.8.2.2
USB host, 4.1.1.1, 4.9
USB system description, 4.1, 4.1:4.1.1.2
assigning addresses. See addresses; bus
enumeration
ASTM-D-4565, 6.6.3, 6.7.1
ASTM-D-4566, 6.6.3, 6.7.1
asynchronous data transfers, 2.0 glossary, 4.9
asynchronous endpoints
connection requirements, 5.10.4.3
feedback for isochronous transfers, 5.10.4.2
overview, 5.10.4.1.1
asynchronous RA, 2.0 glossary, 5.10.4.3. See also
RA (rate adaptation)
asynchronous SRC, 2.0 glossary. See also SRC
Attached device statein bus enumeration process, 9.1.2
overview, 9.1.1.1
visible device state table, 9.1.1 Table 9-1
attaching devices. See dynamic insertion and removal

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attenuation, 7.1.17
attributes of devices in configuration descriptors, 9.6.2
attributes of endpoints in endpoint descriptors, 9.6.4
audio connectivity, 5.10.4.3.1
Audio Device Class Specification Revision 1.0, 9.6
audio devices, defined, 2.0 glossary
available time in framesbulk transfers and, 5.8.4
bus bandwidth reclamation, 5.9.5
control transfers and, 5.5.4
interrupt transfer bus access constraints, 5.7.4
isochronous transfers and, 5.6, 5.6.4
AWG, 2.0 glossary, 6.6.2

Bbabbledefined, 2.0 glossary
EOF2 timing points and, 11.2.2
error detection and recovery, 8.7.4
background of USB development, 3.1:3.3
bAlternateSetting field (interface descriptors), 9.6.3
bandwidth
allocating for pipes, 4.4, 4.7.5
bandwidth reclamation, 5.9.5
defined, 2.0 glossarytransfer management, 4.7.5, 5.9.1:5.9.1.5, 10.3.2
USB system role in, 10.3.2
battery-powered hubs, 7.2.1
bcdDevice field (device descriptors), 9.6.1
bcdUSB field (device descriptors), 9.6.1
bConfigurationValue field (configuration descriptors),
9.6.2
B connectors. See Series "A" and "B" connectors
bDescLength field (hub descriptors), 11.15.2.1
bDescriptorType field
configuration descriptors, 9.6.2
device descriptors, 9.6.1
endpoint descriptors, 9.6.4
hub descriptors, 11.15.2.1, 11.16.2.4, 11.16.2.7
interface descriptors, 9.6.3
string descriptors, 9.6.5
bDeviceClass field, 9.6.1, 11.15.1
bDeviceProtocol field (device descriptors), 9.6.1
bDeviceSubClass field, 9.6.1, 11.15.1
bEndpointAddress field, 9.6.4, 11.15.1
bHubContrCurrent field (hub descriptors), 11.15.2.1
bi-directional communication flow, 5.6.2, 5.8.2
big endian, defined, 2.0 glossarybInterfaceClass field, 9.6.3, 11.15.1
bInterfaceNumber field (interface descriptors), 9.6.3
bInterfaceProtocol field, 9.6.3, 11.15.1
bInterfaceSubClass field, 9.6.3, 11.15.1
bInterval field, 9.6.4, 11.15.1
bit cells, decoding, 7.1.15
bitmaps of hub and port status changes, 11.13.4
bit ordering, 8.1
bits, defined, 2.0 glossary
bit stuffing
bit stuff violations, 8.7.1
calculating transaction times, 5.9.3
defined, 2.0 glossary
overview, 7.1.9282

bit times
bit time designations, 11.3
bit time zero, 11.3
in transaction completion prediction, 11.3.3
bLength field
configuration descriptors, 9.6.2
device descriptors, 9.6.1
endpoint descriptors, 9.6.4
interface descriptors, 9.6.3
string descriptors, 9.6.5
blocking packets in Collision conditions, 11.8.3
blunt cut termination, 6.4.2, 6.4.3
bmAttributes field
configuration descriptors, 9.6.2
endpoint descriptors, 9.6.4
hub descriptors, 11.14, 11.15.1
bMaxPacketSize0 field (device descriptors), 9.6.1
bmRequestType field
hub class requests, 11.16.2
overview, 9.3.1
Setup data format, 9.3
standard device requests, 9.4
bNbrPorts field (hub descriptors), 11.15.2.1
bNumConfigurations field (device descriptors), 9.6.1
bNumEndpoints field, 9.6.3, 11.15.1
bNumInterfaces field (configuration descriptors), 9.6.2
bPwrOn2PwrGood field, 11.11, 11.15.2.1
bRequest field
hub class requests, 11.16.2
overview, 9.3.2
Setup data format, 9.3
standard device requests, 9.4
standard hub requests, 11.16.1
broadcast mode of hub operation, 11.1.2.1
B/S or b/S, defined, 2.0 glossary
bString field (string descriptors), 9.6.5
buffers
buffer impedance, 7.1.1.1
calculating sizes in functions and software, 5.9.4
client pipes and, 10.5.1.2.2
client role in, 10.3.3, 10.5.3
defined, 2.0 glossary
endpoint buffer size, 4.4
hub I/O buffer requirements, 11.12:11.12.2
identifying location and length, 10.3.4
interrupt transfers and, 5.7.3
isochronous transfers and, 5.10.4.2
non-USB isochronous application, 5.10.1
packet buffers, 2.0 glossary
prebuffering data, 5.10.5
rate matching and, 5.10.8
rise and fall times for full-speed buffers, 7.1.2
USBD role in allocating, 10.5.1.2.1
bulk transfers
bus access constraints, 5.8.4
data format, 5.8.1
data sequences, 5.8.5
defined, 2.0 glossary, 5.4
direction, 5.8.2
overview, 4.7.2, 5.8
packet size, 5.8.3, 9.6.4
transaction format, 8.5.1

Universal Serial Bus Specification Revision 1.1

bulk transfers (continued)
transaction organization within IRPs, 5.9.2
USBD pipe mechanism responsibilities, 10.5.3.1.3
bus access for transfersbulk transfer constraints, 5.8.4
bus bandwidth reclamation, 5.9.5
calculating buffer sizes, 5.9.4
calculating bus transaction times, 5.9.3
client software role in, 5.9.1.1
control transfer constraints, 5.5.4
HCD role in, 5.9.1.3
Host Controller role in, 5.9.1.5
interrupt transfer constraints, 5.7.4
isochronous transfer constraints, 5.6.4
transaction list, 5.9.1.4
transaction tracking, 5.9.2
transfer management, 5.9.1:5.9.1.5
transfer type overview, 5.4
USBD role in, 5.9.1.2
bus clock, 5.10.2, 5.10.3, 5.10.8
bus enumeration
defined, 2.0 glossary
device initialization, 10.5.1.1
overview, 4.6.3, 9.1.2
re-enumerating sub-trees, 10.5.4.6
staged power switching in functions, 7.2.1.4
USB System Software role, 4.9
Bus Management services, 10.5.4.2
bus-powered devices and functions
configuration descriptors, 9.6.2
defined, 4.3.1
device states, 9.1.1.2
high-power bus-powered functions, 7.2.1.4
low-power bus-powered functions, 7.2.1.3
power budgeting, 9.2.5.1
bus-powered hubs
configuration, 11.14
defined, 4.3.1, 7.2.1
device states, 9.1.1.2
overview, 7.2.1.1
power switching, 11.11
voltage drop budget, 7.2.2
bus protocol overview, 4.4
Bus_Reset receiver state, 11.6.1, 11.6.1.6
bus states
detecting, 11.16.2.3
evaluating after reset, 7.1.7.1
global suspend, 7.1.7.4.1
Host Controller role in state handling, 10.2.1
signaling levels and, 7.1.7
bus timing/electrical characteristics, 7.3.2
bus topology, 5.2:5.2.5
client-software-to-function relationship, 5.2.5
defined, 4.1
devices, 5.2.2
hosts, 5.2.1
illustrated, 4.1.1
logical bus topology, 5.2.4
physical bus topology, 5.2.3
bus transaction timeout in isochronous transfers,
5.10.7bus turn-around time, 2.0 glossary, 7.1.18, 8.7.2

bypass capacitors, 7.2.4.1, 7.2.4.2
bytes, defined, 2.0 glossaryCcable assemblies, 6.4:6.4.4
cable attenuation, 7.1.17
cable delay
electrical characteristics, 7.3.2 Table 7-9
full-speed cables, 6.4.2
hub differential delay, differential jitter, and SOP
distortion, 7.3.3 Figure 7-42
hub EOP delay and EOP skew, 7.3.3 Figure 7-43
hub signaling timings, 7.1.14
inter-packet delay and, 7.1.18
low-speed cables, 6.4.3, 7.1.1.2
overview, 7.1.16
propagation delay, 6.4.1, 6.7 Table 6-7, 7.1.1.2
skew delay, 6.7 Table 6-7, 7.1.3, 7.3.3 Figure 7-43
cablesattenuation, 7.1.17
cable assemblies, 6.4:6.4.4
cable delay (See cable delay)
captive cables
full-speed captive cable assemblies, 6.4.2
inter-packet delay and, 7.1.18
low-speed captive cable assemblies, 6.4.3
maximum capacitance, 7.1.6
termination, 7.1.5
color choices, 6.4
construction, 6.6.2
description, 6.6.1
detachable cables
cable delay, 7.1.16
connectors and, 6.2
detachable cable assemblies, 6.4.1
inter-packet delay and, 7.1.18
low-speed detachable cables, 6.4.4
maximum capacitance, 7.1.6
termination, 7.1.5
voltage drop budget, 7.2.2
electrical characteristics and standards, 4.2.1,
6.6.3, 6.7, 7.3.2 Table 7-9
end-to-end signal delay, 7.1.19
environmental characteristics, 6.6.4, 6.7
flyback voltage, 7.2.4.2
full-speed cables, 6.4.2
impedance, 6.4.1, 6.4.2, 6.7 Table 6-7
input capacitance, 7.1.6
length, 6.4.1, 6.4.2, 6.4.3
listing, 6.6.5
low-speed cables, 6.4.3, 6.4.4
mechanical configuration and material
requirements, 6.6:6.6.5, 6.7
overview, 6.3
prohibited cable assemblies, 6.4.4
pull-out standards, 6.7 Table 6-7
shielding, 6.6, 6.6.1
termination, 7.1.5
voltage drop budget, 7.2.2

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Universal Serial Bus Specification Revision 1.1

calculations
buffering for rate matching, 5.10.8
buffer sizes in functions and software, 5.9.4
bus transaction times, 5.9.3
capabilities, defined, 2.0 glossary
capacitanceafter dynamic attach, 7.2.4.1
decoupling capacitance, 7.3.2 Table 7-5
input capacitance, 7.1.6, 7.3.2 Table 7-5
low-speed buffers, 7.1.1.2, 7.1.2
low-speed cable capacitive loads, 6.4.3
optional edge rate control capacitors, 7.1.6
pull-up resistors and, 7.1.5
single-ended capacitance, 7.1.1.2
small capacitors, 7.1.6
target maximum droop and, 7.2.4.1
unmated contact capacitance, 7.3.2 Table 7-9capacitive load, 6.7 Table 6-7
captive cables
full-speed captive cable assemblies, 6.4.2
inter-packet delay and, 7.1.18
low-speed captive cable assemblies, 6.4.3
maximum capacitance, 7.1.6
termination, 7.1.5
change bits
device states, 11.13.2
hub and port status change bitmap, 11.13.4
hub status, 11.16.2.5
over-current status change bits, 11.13.5
port status change bits, 11.16.2.6.2:11.16.2.6.2.5
Status Change endpoint defined, 11.13.1
change propagation, host state handling of, 10.2.1
characteristics of devices, 2.0 glossary, 9.6.2
C_HUB_LOCAL_POWER, 11.11, 11.16.2, 11.16.2.1,
11.16.2.5, 11.16.2.6.1.6
C_HUB_OVER_CURRENT, 11.16.2, 11.16.2.1
C_HUB_OVER_POWER, 11.16.2.5
classes of devices. See device classes
Class field, 9.2.3, 9.6.3
class-specific descriptors, 9.5, 11.15.2.1
class-specific requests
hub class-specific requests, 11.16.2:11.16.2.9
time limits for completing, 9.2.6.5
USBDI mechanisms, 10.5.2.8
ClearFeature() request, CLEAR_FEATURE
ClearHubFeature() request, 11.16.2.1
ClearPortFeature() request, 11.16.2.2
endpoint status and, 9.4.5
hub class requests, 11.16.2
hub requests, 11.16.1
overview, 9.4.1
standard device request codes, 9.4
ClearHubFeature() request
clearing hub features, 11.16.2.5
hub class requests, 11.16.2
hub class-specific requests, 11.16.2.1
clearing pipes, 10.5.2.2
ClearPortFeature() request
clearing status change bits, 11.13.2, 11.16.2.6.2
C_PORT_CONNECTION, 11.16.2.6.2.1
C_PORT_ENABLE, 11.16.2.6.2.2
C_PORT_OVER-CURRENT, 11.16.2.6.2.4

284

ClearPortFeature() request (continued)
C_PORT_RESET, 11.16.2.6.2.5
C_PORT_SUSPEND, 11.16.2.6.2.3
hub class requests, 11.16.2, 11.16.2.2
PORT_CONNECTION, 11.16.2.6.1.1
PORT_ENABLE, 11.5.1.4, 11.16.2.6.1.2
PORT_LOW_SPEED, 11.16.2.6.1.7
PORT_OVER_CURRENT, 11.16.2.6.1.4
PORT_POWER, 11.5.1.2, 11.16.2.6.1.6
PORT_RESET, 11.16.2.6.1.5
PORT_SUSPEND, 11.5.1.9
client pipes, 10.5.1.2.2
client softwarein bus topology, 5.2, 5.2.1, 5.2.5
client software-to-function relationships, 5.2, 5.2.5
in communication flow, 5.3
control transfers and, 5.5
defined, 2.0 glossary
as implementation focus area, 5.1
Master Client status, 10.5.3.2.4, 10.5.4.2
notification identification, 10.3.4
role in configuration, 10.3.1
role in data transfers, 10.3.3
service clock and, 5.10.2
in source-to-sink connectivity, 5.10.4.3
in transfer management, 5.9.1, 5.9.1.1
clock model
buffering for rate matching, 5.10.8
bus clock, 5.10.2
clock encoding scheme in electrical specifications
overview, 4.2.1
clock synchronization, 5.10.3
clock-to-clock phase differences, 5.10.3
defined, 5.10
hub clock source, 11.2.1
in non-USB isochronous application, 5.10.1
overview, 5.10.2
sample clock, 5.10.2
service clock, 5.10.2
using SOF tokens as clocks, 5.10.5
clock timings, 7.3.2 Table 7-6, 7.3.2 Table 7-7
CMOS driver circuit, 7.1.1.1
codes. See specific types of codes
Collision conditions, 11.8.3
color choices
cables, 6.4
plugs, 6.5.4.1
receptacles, 6.5.3.1
commanded stalls, 8.4.4
commands. See requests
common mode range for differential input sensitivity,
7.1.4
Communication Cables (UL Subject-444), 6.7.1
Communication Cables (UL Subject-444), 6.6.5
communication flow, 5.3:5.3.2.2
completed operations, 9.2.6
completed transactions, 11.3.3
completion times for hub requests, 11.16.1
compound devices
bus-powered hubs, 7.2.1.1
in bus topology, 5.2.3
defined, 4.8.2.2

Universal Serial Bus Specification Revision 1.1

compound devices (continued)
hub descriptors for, 11.15.2.1
power configuration, 11.14
self-powered hubs, 7.2.1.2
conductor resistance unbalance, 6.6.3
conductors
mechanical specifications, 4.2.2
power and signal conductors in cables, 6.3, 6.6.2
resistance, 6.6.3
configuration
bus enumeration, 4.6.3, 9.1.2
configuration management, 10.5.4.1.1
Configured device state, 9.1.1.5
control transfers and, 5.5.4
descriptors, 9.4.3, 9.5, 9.6.1:9.6.4, 11.15.1 (Seealso descriptors)
device attachment, 4.6.1
device configuration, 10.3.1
device removal, 4.6.2, 10.5.4.1.4
function configuration, 10.3.1
hubs, 11.14
information in device characteristics, 4.8.1
initial device configuration, 10.5.4.1.2
interrupt transfers and, 5.7.4
modifying device configuration, 10.5.4.1.3
multiple configurations, 9.6.1
multiple interfaces, 9.2.3
operations overview, 9.2.3
power distribution and, 7.2.1
remote wakeup capabilities, 9.2.5.2
requests
configuration requests, 5.9.1.2
GetConfiguration() request, 9.4.2
SetConfiguration() request, 9.4.7
required configurations before usage, 10.3.1
USB configuration, 10.3.1
USBDI mechanisms for getting current settings,
10.5.2.4
USBD role in, 5.9.1.2, 10.5.4.1:10.5.4.1.4
Configuration = 0 signal/event, 11.5
configuration descriptors, 9.4.3, 11.15.1
Configured device statein bus enumeration process, 9.1.2
overview, 9.1.1.5
standard device requests and, 9.4.1:9.4.11
visible device state table, 9.1.1 Table 9-1
configuring software, defined, 2.0 glossary
Connect bus state, 7.1.7, 7.1.7.1
connecting devices. See dynamic insertion andremovalconnection status, 11.16.2.6.2, 11.16.2.6.2.1
connectivity
audio connectivity, 5.10.4.3.1
hub fault recovery mechanisms, 11.1.2.3
Hub Repeater responsibilities, 11.1
hubs, 11.1, 11.1.2:11.1.2.3
packet signaling connectivity, 11.1.2.1
resume connectivity, 11.1.2.2
source/sink connectivity, 5.10.4.3
synchronous data connectivity, 5.10.4.3.2

connectors
input capacitance, 7.1.6
inrush current and, 7.2.4.1
interface and mating drawings, 6.5.3, 6.5.4
keyed connector protocol, 6.2
mechanical configuration and material
requirements, 4.2.2, 6.5:6.5.4.3
orientation, 6.5.1
Series "A" and Series "B" plugs, 6.5.4
Series "A" and Series "B" receptacles, 6.5.3
standards for, 6.7
termination data, 6.5.2
USB Icon, 6.5
construction, cable, 6.6.2
contact arcing, minimizing, 7.2.4.1
contact capacitance standards, 6.7 Table 6-7
contact current rating standards, 6.7 Table 6-7
contact materials, 6.5.3.3, 6.5.4.3
control endpoints, 2.0 glossary. See also control
transfers
controlling hubs, defined, 7.1.7.5
control mechanisms
device states and control information, 11.13.2
Host Controller control flow management, 4.9
of USB host, 10.1.2
control pipes, 2.0 glossary. See also control transfers;
message pipes; pipes
control transfers
bus access constraints, 5.5.4
control pipes in device characteristics, 4.8.1
data format, 5.5.1
data sequences, 5.5.5
defined, 2.0 glossary, 5.4
device requests, 9.3
direction, 5.5.2
error handling on last data transaction, 8.5.2.3
full-speed limits, 5.5.4 Table 5-1
low-speed limits, 5.5.4 Table 5-1
overview, 4.7.1, 5.5
packet size, 5.5.3, 9.6.4
protocol stalls, 8.4.4
reporting status results, 8.5.2.1
stages, 2.0 glossary, 5.5
STALL handshakes returned by control pipes,
8.5.2.4transaction format, 8.5.2
transaction organization within IRPs, 5.9.2
USBD pipe mechanism responsibilities, 10.5.3.1.4
variable-length data stage, 8.5.2.2
corrupted transfers and requests
in control transfers, 8.5.2
corrupted ACK handshake, 8.5.2.3, 8.6.4
corrupted CRCs, 10.2.6
corrupted IN tokens, 8.4.5.1
corrupted PIDs, 8.3.1
corrupted SOF packets in isochronous transfers,
5.10.6in data toggle, 8.6.3
error detection and recovery, 8.7:8.7.4
function response to OUT transactions, 8.4.5.3
host response to IN transactions, 8.4.5.2
NAK or STALL handshake, 8.6.3

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Universal Serial Bus Specification Revision 1.1

costs of implementation, 3.3
C_PORT_CONNECTION
clearing, 11.16.2.2
defined, 11.16.2.6.2.1
hub class feature selectors, 11.16.2
Port Change field, 11.16.2.6.2
SetPortFeature() request, 11.16.2.9
C_PORT_ENABLE
ClearPortFeature() request, 11.16.2.2
defined, 11.16.2.6.2.2
hub class feature selectors, 11.16.2
Port Change field, 11.16.2.6.2
SetPortFeature() request, 11.16.2.9
C_PORT_OVER_CURRENT
clearing, 11.16.2.2
defined, 11.16.2.6.2.4
hub class feature selectors, 11.16.2
over-current conditions, 11.11.1, 11.13.5
Port Change field, 11.16.2.6.2
SetPortFeature() request, 11.16.2.9
C_PORT_RESET
clearing, 11.16.2.2
defined, 11.16.2.6.2.5
hub class feature selectors, 11.16.2
Port Change field, 11.16.2.6.2
SetPortFeature() request, 11.16.2.9
C_PORT_SUSPEND
clearing, 11.16.2.2
defined, 11.16.2.6.2.3
hub class feature selectors, 11.16.2
Port Change field, 11.16.2.6.2
resume conditions and, 11.4.4
SetPortFeature() request, 11.16.2.9
CRCs
bit stuffing, 7.1.9
in bulk transfers, 8.5.1
corrupted CRCs, 10.2.6
in data packets, 8.3.5.2, 8.4.3
defined, 2.0 glossary
in error detection, 8.7.1
overview, 8.3.5
protection in isochronous transfers, 5.10.7
resending, 8.6.4
in token packets, 8.3.5.1, 8.4.1
USB robustness and, 4.5, 4.5.1
cross-over points of data lines, 7.1.13.2
cross-over voltage in signaling, 7.1.2
crystal capacitive loading, 7.1.11
CTI, 2.0 glossary, 3.1
current
current averaging profile, 7.2.3
current spikes during suspend/resume, 7.2.3
supply current, 7.3.2 Table 7-5
current frame in hub timing, 11.2.1
current limiting
bus-powered hubs, 7.2.1.1
dynamic attach and detach, 7.2.4.1
in over-current conditions, 11.13.5
power control during suspend/resume, 7.2.3
remote wakeup and, 7.2.3
self-powered functions, 7.2.1.5
cyclic redundancy check. See CRCs

286

D
D+ or D- lines
average voltage, 7.1.2
signaling levels and, 7.1.7
signal termination, 7.1.5
during signal transitions, 7.1.4
single-ended capacitance, 7.1.1.2
standardized contact terminating assignments,
6.5.2
data
data defined, 5.10.4
data encoding/decoding, 7.1.8
data prebuffering, 5.10.5
data processing role of Host Controller, 10.2.4
DATA0/DATA1 PIDs, 8.3.1 Table 8-1
in bulk transfers, 5.8.5, 8.5.1
comparing sequence bits, 8.6.2
in control transfers, 8.5.2
in data packets, 8.4.3
in interrupt transactions, 8.5.3
interrupt transfers and, 5.7.5
synchronization and, 8.6
data field in packets, 8.3.4, 8.4.3
data flow model. See transfers
data flow types. See transfer types
data formats
bulk transfers, 5.8.1
control transfers, 5.5.1
interrupt transfers, 5.7.1
isochronous transfers, 5.6.1, 5.10.4
overview, 5.4
Data J state. See J bus state
Data K bus state. See K bus statedata packets
bus protocol overview, 4.4
data CRCs, 8.3.5.2
in isochronous transfers, 8.5.4
overview, 8.4.3
packet field formats, 8.3:8.3.5.2
data payloadbulk transfer data packets, 5.8.3
calculating transaction times, 5.9.3
defined, 5.3.2
interrupt transfers, 5.7.3
isochronous transfers, 5.6.3
packet size constraints, 5.5.3, 5.6.3
data PIDs, 8.3.1 Table 8-1. See also DATA0/DATA1
PIDsdata rates
adaptive endpoints, 5.10.4.1.3
asynchronous endpoints, 5.10.4.1.1
in buffering calculations, 5.10.8
data-rate tolerance, 7.1.11
defined, 5.10.4
in electrical specifications overview, 4.2.1
feedback for isochronous transfers, 5.10.4.2
full-speed source electrical characteristics, 7.3.2
Table 7-6
low-speed source electrical characteristics, 7.3.2
Table 7-7
overview, 7.1.11

Universal Serial Bus Specification Revision 1.1

data rates (continued)
sample clock and, 5.10.2
synchronous endpoints, 5.10.4.1.2
data retry indicators in control transfers, 5.5.5
data sequencesbulk transfers, 5.8.5
control transfers, 5.5.5
interrupt transfers, 5.7.5
isochronous transfers, 5.6.5
data signaling, 7.1.7.2
data signal rise and fall time. See rise and fall times
data source jitter, 7.1.13.1, 7.1.15
data source signaling, 7.1.13
Data stage
in control transfers, 5.5, 5.5.5, 8.5.2
error handling on last data transaction, 8.5.2.3
length of data, 9.3.5
packet size constraints, 5.5.3
variable-length data stages, 8.5.2.2
data toggle
bulk transfers, 5.8.5
in bulk transfers, 8.5.1
corrupted ACK handshake, 8.6.4
data corrupted or not accepted, 8.6.3
in data packets, 8.4.3
initialization via SETUP token, 8.6.1
in interrupt transactions, 8.5.3
interrupt transfers and, 5.7.5
low-speed transactions, 8.6.5
overview, 8.6
successful data transactions, 8.6.2
data transfers. See data packets; Data stage;transfers
DC electrical characteristics, 7.3.2 Table 7-5
debounce intervals in connection events, 7.1.7.1
debouncing connections, 11.8.2
decoupling capacitance, 7.3.2 Table 7-5
default addresses of devices, 2.0 glossary, 9.1.1.4,
10.5.1.1
Default bus state, 7.1.7.3
Default Control Pipein bus enumeration process, 9.1.2
in communication flow, 5.3
control transfer packet size constraints, 5.5.3
defined, 4.4, 5.3.2
endpoint zero requirements, 5.3.1.1
as message pipe, 5.3.2.2
size description in descriptors, 9.6.1
Default device state
overview, 9.1.1.3
standard device requests and, 9.4.1:9.4.11
visible device state table, 9.1.1 Table 9-1
default pipes, 2.0 glossary, 10.5.1.2.1
delays. See cable delay; differential delay;
propagation delay
delivery rates in isochronous transfers, 4.7.4
DEOP signal/event, 11.7
descriptors
accessing, 11.15.1
in bus enumeration process, 9.1.2
class-specific descriptors, 9.5, 11.15.2.1
configuration descriptors, 9.6.2, 10.3.1, 10.5.2.4

descriptors (continued)
control transfers and, 5.5, 5.5.3
defined, 9.5
device class definitions, 9.7, 9.7.1
device descriptors, 9.4 Table 9-5, 9.6.1:9.6.5
endpoint descriptors, 9.6.4
getting descriptors, 9.4.3, 10.5.2.3
hub descriptors, 11.15:11.15.2.1, 11.16.2.4,
11.16.2.7
interface descriptors, 9.2.3, 9.6.3
isochronous transfer capabilities, 5.10
listing remote wakeup capabilities, 9.2.5.2
overview, 9.5:9.7.3
setting descriptors, 9.4.8, 10.5.2.12
string descriptors, 9.6.5
USBDI mechanisms for getting descriptors,
10.5.2.3
vendor-specific descriptors, 9.5
deserialization of transmissions, 10.2.2
detachable cables
cable delay, 7.1.16
connectors and, 6.2
detachable cable assemblies, 6.4.1
inter-packet delay and, 7.1.18
low-speed detachable cables, 6.4.4
maximum capacitance, 7.1.6
termination, 7.1.5
voltage drop budget, 7.2.2
detached devices, 9.1.1.1, 9.1.2
detaching devices. See dynamic insertion andremovaldetecting bus state, 11.16.2.3
detecting connect and disconnect conditions, 7.1.7.1
detecting errors. See error detection and handling
detecting hub and port status changes, 11.13.2,
11.13.3, 11.13.4
detecting over-current conditions, 7.2.1.2.1
detecting speed of devices. See speed detection
Developing International Software for Windows 95
and Windows NT, 9.6.5
device addresses, 2.0 glossary. See also addresses;
devices
device classes. See also USB device framework
class codes, 9.2.3
defined, 4.8
descriptors, 9.2.3, 9.6.1, 9.7
device characteristics, 4.8.1
device class definitions, 9.7
getting class-specific descriptors, 9.5
hub class-specific requests, 11.16.2:11.16.2.9
interfaces and endpoint usage, 9.7.2
requests, 9.7.3
standard, class, and vendor information, 4.8.1
Device Class Specification for Audio Devices
Revision 1.0, 9.6
device descriptors
descriptor types, 9.4 Table 9-5
device class descriptors, 9.2.3, 9.7
GetDescriptor() request, 9.4.3
getting class-specific descriptors, 9.5
hubs, 11.15.1

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device descriptors (continued)
overview, 9.6.1
standard definitions, 9.6.1:9.6.5
device drivers, 5.10.4.3, 10.3.1
device endpoints, 2.0 glossary, 5.3.1.1. See also
endpoints
device-initiated resume. See remote wakeup
Device layer
descriptors, 9.5:9.7.3
device states, 9.1:9.1.2
generic USB device operations, 9.2:9.2.7
standard device requests, 9.4:9.4.11
in USB device framework, 9
USB device requests, 9.3:9.3.5
Device Management services, 10.5.4.2
Device release numbers, 9.6.1
DEVICE_REMOTE_WAKEUP, 9.4 Table 9-6
DeviceRemovable field (hub descriptors), 11.15.2.1
device resources, 2.0 glossary. See also buffers;
endpoints
devices. See also USB device frameworkaddress assignment, 9.1.2, 9.2.2
characteristics and configuration (See also device
descriptors)
configuration, 4.8.2.2, 9.2.3
data-rate tolerance, 7.1.11
descriptors, 9.5:9.7.3, 9.6.1
device characteristics, 4.8.1
device classes, 4.8, 9.7
device descriptions, 4.8.2:4.8.2.1
device speed, 7.1.5, 7.1.7.1, 11.8.2
host role in configuration, 10.3.1
optional endpoints, 5.3.1.2
USBD role in configuration, 10.5.4.1:10.5.4.1.4
data transfer, 9.2.4
communication flow requirements, 5.3
control transfers and, 5.5
detailed communication flow illustrated, 5.3
differing bus access for transfers, 5.9
jitter budget table, 7.1.15
response to IN transactions, 8.4.5.1
response to OUT transactions, 8.4.5.3
response to SETUP transactions, 8.4.5.4
role in bulk transfers, 8.5.1
device event timings, 7.3.2 Table 7-11
devices defined, 2.0 glossary
dynamic attach and detach, 9.2.1
power distribution, 7.2.4:7.2.4.2
removing, 10.5.2.6, 10.5.4.1.4
USBDI mechanisms, 10.5.2.5, 10.5.2.6
generic USB device operations, 9.2:9.2.7
power distribution, 7.2.1, 9.2.5
bus-powered devices, 4.3.1, 7.2.1.1
dynamic attach and detach, 7.2.4:7.2.4.2
high-power bus-powered functions, 7.2.1.4
low-power bus-powered functions, 7.2.1.3
power supply and, 4.3.1
self-powered devices, 4.3.1, 7.2.1.2, 7.2.1.5
suspend/resume conditions, 7.2.3
voltage drop budget, 7.2.2

288

devices (continued)
requests
host communication, 10.1.1
request errors, 9.2.7
request processing, 9.2.6:9.2.6.5
standard device requests, 9.4:9.4.11
USB device requests, 9.3:9.3.5
statusdevice states, 9.1:9.1.2, 11.13.2
getting device status, 9.4.5
getting port status, 11.16.2.6.1.1
types of devices
compound devices, 4.8.2.2
functions, 4.8.2.2
hubs, 4.8.2.1
mapping physical and virtual devices, 5.10.4.3
virtual devices, 2.0 glossary
in USB topology, 4.1.1.2, 5.2, 5.2.2, 9.0
device software, defined, 2.0 glossary
diagnostic requests, 11.16.2.3
diameter of cables, 6.6.2
dielectric withstanding voltage standards, 6.7 Table 67
Differential 1 bus state, 7.1.7
Differential 2 bus state, 7.1.7
differential data jitter, 7.3.3 Figure 7-39, 7.3.3 Figure
7-42
differential delay, 7.3.2 Table 7-8, 7.3.3 Figure 7-42
differential-ended components in upstream ports, 11.6
differential input receivers, 7.1.4, 7.1.6
differential output drivers, USBD as, 7.1.1
differential signaling, 7.1.7, 7.1.7.2
differential-to-EOP transition skew, 7.3.3 Figure 7-40
dimensional inspection standards, 6.7 Table 6-7
Direction bit, 9.3.1, 9.3.4
direction of communication flow, 5.4
bmRequestType field, 9.3.1
bulk transfers, 5.8.2
bus protocol overview, 4.4
control transfers, 5.5.2
interrupt transfers, 5.7.2
isochronous transfers, 5.6.2
disabled ports, 11.5, 11.5.1.4, 11.16.2.6.1,
11.16.2.6.2
Disabled state, 11.5, 11.5.1.4
disabling features, 9.4.1
discarding packets, 11.3.2
Disconnect_Detect signal/event, 11.5, 11.5.2
Disconnected stateconnect and disconnect signaling, 7.1.7.1
downstream ports, 11.5, 11.5.1.3
signaling levels and, 7.1.7
disconnecting devices. See dynamic insertion andremovaldisconnect timer, 11.5.2
distortion, minimizing in SOP, 7.1.7.2
documents, applicable standards, 6.7.1
down counters in hub timing, 11.2.1

Universal Serial Bus Specification Revision 1.1

downstream ports and hubs
downstream connectivity defined, 11.1.2.1
downstream defined, 2.0 glossary
downstream plugs, 6.2
downstream ports defined, 4.8.2.1
downstream port state machine, 11.5
full- or low-speed drivers and, 7.1.2.1
in hub architecture, 11.1.1
hub delay, 7.3.3 Figure 7-42
hub descriptors, 11.15.2.1
hub EOP delay and EOP skew, 7.3.3 Figure 7-43
input capacitance, 7.1.6
jitter, 7.3.2 Table 7-7
port state descriptions, 11.5.1:11.5.1.11
pull-up and pull-down resistors, 11.12.1
signaling delays, 7.1.14
transceivers, 7.1.7
drain wires, 6.5.2, 6.6.1, 6.6.2
dribble, defined, 7.1.9
drift, 5.10.1, 5.10.3
driver characteristics
full-speed (12Mb/S) driver characteristics, 7.1.1.1
full-speed source electrical characteristics, 7.3.2
Table 7-6
low-speed (1.5Mb/S) driver characteristics, 7.1.1.2
low-speed source electrical characteristics, 7.3.2
Table 7-7
overview, 7.1.1
drivers
defined, 2.0 glossaryrole in configuration, 10.3.1
in source-to-sink connectivity, 5.10.4.3
droop, 7.2.3, 7.2.4.1
dual pin-type receptacles, 6.9
durability standards, 6.7 Table 6-7
DWORD, defined, 2.0 glossary
dynamic insertion and removal, 9.2.1
attaching devices, 4.6.1
defined, 2.0 glossary
detecting insertion and removal, 4.9, 9.2.1
Hub Repeater responsibilities, 11.1
hub support for, 11.1
power control, 7.2.3, 7.2.4:7.2.4.2
power-on and connection events timing, 7.1.7.1
removing devices, 4.6.2
USB robustness and, 4.5

E
E2PROM defined, 2.0 glossary
ease-of-use considerations, 1.1
edges of signals
cable delay, 7.1.16
data source jitter, 7.1.13.1
edge transition density, 8.2
hubs and edge rate control, 11.12.2
optional edge rate control capacitors, 7.1.6
EEPROM, defined, 2.0 glossary
Electrical Connector/Socket Test Procedures, 6.7.1
Electrically Erasable Programmable Read Only
Memory (EEPROM), 2.0 glossary

Electrical Performance Properties of Insulation andJacket for Telecommunication Wire and Cable,
6.7.1
electrical specifications, 6.1, 7
applicable documents, 6.7.1
bus timing/electrical characteristics, 7.3.2
cables, 6.3, 6.4:6.4.4, 6.6:6.6.5
connectors, 6.2, 6.5:6.5.4.3
overview, 4.2.1, 6
PCB reference drawings, 6.9
physical layer specifications, 7.3:7.3.3
power distribution, 7.2.3, 7.2.4:7.2.4.2, 7.2:7.2.1.5
signaling, 7.1:7.1.19
standards for, 6.7, 7.3.1
timing waveforms, 7.3.3
USB grounding, 6.8
embedded hubs, 4.8.2.2, 5.2.3
EMI, USB grounding and, 6.8
enabled ports
connectivity and, 11.1.2.1
downstream ports, 11.5, 11.5.1.6
getting port status, 11.16.2.6.1
PORT_ENABLE bit, 11.16.2.6.1.2
port status change bits, 11.16.2.6.2
Enabled state, 11.5, 11.5.1.6
encoding data, 7.1.8
End-of-Frame (EOF). See EOFs
End-of-Packet (EOP). See EOPs
End-of-Packet bus state, 7.1.7, 7.1.7.2
end-of-packet delimiter. See EOPs
ENDP field, 8.3.2.2, 8.3.5.1, 8.4.1
endpoint addresses, 2.0 glossary, 5.3.1, 9.6.4
endpoint descriptors, 9.4.3, 9.6.1, 9.6.3, 9.6.4
endpoint direction, defined, 2.0 glossary
endpoint field (ENDP), 8.3.2.2, 8.3.5.1, 8.4.1
ENDPOINT_HALT, 9.4 Table 9-6
endpoint numbers, 2.0 glossary, 5.3.1
endpoints
addresses, 9.6.4
characteristics, 5.3.1
description in descriptors, 9.4.3, 9.6.1, 9.6.3, 9.6.4
in device class definitions, 9.7.2
direction of flow, 5.3.1
endpoint address field, 8.3.2.2
endpoint aliasing, 8.3.2
endpoint zero requirements, 4.8.1, 5.3.1.1, 5.3.1.2,
5.3.2
getting endpoint status, 9.4.5
Hub Controller endpoint organization, 11.13.1
in interfaces, 9.2.3, 9.6.2, 9.6.3
logical devices as collections of endpoints, 5.3
message pipes and, 5.3.2.2
non-endpoint zero requirements, 5.3.1.2
overview, 5.3.1
pipes and, 4.4, 5.3.2
programmable data rates, 2.0 glossaryreflected endpoint status, 10.5.2.2
role in data transfers, 4.7
samples, 2.0 glossary
specifying in wIndex field, 9.3.4
stream pipes and, 5.3.2.1
synchronization frame, 9.4.11

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endpoint synchronization type, 5.10.4, 5.10.4.1
endpoint zero
Default Control Pipe and, 5.3.2
in device characteristics, 4.8.1
non-endpoint zero requirements, 5.3.1.2
requirements, 5.3.1.1
end-to-end signal delay, 7.1.19
end users, 2.0 glossary, 3.3
enumeration. See bus enumeration
environmental characteristics for cables, 6.6.4
environmental compliance standards, 6.7
EOF1 or EOF2 signal/event
frame timer and, 11.2.1, 11.2.2
host behavior at end-of-frame, 11.3
in Hub Repeater state machine, 11.7
in transmitter state machine, 11.6.2
EOFs
defined, 2.0 glossary
in frame timer synchronization, 11.2.1
host behavior at end-of-frame, 11.3:11.3.3
Host Controller frame generation, 10.2.3
in transaction completion prediction, 11.3.3
EOI signal/eventin downstream port state machine, 11.5
in internal port state machine, 11.4
in receiver state machine, 11.6.1
in transmitter state machine, 11.6.2
EOP bus state, 7.1.7, 7.1.7.2
EOPs
defined, 2.0 glossary
differential-to-EOP transition skew and EOP width,
7.3.3 Figure 7-40
EOP delimiter, 8.3
EOP width, 7.1.13.2, 7.3.3 Figure 7-40
error detection through bus turn-around timing,
8.7.2
extra bits and, 7.1.9
false EOPs, 2.0 glossary, 8.7.3
handshake packets and, 8.4.4
hub EOP delay and EOP skew, 7.3.3 Figure 7-43
hub/repeater electrical characteristics, 7.3.2 Table
7-8
hub signaling at EOF1, 11.3.1
intervals between IN token and EOP, 11.3.3
propagation delays, 7.1.14
equationsbuffering for rate matching, 5.10.8
buffer sizes in functions and software, 5.9.4
bus transaction times, 5.9.3
error detection and handling. See also corrupted
transfers and requests
babble and loss of activity recovery, 8.7.4
bit stuff violations, 8.7.1
bulk transfers and, 5.8.5, 8.5.1
bus turn-around timing, 8.7.2
control transfers and, 5.5.5, 8.5.2.1
corrupted ACK handshake, 8.5.2.3, 8.6.4
corrupted SOF packets in isochronous transfers,
5.10.6
CRCs, 8.3.5, 8.7.1
data corrupted or not accepted, 8.6.3
error count tally, 10.2.6

290

error detection and handling (continued)
error handling for transfers, 5.4
error handling on last data transaction, 8.5.2.3
false EOPs, 2.0 glossary, 8.7.3
Host Controller role in, 10.2.6
Hub Repeater responsibilities, 11.1
hub role in, 11.1.2.3
interrupt transfers and, 5.7.5
isochronous transfers and, 5.6.4, 5.6.5, 5.10.7
overview, 8.7
packet error categories, 8.7.1
PID check bits, 8.7.1
Port Error conditions, 11.8.1
Request Errors, 9.2.7
sample size and, 5.10.8
short packets and error conditions, 5.3.2
synchronous data connectivity, 5.10.4.3.2
USBD role in, 10.5.4.5
USB robustness and, 4.5.1, 4.5.2
ESD, USB grounding and, 6.8
event notifications, USBD and, 10.5.4.4
exception handling. See error detection and handlingextended descriptor definitions, 9.7.1
extensibility of USB architecture, 4.10
extension cables, 6.4.4
externally-powered hubs, 7.2.1. See also selfpowered hubs
extraction force standards, 6.7 Table 6-7

F
failed data transactions, 8.6.3
false EOPs, 2.0 glossary, 8.7.3
fault detection. See error detection and handlingfeatureshub class feature selectors, 11.16.2
SetFeature() request, 9.4.9
setting hub features, 11.16.2.8
standard feature selectors, 9.4 Table 9-6
feedback for isochronous transfers, 5.10.4.2
ferrite beads, 7.1.6
fields. See names of specific fields
flammability
cables, 6.6.4
Series "A" and Series "B" plugs, 6.5.4.1
Series "A" and Series "B" receptacles, 6.5.3.1
standards, 6.7 Table 6-7
flexibility of USB devices, 3.3
flow control mechanisms
in bus protocol overview, 4.4
handshake packets and, 8.4.4
USB robustness and, 4.5
flyback voltage, 7.2.4.2
format of USB device requests, 9.3
formulasbuffering for rate matching, 5.10.8
buffer sizes in functions and software, 5.9.4
bus transaction times, 5.9.3
frame clock, 5.10.3

Universal Serial Bus Specification Revision 1.1

frame intervals
frame interval adjustment, 7.1.12
full-speed source electrical characteristics, 7.3.2
Table 7-6
low-speed source electrical characteristics, 7.3.2
Table 7-7
repeatability, 7.1.12
frame number field, 8.3.3, 8.4.2
frame numbers, 5.10.6, 8.3.3, 8.4.2, 10.2.3
frame pattern, defined, 2.0 glossary
frames. See also frame timers
allocating bandwidth, 4.7.5, 5.9.1:5.9.1.5, 10.3.2
available time in frames, 5.5.4, 5.6, 5.6.4, 5.7.4,
5.8.4, 5.9.5
bandwidth reclamation, 5.9.5
bit time zero, 11.3
control transfer reserved portions, 5.5.4
defined, 2.0 glossary
frame generation responsibilities of Host Controller,
10.2.3frame intervals, 7.1.12, 7.3.2 Table 7-6, 7.3.2 Table
7-7
frame number field, 8.3.3, 8.4.2
frame numbers, 5.10.6, 8.3.3, 8.4.2, 10.2.3
frame timers, 11.2:11.2.2
frame wander defined, 11.2.2
host behavior at end-of-frame, 11.3
interrupt transfer limitations, 5.7.4
isochronous transfer limitations, 5.6.3, 5.6.4
Master Client status, 10.5.3.2.4, 10.5.4.2
one transaction per frame in isochronous transfers,
5.10.7
organization of transactions within, 5.9.2
samples per frame in isochronous transfers,
5.10.4.2
SOF packets, 8.4.2
SOF tracking, 5.10.6
frame timers
frame wander, 11.2.2
hub frame timer, 11.2:11.2.2
timing skew, 11.2.2
frame wander, defined, 11.2.2
frequency-locked clocks, 5.10.3
Fs. See SRC
Fsus state, 11.4, 11.4.3
full-duplex, defined, 2.0 glossary
full-speed buffers, 7.1.2
full-speed cables
cable delay, 7.1.16
captive cable assemblies, 6.4.2
construction, 6.6, 6.6.2
description, 6.6.1
listing, 6.6.5
specifications, 6.3
standards for, 6.6.3, 6.6.4, 6.7
full-speed driver characteristics (12Mb/S), 7.1.1.1
full-speed functions and hubs
bulk transfers and, 5.8.4
cable and resistor connections, 7.1.5
connect detection, 7.1.7.1
control transfers and, 5.5.3, 5.5.4, 5.5.4 Table 5-1
data-rate tolerance, 7.1.11

full-speed functions and hubs (continued)
detachable cables and, 6.4.1
full-speed source electrical characteristics, 7.3.2
Table 7-6
full- vs. low-speed port behavior, 11.8.4
getting port status, 11.16.2.6.1
hub/repeater electrical characteristics, 7.3.2 Table
7-8
hub support for, 11.1
input capacitance, 7.1.6
interrupt transfers and, 5.7.3, 5.7.4 Table 5-4
isochronous transfers and, 5.6.4
optional endpoints, 5.3.1.2
signal termination, 7.1.5
SOF PID and, 8.4.2
speed detection and, 11.8.2
Transmit state and, 11.5.1.7
full-speed signaling
calculating transaction times, 5.9.3
data rates, 4.2.1
data source jitter, 7.1.13.1
edge rates, 11.12.2
EOP width, 7.1.13.2
full-speed loads, 7.1.2
intervals between IN token and EOP, 11.3.3
J and K states, 7.1.7
jitter budget table, 7.1.15
propagation delays, 7.1.14
Full Suspend (Fsus) state, 11.4, 11.4.3
function address field (ADDR), 8.3.2.1
functional stall, 8.4.4, 8.5.2.4
Function layer
detailed communication flow, 5.3
illustrated, 5.1
interlayer communications model, 10.1.1
functions. See also devices
address assignment, 9.1.2, 9.2.2
characteristics and configuration (See also device
descriptors)
configuration, 4.8.2.2, 9.2.3
data-rate tolerance, 7.1.11
descriptors, 9.5:9.7.3, 9.6.1
device characteristics, 4.8.1
device classes, 4.8, 9.7
device speed, 7.1.7.1, 11.8.2
host role in configuration, 10.3.1
optional endpoints, 5.3.1.2
data transfer, 9.2.4
communication flow requirements, 5.3
control transfers and, 5.5
detailed communication flow illustrated, 5.3
differing bus access for transfers, 5.9
jitter budget table, 7.1.15
response to IN transactions, 8.4.5.1
response to OUT transactions, 8.4.5.3
response to SETUP transactions, 8.4.5.4
role in bulk transfers, 8.5.1
device event timings, 7.3.2 Table 7-11
devices defined, 2.0 glossary291Universal Serial Bus Specification Revision 1.1

dynamic attach and detach, 9.2.1
power distribution, 7.2.4:7.2.4.2
removing, 10.5.2.6, 10.5.4.1.4
USB mechanisms, 10.5.2.5, 10.5.2.6
generic USB device operations, 9.2:9.2.7
overview, 4.8.2.2
power distribution, 7.2.1, 9.2.5
bus-powered devices, 4.3.1, 7.2.1.1
dynamic attach and detach, 7.2.4:7.2.4.2
high-power bus-powered functions, 7.2.1.4
low-power bus-powered functions, 7.2.1.3
power supply and, 4.3.1
self-powered functions, 7.2.1.2, 7.2.1.5
suspend/resume conditions, 7.2.3
voltage drop budget, 7.2.2
requests
host communication with, 10.1.1
request errors, 9.2.7
request processing, 9.2.6:9.2.6.5
standard device requests, 9.4:9.4.11
USB device requests, 9.3:9.3.5
status, 9.1:9.1.2, 9.4.5
types of devices
compound devices, 4.8.2.2
functions, 4.8.2.2
hubs, 4.8.2.1
mapping physical and virtual devices, 5.10.4.3
virtual devices, 2.0 glossary
in USB topology, 4.1.1.2, 5.2.2, 5.2.3, 9.0function-to-host transfers. See IN PID

Ggang-mode power control, 11.15.2.1
garbling messages in Collision conditions, 11.8.3
Generate End of Packet Towards Upstream Port state(GEOPTU), 11.6.2, 11.6.2.5
Generate Resume state, 11.4, 11.4.4
generic USB device operations, 9.2:9.2.7
GEOPTU state, 11.6.2, 11.6.2.5
GetBusState() request, 11.16.2, 11.16.2.3
GetConfiguration() request, GET_CONFIGURATION
hub requests, 11.16.1
overview, 9.4.2
returning interface descriptors, 9.6.3
standard device request codes, 9.4
GetDescriptor() request, GET_DESCRIPTOR,
11.15.1
endpoint descriptors, 9.6.4
GetDescriptor(CONFIGURATION) request, 9.5,
9.6.4
GetHubDescriptor() request, 11.16.2.4
hub class requests, 11.16.2
hub descriptors, 11.16.2.4
hub requests, 11.16.1
interface descriptors, 9.6.3
overview, 9.4.3
standard device request codes, 9.4
GetHubDescriptor() request, 11.16.2, 11.16.2.4
GetHubStatus() request, 11.16.2, 11.16.2.5

292

GET_INTERFACE
hub requests, 11.16.1
overview, 9.4.4
standard device request codes, 9.4
GetInterface() request
alternate settings for interfaces, 9.2.3
interface descriptors, 9.6.3
overview, 9.4.4
GetPortStatus() request, 11.16.2,
11.16.2.6:11.16.2.6.2.5
GET_STATE, 11.16.2, 11.16.2.3
GetStatus() request, GET_STATUS, 9.4.5
GetHubStatus() request, 11.16.2.5
GetPortStatus() request, 11.16.2.6
hub class requests, 11.16.2
overview, 9.4.5
standard device request codes, 9.4
global resumes
frame timer synchronization, 11.2.1
hub support, 11.9
signaling, 7.1.7.5
global suspend, 7.1.7.4.1, 11.9
glossary, 2.0
GND leads
cable electrical characteristics, 7.3.2 Table 7-9
captive cable assemblies, 6.4.2, 6.4.3
detachable cables, 6.4.1
electrical specifications overview, 4.2.1
standardized contact terminating assignments,
6.5.2
GResume state, 11.4, 11.4.4
grounding, 6.8

Hhalted pipes, 10.5.2.2
Halt featurebulk transfers, 5.8.5
control transfers, 5.5.5, 8.5.2.4
functional stalls, 8.4.4
GetStatus() request, 9.4.5
interrupt transfers, 5.7.5, 8.5.3
isochronous transfers, 5.6.5
responses to standard device requests, 9.4
handshakes, 8.3.1 Table 8-1. See also ACKs; NAKs;
STALLs
ACK PID, 8.3.1 Table 8-1
bulk transfers, 8.5.1
bus protocol overview, 4.4
defined, 2.0 glossaryfunction response to IN transactions, 8.4.5.1
function response to OUT transactions, 8.4.5.3
function response to SETUP transactions, 8.4.5.4
handshake responses, 8.4.5, 8.4.5:8.4.5.4
host response to IN transactions, 8.4.5.2
isochronous transfers, 5.6.5, 5.10.7
NAK PID, 8.3.1 Table 8-1
overview, 8.4.4
packet field formats, 8.3:8.3.5.2
STALL PID, 8.3.1 Table 8-1
total allocation of bit times, 11.3.3
hardwired cable assemblies, 6.4.2

Universal Serial Bus Specification Revision 1.1

HCD (Host Controller Driver)defined, 2.0 glossary, 5.3
HCDI (Host Controller Driver Interface), 10.1.1,
10.4
overview, 10.4
software interface overview, 10.3
in transfer management, 5.9.1, 5.9.1.3
in USB topology, 5.2.1, 10.1.1
HCDI (Host Controller Driver Interface), 10.1.1, 10.4
high-powered devices
bus-powered functions, 7.2.1, 7.2.1.4
high-power ports, 7.2.1
voltage drop budget, 7.2.2
host, 10in bus topology, 4.1.1.1, 5.2, 5.2.1
collecting status and activity statistics, 10.1.4
components, 10.1.1
control mechanisms, 10.1.2
EOF1 and EOF2 timing points, 11.2.2
host behavior at end-of-frame, 11.3:11.3.3
host-to-hub communications, 11.1
resource management, 10.3.2
responsibilities and capabilities, 10.1.1
role in assigning addresses, 9.2.2
role in configuration, 9.2.3, 10.3.1
synchronizing hub frame timer to host frame
period, 11.2
data flow, 10.1.3
common data definitions, 10.3.4
data-rate tolerance, 7.1.11
data transfer mechanisms, 10.1.3, 10.3.3
detailed communication flow illustrated, 5.3
host response to IN transactions, 8.4.5.2
interlayer communications model, 10.1.1
role in bulk transfers, 8.5.1
defined, 2.0 glossary, 4.9
electrical considerations, 10.1.5
jitter budget table, 7.1.15
over-current protection, 7.2.1.2.1
over-current recovery, 11.13.5
hardware and software, 10.0
Host Controller Driver (HCD), 10.4 (See also HCD)
Host Controller responsibilities, 4.9, 10.2 (See also
Host Controller)
host tolerance, hub frame timer and, 11.2
operating system environment guides, 10.6
overview of USB Host, 10.1:10.1.5
power management overview, 4.3.2
software mechanisms, 10.3:10.3.4
status in USBD pipe state, 10.5.2.2
Universal Serial Bus Driver (USBD), 10.5:10.5.5(See also USBD (USB Driver))USB System Software responsibilities, 4.9 (Seealso USB System Software)
Host Controller, 4.9
in bus topology, 5.2.1
calculating buffer sizes in functions and software,
5.9.4
data transfer mechanisms, 10.1.3
bulk transfers, 5.8.3
control transfers, 5.5.3, 5.5.4
data processing, 10.2.4

Host Controller
data transfer mechanisms (continued)
data-rate tolerance, 7.1.11
interrupt transfers, 5.7.3
role in transfer management, 5.9.1, 5.9.1.5
tracking transactions, 5.9.2
transaction list, 5.9.1.4
transmission error handling, 10.2.6
defined, 2.0 glossary, 4.9
frame generation, 10.2.3
HCD and HCDI overview, 10.4 (See also HCD;
HCDI)
host behavior at end-of-frame, 11.3
host-system interface, 10.2.9
as implementation focus area, 5.1
implemented in USB Bus interface, 10.1.1
multiple Host Controllers, 4.10
passing preboot control to operating system, 10.5.5
port resets, 10.2.8.1
protocol engine, 10.2.5
remote wakeup and, 10.2.7
requirements, 10.2
root hub and, 10.2.8
serializer/deserializer, 10.2.2
state handling, 10.2.1
status and activity monitoring, 10.1.4
USB System interaction, 10.1.1
Host Controller Driver. See HCD (Host Controller
Driver)Host Controller Driver Interface (HCDI), 10.1.1, 10.4
host resources, 2.0 glossary
host side bus interface. See Host Controller
host softwarein bus topology, 5.2.1
as component of USB System, 10.1.1
pipes and, 10.5.1.2
status and activity monitoring, 10.1.4
host-to-function transfers. See OUT PID
hot plugging. See dynamic insertion and removal
Hub Change field, 11.4.4, 11.16.2.5
hub class definitions
additional endpoints, 11.13.1
feature selectors, 11.16.2
request codes, 11.16.2
root hub and, 10.4
Hub Controller, 11.13:11.13.15
control commands, 11.1
defined, 4.8.2.1
endpoint organization, 11.13.1
hub and port change information processing,
11.13.3, 11.13.4
in hub architecture, 11.1.1, 11.13.2
internal port connection, 11.4
over-current reporting and recovery, 11.13.5
power distribution and, 7.2.1.1
role in host-to-hub communications, 11.1
status commands, 11.1
hub descriptors, 11.13.2
hub drivers, 10.3.1
hub frame timer, 11.2:11.2.2
hub oscillator tolerance, 11.2

293

Universal Serial Bus Specification Revision 1.1

Hub Repeater
Collision conditions, 11.8.3
connectivity setup and tear-down, 11.1
defined, 4.8.2.1
dynamic insertion and removal, 11.1
electrical characteristics, 7.3.2 Table 7-8
fault detection and recovery, 11.1
in hub architecture, 11.1.1
hub signaling timings, 7.1.14
internal port connection, 11.4
packet signaling connectivity, 11.1.2.1
repeater state descriptions, 11.2.1, 11.7:11.7.4
Wait for End of Packet (WFEOP), 11.7.4
Wait for End of Packet from Upstream Port state(WFEOPFU), 11.7.2
Wait for Start of Packet (WFSOP), 11.7.3
Wait for Start of Packet from Upstream Port state(WFSOPFU), 11.7.1
Hub Repeater state machine, 11.2.1, 11.7
hubs, 11. See also Hub Controller; Hub Repeater;
ports
architecture, 4.1.1.2, 11.1
bus statesbus state evaluation, 11.8:11.8.4.1
collision, 11.8.3
connect/disconnect detection, 11.1
full- vs. low-speed behavior, 11.8.4
getting bus state, 11.16.2.3
low-speed keep-alive, 7.1.7, 11.8.4.1
port error, 11.8.1
reset behavior, 11.10, 11.10.1
speed detection, 11.8.2
status change detection, 11.13.2, 11.13.4
in bus topology, 5.2.3, 5.2.4
characteristics and configuration, 11.14
clearing features, 11.16.2.5
data-rate tolerance, 7.1.11
descriptors, 11.13.2, 11.15, 11.15:11.15.2.1,
11.16.2.7
full- vs. low-speed behavior, 11.8.4
input capacitance, 7.1.6
speed detection of devices, 11.8.2
typical configuration illustrated, 4.8.2.1
connectivity behavior, 11.1, 11.1.2:11.1.2.3
controlling hubs, 7.1.7.5
defined, 2.0 glossary
downstream ports, 11.5:11.5.1.11
dynamic insertion and removal role, 4.6.1, 4.6.2
embedded hubs, 4.8.2.2
fault detection and recovery, 11.1
Hub Controller, 4.8.2.1, 11.1, 11.13:11.13.15 (Seealso Hub Controller)
hub drivers, 10.3.1
Hub Repeater, 4.8.2.1, 7.3.2 Table 7-8, 11.1,
11.7:11.7.4 (See also Hub Repeater)
hub tier, 2.0 glossaryintermediate hubs, 7.1.7.5
internal ports, 11.4:11.4.4
I/O buffer requirements, 11.12:11.12.2
overview, 4.8.2.1, 11.1:11.1.2.3

294

hubs (continued)
power management, 11.1
bus-powered hubs, 4.3.1, 7.2.1.1
hub port power control, 11.11
multiple gangs, 11.11.1
over-current reporting and recovery, 11.13.5
power source and sink requirements, 7.2.1
self-powered hubs, 7.2.1.2
surge limiting, 7.2.4.1
requests, 11.16:11.16.2.9
root hubs, 2.0 glossary
signaling and timingedge rate control, 11.12.2
host behavior at end-of-frame, 11.3:11.3.3
hub differential delay, 7.3.3 Figure 7-42
hub EOP delay and EOP skew, 7.3.3 Figure 743
hub event timings, 7.3.2 Table 7-10
hub frame timer, 11.2, 11.2:11.2.2
hub oscillator tolerance, 11.2
hub signaling timings, 7.1.14
hub switching skews, 7.1.9
jitter budget table, 7.1.15
low-speed keep-alive strobe, 7.1.7, 11.8.4.1
power-on and connection events timing, 7.1.7.1
reset signaling, 7.1.7.3
resume signaling, 7.1.7.5
signaling delays, 7.1.14
suspend and resume signaling, 11.9
tracking frame intervals, 7.1.12
upstream ports, 11.6, 11.6:11.6.2.6
Hub State Machine, 11.1.1
Hub Status field, 11.16.2.5
humidity life standards, 6.7 Table 6-7
hybrid powered hubs, 7.2.1.2
hysteresis in single-ended receivers, 7.1.4

I
iConfiguration field (configuration descriptors), 9.6.2
Icon for USB plugs and receptacles, 6.5, 6.5.1
Idle bus statedata signaling overview, 7.1.7.2
hub connectivity and, 11.1.2.1
Idle-to-K state transition, 7.1.14
NRZI data encoding, 7.1.8
signaling levels and, 7.1.7
idle pipes, 5.3.2, 10.5.2.2
idProduct field (device descriptors), 9.6.1
idVendor field (device descriptors), 9.6.1
iInterface field (interface descriptors), 9.6.3
iManufacturer field (device descriptors), 9.6.1
impedance
cable impedance tests, 6.7 Table 6-7
detachable cable assemblies, 6.4.1
differential cable impedance, 7.3.2 Table 7-9
full-speed captive cable assemblies, 6.4.2
full-speed connections, 7.1.1.1
input impedance of ports, 7.1.6
zero impedance voltage sources, 7.1.1
implementer viewpoints of data flow models, 5.1
Inactive state, 11.4, 11.4.1, 11.6.2, 11.6.2.1
in-band signaling, 10.1.2

Universal Serial Bus Specification Revision 1.1

initial frequency inaccuracies, 7.1.11
initialization of USBD, 10.5.1.1
injection molded thermoplastic insulator material,
6.5.3.1, 6.5.4.1
inner shielding in cables, 6.6.2
IN PID, 8.3.1 Table 8-1
ACK handshake and, 8.4.4
ADDR field, 8.3.2.1
bit times and, 11.3.3
bulk transfers, 8.5.1
control transfers, 8.5.2, 8.5.2.1
ENDP field, 8.3.2.2
error handling on last data transaction, 8.5.2.3
function response to, 8.4.5.1
host response to, 8.4.5.2
interrupt transactions, 8.5.3
intervals between IN token and EOP, 11.3.3
isochronous transfers, 8.5.4
low-speed transactions, 8.6.5
NAK handshake and, 8.4.4
prebuffering data, 5.10.5
STALL handshake and, 8.4.4
token CRCs, 8.3.5.1
token packets, 8.4.1
input capacitance, 7.1.2, 7.3.2 Table 7-5
input characteristics (signaling), 7.1.6
input impedance of ports, 7.1.6
input levels, 7.3.2 Table 7-5
inputs (Series "B" receptacles), 6.2
input sensitivity of differential input receivers, 7.1.4
inrush current limiting
bus-powered hubs, 7.2.1.1
dynamic attach and detach, 7.2.4.1
remote wakeup and, 7.2.3
self-powered functions, 7.2.1.5
suspend/resume and, 7.2.3
inserting devices. See dynamic insertion and removal
insertion force standards, 6.7 Table 6-7
instancing of USBD, 10.5
insulation
cables, 6.6.2
insulator materials, 6.5.3.1, 6.5.4.1
resistance standards, 6.7 Table 6-7
interconnect model, 4.1, 5.10.4.3
interface class codes, 9.6.3
interface descriptors
GetDescriptor() request, 9.4.3
hubs, 11.15.1
overview, 9.6.3
interface numbers, 9.2.3, 9.6.3
interfaces
alternate interfaces, 9.2.3, 10.5.2.10
alternate settings, 9.6.3
in configuration descriptors, 9.6.2
defined, 9.2.3
in device class definitions, 9.7.2
as endpoint sets, 5.3
getting interface status, 9.4.4, 9.4.5
host-system interface, 10.2.9
interface class codes, 9.6.3
interface descriptors, 9.4.3, 9.6.3, 11.15.1
interface numbers, 9.2.3, 9.6.3

interfaces (continued)
interface subclass codes, 9.6.3
plug interface and mating drawings, 6.5.4
setting interface state, 9.4.10, 10.5.2.1
specifying in wIndex field, 9.3.4
interfaces of plugs, 6.5.3
interface state control, 10.5.2.1
interface subclass codes, 9.6.3
interlayer communications model, 4.1, 10.1.1
intermediate hubs, 7.1.7.5
internal clock source jitter, 7.1.13.1
internal ports
Full Suspend (Fsus) state, 11.4.3
Generate Resume (GResume) state, 11.4.4
Inactive state, 11.4.1
Suspend Delay state, 11.4.2
internal port state machine, 11.4
inter-packet delay, 7.1.18
interrupt endpoints, 11.13.1
interrupt requests, defined, 2.0 glossaryinterrupt transfers
bus access constraints, 5.7.4
data format, 5.7.1
data sequences, 5.7.5
defined, 2.0 glossary, 5.4
direction, 5.7.2
full-speed transfer limits, 5.7.4 Table 5-4
low-speed transfer limits, 5.7.4 Table 5-4
overview, 4.7.3, 5.7
packet size, 5.7.3, 9.6.4
transaction format, 8.5.3
transaction organization within IRPs, 5.9.2
USBD pipe mechanism responsibilities, 10.5.3.1.2
intervals
debounce intervals in connection events, 7.1.7.1
frame intervals, 7.1.12, 7.3.2 Table 7-6, 7.3.2 Table
7-7
Resetting state and Resuming state intervals,
11.5.1.9
resume and recovery intervals for devices, 9.2.6.2
service and polling intervals, 2.0 glossary, 9.6.4,
10.3.3timeout intervals, 8.7.3
between IN token and EOP, 11.3.3
interwoven tinned copper wire, 6.6.2
I/O buffers. See buffers
I/O Request Packets. See IRPs
iProduct field (device descriptors), 9.6.1
IRPs. See also requests; transfers
aborting/retiring, 5.3.2, 10.5.3.2.1
class-specific requests, 9.2.6.5
client software role in, 5.9.1.1
defined, 2.0 glossary, 5.3.2
HCD tracking of, 5.9.1.3
multiple data payloads in, 5.3.2
pipes and, 5.3.2
queuing IRPs, 10.5.3.2.3
request processing overview, 9.2.6
reset/resume recovery time, 9.2.6.2
set address processing, 9.2.6.3
STALLS and, 5.3.2
standard device requests, 9.2.6.4, 9.4:9.4.11

295

Universal Serial Bus Specification Revision 1.1

IRPs (continued)
timing, 9.2.6.1, 9.2.6.3, 9.2.6.4
transaction organization within IRPs, 5.9.2
USB device requests, 9.3:9.3.5
USBD role in, 10.1.1
IRQs, defined, 2.0 glossary
iSerialNumber field (device descriptors), 9.6.1
isochronous data, defined, 2.0 glossary
isochronous devices, defined, 2.0 glossary
isochronous sink endpoints, defined, 2.0 glossary
isochronous source endpoints, defined, 2.0 glossary
isochronous transfers
buffering for rate matching, 5.10.8
bus access constraints, 5.6.4
clock model, 5.10.2
clock synchronization, 5.10.3
connectivity, 5.10.4.3
data format, 5.6.1
data prebuffering, 5.10.5
data sequences, 5.6.5
defined, 2.0 glossary, 5.4
direction, 5.6.2
endpoint synchronization frame, 9.4.11
error handling, 5.10.7
feedback, 5.10.4.2
illustrated, 5.10.2
isochronous device framework, 5.10.4
non-USB example isochronous application, 5.10.1
overview, 4.7.4, 5.6
packet size, 5.6.3, 9.6.4
SOF tracking, 5.10.6
special considerations, 5.10:5.10.8
synchronization types, 5.10.4.1.1, 5.10.4.1.2,
5.10.4.1.3
transaction format, 8.5.4
transaction organization within IRPs, 5.9.2
USBD pipe mechanism responsibilities, 10.5.3.1.1
USB features and, 3.3
USB System Software role, 4.9
ISO transfers. See isochronous transfers
ITCW (interwoven tinned copper wire), 6.6.2

Jjacketing in cables, 6.6.2
J bus statedata signaling overview, 7.1.7.2
J-to-K state transition, 7.1.14
NRZI data encoding, 7.1.8
signaling levels and, 7.1.7
jitterclock jitter, 5.10.3
data source jitter, 7.1.13.1
defined, 2.0 glossary
differential data jitter, 7.3.3 Figure 7-39
differential jitter, 7.3.3 Figure 7-42
full-speed source electrical characteristics, 7.3.2
Table 7-6
hub/repeater electrical characteristics, 7.3.2 Table
7-8
internal clock source jitter, 7.1.13.1
low-speed source electrical characteristics, 7.3.2
Table 7-7

296

jitter (continued)
in non-USB isochronous application, 5.10.1
output driver jitter, 7.1.15
receiver data jitter, 7.1.15, 7.3.3 Figure 7-41
service jitter, 2.0 glossary
J signal/event
differential transmissions, 11.6
downstream port state machine, 11.5
receiver state machine, 11.6.1
transmitter state machine, 11.6.2

K
kB/S and kb/S, defined, 2.0 glossary
K bus statedata signaling overview, 7.1.7.2
Idle-to-K state transition, 7.1.14
K-to-J state transition, 7.1.14
NRZI data encoding, 7.1.8
signaling levels and, 7.1.7
keep-alive strobe, 7.1.7, 7.1.7.4, 11.8.4.1
keyed connector protocol, 6.2
K signal/event
differential transmissions, 11.6
downstream port state machine, 11.5
receiver state machine, 11.6.1
transmitter state machine, 11.6.2

L
LANGID code array, 9.6.5
language IDs in string descriptors, 9.6.5
latency constraints for transfers, 5.4
latest host packet, 11.3.1
least-significant bit, 2.0 glossary, 8.1
length of cables, 6.4.1, 6.4.2, 6.4.3
listing, UL listing for cables, 6.6.5
little endian, defined, 2.0 glossary
LOA, 2.0 glossary, 8.7.4
load capacitance, 6.4.3, 6.7 Table 6-7
Local Power Source field, 11.16.2.5
Local Power Status Change field, 11.16.2.5
Local Power Status field, 11.16.2.6.1.6
local power supplies, 7.2.1.2, 7.2.1.5
locking hub frame timer, 11.2.1
Lock signal/event, 11.7
logical bus topology, 5.2, 5.2.4
logical devices
in bus topology, 5.2.2
as collections of endpoints, 5.3
detailed communication flow illustrated, 5.3
unique addresses and endpoints, 5.3.1
Logical Power Switching Mode field, 11.11, 11.11.1,
11.15.2.1
logo location on connectors, 6.5.1
LOI, 6.5.3.1, 6.5.4.1
loss of bus activity. See LOA
low level contact resistance standards, 6.7 Table 6-7
low-power bus-powered functions, 7.2.1, 7.2.1.3
low-power hubs, 7.2.2
low-power ports, 7.2.1
low-speed buffers, 7.1.2

Universal Serial Bus Specification Revision 1.1

low-speed cables
cable environmental characteristics, 6.6.4
captive cable assemblies, 6.4.3
configuration overview, 6.6
construction, 6.6.2
description, 6.6.1
detachable cables, 6.4.4
listing, 6.6.5
specifications, 6.3
standards for, 6.6.3, 6.7
low-speed driver characteristics (1.5Mb/S), 7.1.1.2
low-speed functions and hubs
cable and resistor connections, 7.1.5
connect detection, 7.1.7.1
control transfers and, 5.5.3, 5.5.4, 5.5.4 Table 5-1
data-rate tolerance, 7.1.11
detachable cables and, 6.4.1
detecting, 11.16.2.6.1.7
full-speed captive cable assemblies, 6.4.2
full- vs. low-speed port behavior, 11.8.4
getting port status, 11.16.2.6.1
hub/repeater electrical characteristics, 7.3.2 Table
7-8
hub support for, 11.1
input capacitance, 7.1.6
interrupt transfers and, 5.7.3, 5.7.4 Table 5-4
low-speed captive cable assemblies, 6.4.3
optional endpoints, 5.3.1.2
signal termination, 7.1.5
speed detection and, 11.8.2
Transmit state and, 11.5.1.7
low-speed keep-alive strobe, 7.1.7, 7.1.7.4, 11.8.4.1
low-speed signaling
calculating transaction times, 5.9.3
data rates, 4.2.1
data source jitter, 7.1.13.1
data toggle synchronization and retry, 8.6.5
edge rates, 11.12.2
EOP width, 7.1.13.2
intervals between IN token and EOP, 11.3.3
J and K states, 7.1.7
jitter budget table, 7.1.15
low-speed loads, 7.1.2
propagation delays, 7.1.14
transactions illustrated, 8.6.5
low-speed source electrical characteristics, 7.3.2
Table 7-7
LSb and LSB
in bit ordering, 8.1
defined, 2.0 glossary
LS signal/event, 11.5

M
male plug contact materials, 6.5.4.3
Manufacturer's logo location, 6.5.1
Manufacturer's names in device descriptors, 9.6.1
mapping physical and virtual devices, 5.10.4.3
marking on cables, 6.6.2
Master Client status, 5.10.4.1.1, 10.5.3.2.4, 10.5.4.2
master clock, 5.10.1
mastership control over SOF, 5.10.4.1.1, 10.5.3.2.4,
10.5.4.2

material requirements
cables, 6.6:6.6.5
connectors, 6.5:6.5.4.3
mating area materials, 6.5.3.3, 6.5.4.3
MaxPower field, 9.6.2, 11.14
MB/S, defined, 2.0 glossary
Mb/S, defined, 2.0 glossary
mechanical specifications, 6, 6.1
applicable documents, 6.7.1
architectural overview, 6.1
cable assembly, 6.4:6.4.4
cables, 6.3, 6.6:6.6.5
connectors, 6.2, 6.5:6.5.4.3
overview, 4.2.2, 6
PCB reference drawings, 6.9
standards for, 6.7, 6.7.1
USB grounding, 6.8
message pipes. See also control pipes; pipes
in bus protocol overview, 4.4
defined, 2.0 glossary, 5.3.2
overview, 5.3.2.2
microphone non-USB isochronous application, 5.10.1
microphone USB isochronous application, 5.10.2
modifying device configuration, 10.5.4.1.3
monotonic transitions, 7.1.2
most-significant bit, 2.0 glossary, 8.1
MSb and MSB, 2.0 glossary, 8.1
multiple gangs, hubs and, 11.11.1

NNAKs
in bulk transfers, 8.5.1
busy endpoints, 5.3.2
in control transfers, 8.5.2.1
data corrupted or not accepted, 8.6.3
defined, 2.0 glossaryfunction response to IN transactions, 8.4.5.1
function response to OUT transactions, 8.4.5.3
in interrupt transfers, 5.7.4, 8.5.3
overview, 8.3.1 Table 8-1, 8.4.4
NEC Article 800 for communications cables, 6.6.4
next frame in hub timing, 11.2.1
No Acknowledge packet. See NAKs
nominal cable diameter, 6.6.2
nominal cable temperatures, 6.6.4
nominal twist ratio in signal pair, 6.6.2
non-acceptable cables, 6.4.4
Non Return to Zero Invert. See NRZI encoding
non-twisted power pair in cables, 6.6.1, 6.6.2
non-USB isochronous application, 5.10.1
Not Configured state, 11.5, 11.5.1.1
NRZI encoding, 7.1.8
bit stuffing, 7.1.9
defined, 2.0 glossary
in electrical specifications overview, 4.2.1
sync pattern, 7.1.10

O
objects, defined, 2.0 glossary
offsets between host and hub, 11.2
one-way propagation delay, 7.1.16, 7.3.2 Table 7-6,
7.3.2 Table 7-7297Universal Serial Bus Specification Revision 1.1

open architecture, USB development and, 1.2
open-circuit voltage, 7.1.1
operating systems
companion specifications, 10.6
device configuration, 10.3.1
interaction with USBD, 10.5
passing preboot control to, 10.5.5
operating temperatures for cables, 6.6.4
operations, generic USB device operations, 9.2:9.2.7
optional edge rate control capacitors, 7.1.6
out-of-band signaling, 10.1.2
OUT PID, 8.3.1 Table 8-1
ACK handshake and, 8.4.4
ADDR field, 8.3.2.1
in bulk transfers, 8.5.1
in control transfers, 8.5.2, 8.5.2.1
in data toggle, 8.6.1
ENDP field, 8.3.2.2
function response to OUT transactions, 8.4.5.3
in interrupt transfers, 8.5.3
in isochronous transfers, 8.5.4
NAK handshake and, 8.4.4
prebuffering data, 5.10.5
STALL handshake and, 8.4.4
token CRCs, 8.3.5.1
in token packets, 8.4.1
output driver jitter, 7.1.15
output levels, 7.3.2 Table 7-5
output receptacles, 6.2
output rise and fall times, 7.1.2
outside plating, 6.5.3.2, 6.5.4.2
over-current conditions
C_PORT_OVER-CURRENT bit, 11.16.2.6.2.4
getting port status, 11.16.2.6.1
over-current gangs, 11.11.1
over-current protection in self-powered hubs,
7.2.1.2.1
PORT_OVER-CURRENT bit, 11.16.2.6.1.4
port status change bits, 11.16.2.6.2
protection mode descriptors, 11.15.2.1
reporting and recovery, 11.13.5
signaling, 11.11.1
Over-Current Indicator Change field, 11.16.2.5
Over-Current Indicator field, 11.16.2.5
Over-current Reporting Mode field, 11.11.1
Over-current signal/event, 11.5
over-sampling state machine DPLLs, 7.1.15
oxygen index, 6.5.3.1, 6.5.4.1

P
packet buffers, defined, 2.0 glossary
packet field formats, 8.3
address fields, 8.3.2:8.3.2.2
cyclic redundancy checks (CRC), 8.3.5:8.3.5.2
data field, 8.3.4, 8.4.3
frame number field, 8.3.3, 8.4.2
packet identifier field, 8.3.1 (See also PIDs)
packet formats
data packets, 4.4, 8.3.5.2, 8.3:8.3.5.2, 8.4.3, 8.5.1,
8.5.4
handshake packets, 8.4.4 (See also handshakes)
handshake responses, 8.4.5:8.4.5.4

298

packet formats (continued)
overview, 8.4
packet field formats, 8.3:8.3.5.2
SOF packets, 5.10.2, 5.10.4.1.1, 5.10.4.1.2, 8.4.2
token packets, 4.4, 8.3.5.1, 8.4.1, 8.5.1, 8.5.4
packet identifier field (PID). See PIDs
packet IDs. See PIDs
packet nullification, 11.3.2
packets. See also packet field formats; packet
formats; packet size
bit stuffing, 7.1.9
blocking in Collision conditions, 11.8.3
bus protocol overview, 4.4
data packets defined, 4.4
data signaling overview, 7.1.7.2
defined, 2.0 glossary
error detection and recovery, 8.7:8.7.4
handshake packets defined, 4.4
hub connectivity and, 11.1.2.1
inter-packet delay, 7.1.18
one transaction per frame in isochronous transfers,
5.10.7
packet field formats, 8.3:8.3.5.2
packet formats, 8.4:8.4.5.4
packet nullification, 11.3.2
packet size (See packet size)
packet voltage levels, 7.1.7.2
short packets, 5.3.2
splitting samples across packets, 5.10.8
SYNC field, 8.2
token packets defined, 4.4transaction formats, 8.5:8.5.4
packet sizein buffering calculations, 5.10.8
bulk transfer constraints, 5.8.3
characteristics for transfers, 5.4
control transfer constraints, 5.5.3
determining missing packet size, 5.10.7
in device descriptors, 9.6.1
in endpoint descriptors, 9.6.4
interrupt transfer constraints, 5.7.3
isochronous transfer constraints, 5.6.3
packet voltage levels, 7.1.7.2
partitioning of power, 7.2.1.1
pattern synchronization, 9.4.11
PBT (polybutylene terephthalate), 6.5.3.1, 6.5.4.1
PCB reference drawings, 6.9
PC industry, USB and, 3.3
PC-to-telephone interconnects, 1.1
performance criteria for electrical, mechanical andenvironmental compliance, 6.7
period transfers. See interrupt transfers; isochronoustransfers
peripheral devices, 4.8.2.2. See also devices;functionsper-port current limiting, 11.13.5
per-port power switching, 11.11, 11.13.5
PET (polyethylene terephthalate), 6.5.3.1, 6.5.4.1
phase delay for SOF packets, 11.3.1
phase differences, clock synchronization and, 5.10.3
phase-locked clocks, 5.10.3
Phase Locked Loop, defined, 2.0 glossaryUniversal Serial Bus Specification Revision 1.1

phases, defined, 2.0 glossary
Physical and Environmental Performance Properties
of Insulation and Jacket for Telecommunication
Wire and Cable, 6.7.1
physical bus topology, 5.2, 5.2.3, 6.1
physical devices
connectivity illustrated, 5.10.4.3
defined, 2.0 glossary
as implementation focus area, 5.1
logical components in bus topology, 5.2.2
physical interface, 4.2:4.2.2
physical shock standards, 6.7 Table 6-7
PID errors, defined, 8.3.1
PIDs
corrupted PIDs, 8.3.1
in data packets, 8.4.3
data PIDs
DATA0/DATA1 PIDs, 8.3.1 Table 8-1
bulk transfers, 5.8.5
in bulk transfers, 8.5.1
comparing sequence bits, 8.6.2
in control transfers, 8.5.2
in data packets, 8.4.3
in interrupt transactions, 8.5.3
interrupt transfers and, 5.7.5
synchronization and, 8.6
defined, 2.0 glossary
full- vs. low-speed port behavior, 11.8.4
in handshake packets, 8.4.4
handshake PIDs, 8.3.1 Table 8-1
ACK PID
in bulk transfers, 8.5.1
in control transfers, 8.5.2, 8.5.2.1
corrupted ACK handshake, 8.5.2.3, 8.6.4
in data toggle, 8.6, 8.6.1, 8.6.2
defined, 2.0 glossaryfunction response to OUT transactions,
8.4.5.3
host response to IN transactions, 8.4.5.2
overview, 8.3.1 Table 8-1, 8.4.4
in request processing, 9.2.6
NAK PID
in bulk transfers, 8.5.1
busy endpoints, 5.3.2
in control transfers, 8.5.2.1
data corrupted or not accepted, 8.6.3
defined, 2.0 glossaryfunction response to IN transactions, 8.4.5.1
function response to OUT transactions,
8.4.5.3
in interrupt transfers, 5.7.4, 8.5.3
overview, 8.3.1 Table 8-1, 8.4.4
STALL PID
in bulk transfers, 5.8.5, 8.5.1
in control transfers, 8.5.2.1
data corrupted or not accepted, 8.6.3
functional and commanded stalls, 8.4.4
function response to IN transactions, 8.4.5.1
function response to OUT transactions,
8.4.5.3
in interrupt transfers, 5.7.5, 8.5.3
overview, 8.3.1 Table 8-1, 8.4.4

PIDs
handshake PIDs
STALL PID (continued)
protocol stalls, 8.4.4
Request Error responses, 9.2.7
responses to standard device requests, 9.4
returned by control pipes, 8.5.2.4
overview, 8.3.1
PID check bits, 8.7.1
PID errors, 8.3.1
special PIDs
PRE PID
inter-packet delays and, 7.1.18
low-speed port behavior and, 11.8.4
low-speed transactions, 8.6.5
overview, 8.3.1 Table 8-1
Transmit state and, 11.5.1.7
in start-of-frame packets, 8.4.2
in token packets, 8.4.1
token PIDs, 8.3.1 Table 8-1
OUT PID
ACK handshake and, 8.4.4
ADDR field, 8.3.2.1
in bulk transfers, 8.5.1
in control transfers, 8.5.2, 8.5.2.1
in data toggle, 8.6.1
ENDP field, 8.3.2.2
function response to OUT transactions,
8.4.5.3
in interrupt transfers, 8.5.3
in isochronous transfers, 8.5.4
NAK handshake and, 8.4.4
overview, 8.3.1 Table 8-1
prebuffering data, 5.10.5
STALL handshake and, 8.4.4
token CRCs, 8.3.5.1
in token packets, 8.4.1
IN PID
ACK handshake and, 8.4.4
ADDR field, 8.3.2.1
bit times and, 11.3.3
bulk transfers, 8.5.1
control transfers, 8.5.2, 8.5.2.1
ENDP field, 8.3.2.2
error handling on last data transaction,
8.5.2.3
function response to, 8.4.5.1
host response to, 8.4.5.2
interrupt transactions, 8.5.3
intervals between IN token and EOP, 11.3.3
isochronous transfers, 8.5.4
low-speed transactions, 8.6.5
NAK handshake and, 8.4.4
overview, 8.3.1 Table 8-1
prebuffering data, 5.10.5
STALL handshake and, 8.4.4
token CRCs, 8.3.5.1
token packets, 8.4.1

299

Universal Serial Bus Specification Revision 1.1

PIDs
token PIDs (continued)
SETUP PIDACK handshake and, 8.4.4
ADDR field, 8.3.2.1
in control transfers, 8.5.2
in data toggle, 8.6.1
ENDP field, 8.3.2.2
function response to SETUP transactions,
8.4.5.4
overview, 8.3.1 Table 8-1
token CRCs, 8.3.5.1
in token packets, 8.4.1
SOF PID (See also SOFs)
frame number field, 8.3.3
overview, 8.3.1 Table 8-1
start-of-frame packets, 8.4.2
pinsdual pin-type receptacles, 6.9
inrush current and, 7.2.4.1
single pin-type receptacles, 6.9
pipes
aborting or resetting, 10.5.2.2, 10.5.3.2.1
active, stalled, or idle status, 10.5.2.2
allocating bandwidth for, 4.7.5
characteristics and transfer types, 5.4
client pipes, 10.5.1.2.2
data transfer mechanisms, 10.1.3
Default Control Pipe, 4.4 (See also Default Control
Pipe)default pipes, 10.5.1.2.1
defined, 2.0 glossary, 4.4
in device characteristics, 4.8.1
identification, 10.3.4
Master Client status, 5.10.4.1.1, 10.5.3.2.4,
10.5.4.2
overview, 5.3.2, 10.5.1, 10.5.3
pipe state control, 10.5.2.2
pipe usage, 10.5.1.2
Policies, 10.3.1, 10.3.3, 10.5.3.2.2
queuing IRPs, 10.5.3.2.3
role in data transfers, 4.7
service and polling intervals, 10.3.3
stream and message pipes, 4.4, 5.3.2, 5.3.2.1,
5.3.2.2
supported pipe types, 10.5.3.1:10.5.3.1.4
USBD pipe mechanism responsibilities,
10.5.1:10.5.3.2.4
USB robustness and, 4.5
pipe state control, 10.5.2.2
pipe status, 10.5.2.2
platingplug contact materials, 6.5.4.3
plug shell materials, 6.5.4.2
receptacle contact materials, 6.5.3.3
receptacle shell materials, 6.5.3.2
PLL, defined, 2.0 glossary
plugsinterface and mating drawings, 6.5.3
keyed connector protocol, 6.2
materials, 6.5.4.1, 6.5.4.2, 6.5.4.3
orientation, 6.5.1

300

plugs (continued)
Series "A" and Series "B" plugs, 6.5.4
standards for, 6.7
termination data, 6.5.2
USB Icon, 6.5, 6.5.1
Policies
defined, 10.3.1
setting, 10.3.3
USBDI mechanisms, 10.5.3.2.2
pollingdefined, 2.0 glossary
endpoints, 9.6.4
setting intervals for pipes, 10.3.3
polybutylene terephthalate (PBT), 6.5.3.1, 6.5.4.1
polyethylene terephthalate (PET), 6.5.3.1, 6.5.4.1
POR signal/event
Bus_Reset state and, 11.6.1.6
defined, 2.0 glossary
in receiver state machine, 11.6.1
Port Change field, 11.4.4, 11.16.2.6.2
PORT_CONNECTION
defined, 11.16.2.6.1.1
hub class feature selectors, 11.16.2
Port Status field, 11.16.2.6.1
PORT_ENABLE
clearing, 11.16.2.2
defined, 11.16.2.6.1.2
hub class feature selectors, 11.16.2
Port Error condition, 11.16.2.6.2.2
Port Status field, 11.16.2.6.1
Port Error condition, 11.8.1, 11.16.2.6.2.2
Port_Error signal/event, 11.5
PORT_LOW_SPEED
defined, 11.16.2.6.1.7
hub class feature selectors, 11.16.2
Port Status field, 11.16.2.6.1
speed detection and, 11.8.2
PORT_OVER_CURRENT
defined, 11.16.2.6.1.4
hub class feature selectors, 11.16.2
over-current conditions and, 11.11.1, 11.13.5
Port Status field, 11.16.2.6.1
PORT_POWER
clearing, 11.16.2.2
defined, 11.16.2.6.1.6
hub class feature selectors, 11.16.2
Port Status field, 11.16.2.6.1
SetPortFeature() request, 11.16.2.9
shared power switching and, 11.11.1
PortPwrCtrlMask field
hub descriptors for, 11.15.2.1
multiple gangs and, 11.11.1
power switching settings, 11.11
PORT_RESET
defined, 11.16.2.6.1.5
hub class feature selectors, 11.16.2
Port Status field, 11.16.2.6.1
SetPortFeature() request, 11.16.2.9

Universal Serial Bus Specification Revision 1.1

ports. See also hubs
in bus topology, 5.2.3
clearing features, 11.16.2.2
data source signaling, 7.1.13
defined, 4.8.2.1
disconnect timer, 11.5.2
downstream ports, 4.8.2.1, 11.5:11.5.1.11
full- vs. low-speed port behavior, 11.8.4
getting bus state, 11.16.2.3
in hub architecture, 11.1.1
hub configuration and, 11.14
hub descriptors, 11.15:11.15.2.1
hub internal ports, 11.4:11.4.4
input capacitance, 7.1.6
over-current reporting and recovery, 11.13.5
port expansion considerations in USB
development, 1.1
power control, 11.11
resetting, 10.2.8.1
root ports, 2.0 glossary
setting port features, 11.16.2.9
statusbus state evaluation, 11.8:11.8.4.1
detecting status changes, 11.13.2, 11.13.3
hub and port status change bitmap, 11.13.4
port status bits, 11.16.2.6.1:11.16.2.6.2.5
upstream ports, 4.8.2.1, 11.6:11.6.2.6
port status change bits, 11.16.2.6.1:11.16.2.6.2.5
Port Status field, 11.16.2.6.1
PORT_SUSPEND
clearing, 11.16.2.2
defined, 11.16.2.6.1.3
hub class feature selectors, 11.16.2
Port Status field, 11.16.2.6.1
SetPortFeature() request, 11.16.2.9
power budgeting, 7.2.1.4, 9.2.5.1
power conductors in cables, 6.3
power control circuits in bus-powered hubs, 7.2.1.1
power distribution and management. See also overcurrent conditions; power switchingclasses of devices, 7.2.1:7.2.1.5
bus-powered devices or hubs, 4.3.1, 7.2.1.1
high-power bus-powered functions, 7.2.1.4
low-power bus-powered functions, 7.2.1.3
self-powered devices or hubs, 4.3.1, 7.2.1.2,
7.2.1.5
configuration characteristics
hub descriptors for power-on sequence,
11.15.2.1
information in device characteristics, 4.8.1
power consumption in configuration descriptors,
9.6.2
power source capability in configuration, 9.1.1.2
dynamic attach and detach, 7.2.3, 7.2.4:7.2.4.2
Host Controller role in, 4.9
host role in, 10.1.5
hub support for, 11.1
loss of power, 7.2.1.2
over-current conditions, 7.2.1.2.1, 11.13.5 (Seealso over-current conditions)overview, 4.3.1, 4.3.2, 9.2.5
power budgeting, 7.2.1.4, 9.2.5.1

power distribution and management (continued)
power statuscontrol during suspend/resume, 7.2.3
device states, 9.1.1.2
port power states, 11.16.2.6.1.6, 11.16.2.9
power switching, 11.11, 11.11.1
remote wakeup, 7.2.3, 9.2.5.2
USB System role, 10.5.4.3
USB System Software role, 4.9
voltage drop budget, 7.2.2
Powered device state, 9.1.1 Table 9-1, 9.1.1.2
Powered Off state, 11.5, 11.5.1.2
powered-on ports, 11.16.2.9
Power On Reset. See POR signal/event
power-on sequence, 11.15.2.1
power pair construction, 6.6.2
power pins, 7.2.4.1
Power_source_off signal/event, 11.5
power switchingbus-powered hubs, 7.2.1.1
getting port status, 11.16.2.6.1
hub descriptors for, 11.15.2.1
hub port power control, 11.11
power-on and connection events timing, 7.1.7.1
power switching gangs, 11.11.1
staged power switching, 7.2.1.4
preamble packet. See PRE PID
preboot control, passing to operating systems, 10.5.5
prebuffering data, 5.10.5
prepared termination, 6.4.2, 6.4.3
PRE PID, 8.3.1 Table 8-1
inter-packet delays and, 7.1.18
low-speed port behavior and, 11.8.4
low-speed transactions, 8.6.5
Transmit state and, 11.5.1.7
product descriptions in device descriptors, 9.6.1
Product IDs in device descriptors, 9.6.1
programmable data rate, defined, 2.0 glossary
prohibited cable assemblies, 6.4.4
propagation delay
cable delay, 7.1.16
detachable cables, 6.4.1
end-to-end signal delay, 7.1.19
full- and low-speed signals, 7.1.14
full-speed cables, 6.4.2
full-speed source electrical characteristics, 7.3.2
Table 7-6
low-speed cables, 6.4.3, 7.1.1.2
low-speed source electrical characteristics, 7.3.2
Table 7-7
tests, 6.7 Table 6-7
propagation delay skew, 6.7 Table 6-7
protected fields in packets, 8.3.5
protocol codes
defined, 9.2.3
in device descriptors, 9.6.1
in interface descriptors, 9.6.3
protocol engine requirements of Host Controller,
10.2.5
protocol errors, detecting, 10.2.6
Protocol field, 9.2.3

301

Universal Serial Bus Specification Revision 1.1

Protocol layer, 8
bit ordering, 8.1
bus protocol, 4.4
data toggle synchronization and retry, 8.6:8.6.5
error detection and recovery, 8.7:8.7.4
packet field formats, 8.3:8.3.5.2
packet formats, 8.4:8.4.5.4
SYNC field, 8.2
transaction formats, 8.5:8.5.4
protocols
defined, 2.0 glossary
protocol codes, 9.2.3, 9.6.1, 9.6.3
protocol stall, 8.4.4, 8.5.2.4
pull-up and pull-down resistors
buffer impedance measurement, 7.1.1.1
hub requirements, 11.12.1
power control during suspend/resume, 7.1.7.4,
7.2.3
signal termination, 7.1.5

Qqueuing IRPs, 10.5.3.2.3

R
RA (rate adaptation)
asynchronous RA, 2.0 glossary
audio connectivity and, 5.10.4.3.1
defined, 2.0 glossary
in source-to-sink connectivity, 5.10.4.3
synchronous data connectivity, 5.10.4.3.2
synchronous RA, 2.0 glossary
random vibration standards, 6.7 Table 6-7
rate adaptation. See RA (rate adaptation)
rate matchers
asynchronous endpoints, 5.10.4.1.1
buffering for rate matching, 5.10.8
client software role, 5.10.4.3
in non-USB isochronous application, 5.10.1
synchronous endpoints, 5.10.4.1.2
ratings, full-speed, 6.4.1, 6.4.2
read/write sequencesin bulk transfers, 8.5.1
in control transfers, 8.5.2, 8.5.2.1
real-time clock, 5.10.1
real-time data transfers. See isochronous transfers
receivers
receive phase of signaling, 7.1.1
receiver characteristics, 7.1.4
receiver jitter, 7.1.15, 7.3.2 Table 7-6, 7.3.2 Table
7-7, 7.3.3 Figure 7-41
receiver sequence bits, 8.6, 8.6.2
receiver state descriptions, 11.6.1
receiver state machine, 11.6, 11.6.1
receiver state machine, 11.6, 11.6.1
ReceivingJ state, 11.6.1, 11.6.1.1
ReceivingK state, 11.6.1, 11.6.1.3
ReceivingSE0 state, 11.6.1, 11.6.1.5
receptacles
interface and mating drawings, 6.5.3
keyed connector protocol, 6.2
materials, 6.5.3.1, 6.5.3.2, 6.5.3.3
PCB reference drawings, 6.9

302

receptacles (continued)
Series "A" and Series "B" receptacles, 6.5.3
standards for, 6.7
termination data, 6.5.2
USB Icon, 6.5, 6.5.1
Recipient bits, 9.4.5
recovering from errors. See error detection andhandling
recovery intervals for devices, 9.2.6.2
re-enumerating sub-trees, 10.5.4.6
reflected endpoint status, 10.5.2.2
registers in hub timing, 11.2.1
regulators in bus-powered hubs, 7.2.1.1
regulatory requirements for USB devices, 7.3.1
reliable delivery in isochronous transfers, 5.10
remote wakeup
in configuration descriptors, 9.6.2
Host Controller role, 10.2.7
inrush current and, 7.2.3
overview, 9.2.5.2
resume signaling, 7.1.7.5, 9.1.1.6
timing relationships, 11.9
USB System role in, 10.5.4.6
Remote Wakeup field, 9.4.5
removable devices, 11.15.2.1
removing devices. See dynamic insertion and removal
RepeatingSE0 state, 11.6.2, 11.6.2.3
replacing configuration information, 10.5.4.1.3
reporting rates for feedback, 5.10.4.2
request codes, 9.4 Table 9-4, 11.16.2
Request Errors, 9.2.7
requests. See also PIDs; names of specific requests
bRequest field, 9.3.2
class-specific requests, 9.2.6.5, 10.5.2.8,
11.16:11.16.2.9
completion times for hub requests, 11.16.1
control transfers and, 5.5
defined, 2.0 glossary
in device class definitions, 9.7.3
hub standard and class-specific requests,
11.16:11.16.2.9
information requirements for, 10.3.4
overview, 9.2.6
port status reporting, 11.13.3
request processing timing, 9.2.6.1
reset/resume recovery time, 9.2.6.2
set address processing, 9.2.6.3
standard device requests, 9.2.6.4, 9.4:9.4.11
standard feature selectors, 9.4 Table 9-6
standard hub requests, 11.16:11.16.2.9
standard request codes, 9.4 Table 9-4
USBD command mechanisms, 10.5.2:10.5.2.12
USB device requests, 9.3:9.3.5
vendor-specific requests, 10.5.2.9
required data sequences for transfers, 5.4
reserved portions of frames, 5.5.4
Reset bus state
downstream ports, 11.5, 11.5.1.5
in power-on and connection events, 7.1.7.1
reset signaling, 7.1.7.3
signaling levels and, 7.1.7

Universal Serial Bus Specification Revision 1.1

reset conditionin bus enumeration process, 9.1.2
C_PORT_RESET bit, 11.16.2.6.2.5
Default device state and, 9.1.1.3
device characteristics, 9.2.1
getting port status, 11.16.2.6.1
hub reset behavior, 11.10
PORT_RESET bit, 11.16.2.6.1.5
port status change bits, 11.16.2.6.2
remote wakeup and, 10.5.4.6
reset recovery time, 7.1.7.3, 9.2.6.2
resetting pipes, 10.5.2.2
SetPortFeature(PORT_RESET) request, 11.16.2.9
USB System and, 10.2.8.1
reset signaling, 7.1.7.3
resistance ratings, 6.6.3
resistors
pull-up and pull-down resistors, 7.1.1.1, 7.1.5,
7.1.7.4, 7.2.3, 11.12.1
series damping resistors, 7.1.1.1
resonators, data-rate tolerance and, 7.1.11
resource management, USB System role in, 10.3.2
Restart_S/Restart_E states, 11.5, 11.5.1.11
Resume bus state
downstream ports, 11.5, 11.5.1.9
overview, 7.1.7.5
receivers, 11.6.1, 11.6.1.4
reset signaling and, 7.1.7.3
signaling levels and, 7.1.7
Resume_Event signal/event, 11.4
resume intervals for devices, 9.2.6.2
resume signaling
hub support, 11.1.2.2, 11.9
power control during suspend/resume, 7.2.3
remote wakeup and, 10.5.4.6
resume conditions in Hub Controller, 11.4.4
single-ended transmissions, 11.6
retire, defined, 2.0 glossary
retiring IRPs. See aborting/retiring transfers
RFI, USB grounding and, 6.8
rise and fall times
data source jitter, 7.1.13.1
driver usage, 7.1.2.1
full-speed connections, 7.1.1.1
full-speed source electrical characteristics, 7.3.2
Table 7-6
low-speed source electrical characteristics, 7.3.2
Table 7-7
overview, 7.1.2
SE0 from low-speed devices, 7.1.14
robustness of USB, 3.3, 4.5:4.5.2
root hubin bus topology, 5.2.3
defined, 2.0 glossary
HCDI presentation of, 10.4
Host Controller and, 10.2.8, 10.2.8.1
state handling, 10.2.1
root port hub, defined, 7.2.1
root ports, 2.0 glossary, 11.9
Rptr_Enter_WFEOPFU signal/event, 11.5
Rptr_Exit_WFEOPFU signal/event, 11.5
Rptr_WFEOP signal/event, 11.6.2

Rx_Bus_Reset signal/event, 11.6.2, 11.7
Rx_Resume signal/event, 11.5, 11.7
Rx_Suspend signal/event, 11.4, 11.5, 11.6.2, 11.7

S
sample clock
buffering for rate matching, 5.10.8
defined, 5.10.2
synchronous endpoints, 5.10.4.1.2
sampled analog devices, 5.10.4
Sample Rate Conversion. See SRC
samples
defined, 2.0 glossary
sample size in buffering calculations, 5.10.8
samples per frame in isochronous transfers,
5.10.4.2
schedulingaccess to USB interconnect, 4.1
transaction schedule in bus protocol overview, 4.4
SE0sent signal/event, 11.6.2
SE0 signal/eventin data signaling, 7.1.7.2
downstream port state machine, 11.5
Not Configured state, 11.5.1.1
propagation delays, 7.1.14
pull-down resistors and, 7.1.7.1
receiver state machine, 11.6.1
reset signaling, 7.1.7.3
SE0 interval of EOP, 7.3.2 Table 7-6, 7.3.2 Table
7-7
signaling levels and, 7.1.7
single-ended transmissions, 11.6
SE0 width, 7.1.13.2, 7.1.14, 7.3.2 Table 7-6, 7.3.2
Table 7-7
selective resume signaling, 11.9
selective suspend signaling
defined, 9.1.1.6
hub support, 11.9
overview, 7.1.7.4.2
self-powered devices and functions
configuration descriptors, 9.6.2
defined, 4.3.1, 7.2.1
device states, 9.1.1.2
overview, 7.2.1.5
Self Powered field, 9.4.5
self-powered hubs
configuration, 11.14
defined, 7.2.1
device states, 9.1.1.2
over-current protection, 7.2.1.2.1
overview, 7.2.1.2
power switching, 11.11
self-recovery, USB robustness and, 4.5
SendEOP state, 11.5, 11.5.1.10
SendJ state, 11.6.2, 11.6.2.4
Send Resume state (Sresume), 11.6.2, 11.6.2.6
sequence of transactions in frames, 5.9.2
Serial Interface Engine (SIE), 10.1.1, 10.2.2
serializer/deserializer, 10.2.2
serial numbers in device descriptors, 9.6.1

303

Universal Serial Bus Specification Revision 1.1

Series "A" and "B" connectors
detachable cables and, 6.4.1
keyed connector protocol, 6.2
plugs
injection molded thermoplastic insulator
material, 6.5.4.1
interface drawings, 6.5.4
plug (male) contact materials, 6.5.4.3
plug shell materials, 6.5.4.2
receptacles
injection molded thermoplastic insulator
material, 6.5.3.1
interface and mating drawings, 6.5.3
PCB reference drawings, 6.9
receptacle contact materials, 6.5.3.3
receptacle shell materials, 6.5.3.2
standards for, 6.7
USB Icon, 6.5, 6.5.1
series damping resistors, 7.1.1.1
service, defined, 2.0 glossary
service clock, 5.10.2, 5.10.8
service intervals, 2.0 glossary, 10.3.3
service jitter, defined, 2.0 glossary
service periods of data, 5.10.1
service rates, defined, 2.0 glossary
SET_ADDRESS
hub requests, 11.16.1
overview, 9.4.6
standard device request codes, 9.4
SetAddress() request
overview, 9.4.6
reset recovery time and, 7.1.7.3
time limits for completing processing, 9.2.6.3
SetConfiguration() request, SET_CONFIGURATION
hub requests, 11.16.1
overview, 9.4.7
Powered-off state and, 11.5.1.2
setting configuration in descriptors, 9.6.2
standard device request codes, 9.4
SetDescriptor() request, SET_DESCRIPTOR
getting endpoint descriptors, 9.6.4
hub class requests, 11.16.2
hub requests, 11.16.1
interface descriptors and, 9.6.3
overview, 9.4.8
SetHubDescriptor() request, 11.16.2.7
standard device request codes, 9.4
SetDeviceFeature(DEVICE_REMOTE_WAKEUP)
request, 10.5.4.6
SetFeature() request, SET_FEATURE, 9.4.5, 9.4.9
hub class requests, 11.16.2
hub requests, 11.16.1
overview, 9.4.9
SetHubFeature() request, 11.16.2.8
SetPortFeature() request, 11.16.2.9
standard device request codes, 9.4
SetHubDescriptor() request, 11.16.2, 11.16.2.7
SetHubFeature() request, 11.16.2, 11.16.2.5,
11.16.2.8
SetInterface() request, SET_INTERFACE, 9.2.3, 9.4,
9.4.10, 9.6.3, 11.16.1

304

SetPortFeature() request
hub class requests, 11.16.2, 11.16.2.9
PORT_CONNECTION, 11.16.2.6.1.1
PORT_ENABLE, 11.16.2.6.1.2
PORT_LOW_SPEED, 11.16.2.6.1.7
PORT_OVER_CURRENT, 11.16.2.6.1.4
PORT_POWER
Disconnected state and, 11.5.1.3
port power settings, 11.11
port power states, 11.16.2.6.1.6
requirements, 11.16.2.9
PORT_RESET
completion, 9.2.6
C_PORT_ENABLE bit, 11.16.2.6.2.2
evaluating device speed during, 11.8.2
initiating port reset, 11.16.2.6.1.5
in port enabling, 11.16.2.6.1.2
requirements, 11.16.2.9
Resetting state and, 11.5.1.5
PORT_SUSPEND, 10.5.4.6
requirements, 11.16.2.9
selective suspend, 7.1.7.4.2
suspending ports, 11.5.1.8, 11.16.2.6.1.3
power-off conditions and, 11.14
SETUP PID, 8.3.1 Table 8-1
ACK handshake and, 8.4.4
ADDR field, 8.3.2.1
in control transfers, 8.5.2
in data toggle, 8.6.1
ENDP field, 8.3.2.2
function response to, 8.4.5.4
token CRCs, 8.3.5.1
in token packets, 8.4.1
Setup stage
in control transfer data sequences, 5.5.5
in control transfers, 5.5, 8.5.2
data format for USB device requests, 9.3
shellconductors, 6.5.2
plug shell materials, 6.5.4.2
receptacle shell materials, 6.5.3.2
shieldinggrounding, 6.8
low-speed and full-speed cables, 6.6
outer and inner cable shielding, 6.6.1
shielded cables illustrated, 6.4.1
standardized contact terminating assignments,
6.5.2
short circuits, USB withstanding capabilities, 7.1.1
short packets
defined, 9.4.3
detecting, 10.2.6
multiple data payloads and, 5.3.2
SIE (Serial Interface Engine), 10.1.1, 10.2.2
signal conductors in cables, 6.3
signal edges. See edges of signals
signaling
bit stuffing, 7.1.9
cable attenuation, 7.1.17
connect and disconnect signaling, 7.1.7.1
data encoding/decoding, 7.1.8

Universal Serial Bus Specification Revision 1.1

signaling (continued)
data rate, 7.1.11
data signaling, 7.1.7.2
delay
bus turn-around time and inter-packet delay,
7.1.18cable delay, 7.1.16
cable skew delay, 7.1.3
maximum end-to-end signal delay, 7.1.19
frame interval and frame interval adjustment,
7.1.12
hub signaling timings, 7.1.14
in-band and out-of-band, 10.1.2
input characteristics, 7.1.6
jitter, 7.1.13.1, 7.1.15 (See also jitter)
low-speed (1.5Mb/S) driver characteristics, 7.1.1.2
overview, 7.1
receiver characteristics, 7.1.4, 7.1.15
reset signaling, 7.1.7.3
resume signaling, 7.1.7.5
rise and fall time, 7.1.2, 7.1.2.1
signal attenuation, 7.1.17
signal edges (See edges of signals)
signaling levels, 7.1.7:7.1.7.5
signal integrity, 4.5
signal termination, 7.1.5
source signaling, 7.1.13, 7.1.13.1, 7.1.13.2
suspend signaling, 7.1.7.4
sync pattern, 7.1.10
USB driver characteristics, 7.1.1
signal matching, 7.1.2
signal pair attenuation, 6.4.1, 6.7 Table 6-7
signal pair construction, 6.6.2
signal pins, 7.2.4.1
signal swing, 7.1.2
signal termination, 7.1.5
Single-ended 0 bus state (SEO)in data signaling, 7.1.7.2
pull-down resistors and, 7.1.7.1
reset signaling, 7.1.7.3
signaling levels and, 7.1.7
single-ended capacitance, 7.1.1.2
single-ended components in upstream ports, 11.6
single-ended receivers, 7.1.4, 7.1.6
single pin-type receptacles, 6.9
sink endpoints
adaptive sink endpoints, 5.10.4.1.3
audio connectivity, 5.10.4.3.1
connectivity overview, 5.10.4.3
feedback for isochronous transfers, 5.10.4.2
synchronization types, 5.10.4.1
synchronous data connectivity, 5.10.4.3.2
skew
cable skew delay, 6.7 Table 6-7, 7.1.3, 7.3.2 Table
7-9
differential-to-EOP transition skew, 7.3.3 Figure 740
hub EOP delay and EOP skew, 7.3.3 Figure 7-43
hub/repeater electrical characteristics, 7.3.2 Table
7-8
hub switching skew, 7.1.9
Idle-to-K state transition, 7.1.14

skew (continued)
minimizing signal skew, 7.1.1
timing skew accumulating, 11.2.2
slips in synchronous data, 5.10.4.3.2
small capacitors, 7.1.6
SOF PID, 8.3.1 Table 8-1
frame number field, 8.3.3
start-of-frame packets, 8.4.2
SOFs
bus clock and, 5.10.2
defined, 2.0 glossary
in downstream port state machine, 11.5
error recovery in isochronous transfers, 5.10.7
frame intervals, 7.1.12
frame timer synchronization, 11.2, 11.2.1
Host Controller frame generation, 10.2.3
loss of consecutive SOFs, 11.2.2
mastership control and synchronization, 5.10.4.1.1,
5.10.4.1.2
overview, 8.4.2
tracking, 5.10.6, 5.10.7
using as clocks, 5.10.5
soft-start circuits, 7.2.4.1
software interfaces. See client software; HCDI; host;
USBDI; USB System software
solderability standards, 6.7 Table 6-7
solder tails, 6.5.3.3, 6.5.4.3
SOP bus state, 7.1.7, 7.1.7.2
SOP_FD signal/event
generating, 11.4.4
in Hub Repeater state machine, 11.7
SOP_FU signal/event, 11.7
SOPs, 8.3
error detection through bus turn-around timing,
8.7.2
frame timer synchronization and, 11.2.1
Idle-to-K state transition, 7.1.14
SOP distortion, 7.3.3 Figure 7-42
timeout periods and, 7.1.19
source endpoints
adaptive source endpoints, 5.10.4.1.3
audio connectivity, 5.10.4.3.1
connectivity overview, 5.10.4.3
feedback for isochronous transfers, 5.10.4.2
synchronization types, 5.10.4.1
synchronous data connectivity, 5.10.4.3.2
source jitter, 7.3.2 Table 7-6, 7.3.2 Table 7-7
source/sink connectivity, 5.10.4.3
special PIDs (PRE PID), 8.3.1 Table 8-1
inter-packet delays and, 7.1.18
low-speed port behavior and, 11.8.4
low-speed transactions, 8.6.5
Transmit state and, 11.5.1.7
specific-sized data payloads, 5.3.2
speed detection
attached devices, 11.8.2
detecting low-speed functions and hubs,
11.16.2.6.1.7
detecting speed of devices, 7.1.7.1
termination and, 7.1.5
Speed_eval state, 11.8.2
SPI, defined, 2.0 glossary305

Universal Serial Bus Specification Revision 1.1

splitting sample across packets, 5.10.8
SRC
asynchronous SRC, 2.0 glossary
audio connectivity and, 5.10.4.3.1
defined, 2.0 glossary
synchronous SRC, 2.0 glossary
Sresume state, 11.6.2, 11.6.2.6
staged power switching, 7.2.1.4
stages in control transfers, defined, 2.0 glossary, 5.5.
See also Data stage; Setup stage; Status
stage
STALLs, 8.3.1 Table 8-1
in bulk transfers, 5.8.5, 8.5.1
in control transfers, 8.5.2.1
data corrupted or not accepted, 8.6.3
functional and commanded stalls, 8.4.4
function response to IN transactions, 8.4.5.1
function response to OUT transactions, 8.4.5.3
in interrupt transfers, 5.7.5, 8.5.3
overview, 8.4.4
protocol stalls, 8.4.4
Request Error responses, 9.2.7
responses to standard device requests, 9.4
returned by control pipes, 8.5.2.4
standard device information, 4.8.1
standard device requests, 9.2.6.4, 9.4:9.4.11, 11.16.1
standards (applicable documents), 6.7.1
standard USB descriptor definitions, 9.6.1:9.6.5
Start-of-Frame. See SOFs
Start-of-Packet. See SOPs
Start-of-Packet bus state (SOP), 7.1.7, 7.1.7.2
start-of-packet delimiter. See SOPs
star topology, 5.2.3
state handling. See bus states; status
static output swing of USBD, 7.1.1
status. See also status change bits
device states, 10.5.2.7, 11.13.2
Host Controller role in, 4.9
host's role in monitoring status and activity, 10.1.4
hub and port status change bitmap, 11.13.4
hub status, 11.16.2.5
notification of completion status, 10.3.4
port change information processing, 11.13.3
port status, 11.16.2.6:11.16.2.6.2.5
port status change bits, 11.16.2.6.2:11.16.2.6.2.5
USBD event notifications, 10.5.4.4
USBD status reporting and error recovery, 10.5.4.5
status change bits. See also Status Change endpoint
detecting changes, 11.13.2
device states, 11.13.2
hub and port status change bitmap, 11.13.4
hub status, 11.16.2.5
over-current status change bits, 11.13.5
port status change bits, 11.16.2.6.2:11.16.2.6.2.5
Status Change endpoint
defined, 11.13.1
device states and, 11.13.2
hub and port status change bitmap, 11.13.4
hub configuration and, 11.14
hub descriptors, 11.15.1

306

Status stage
in control transfers, 5.5, 5.5.5, 8.5.2
reporting status results, 8.5.2.1
storage temperatures for cables, 6.6.4
stranded tinned conductors, 6.6.2
streaming real time transfers. See isochronoustransfers
stream pipes
bulk transfers and, 5.8.2
in bus protocol overview, 4.4
defined, 2.0 glossary, 5.3.2
interrupt transfers and, 5.7.2
isochronous transfers and, 5.6.2
overview, 5.3.2.1
string descriptors
GetDescriptor() request, 9.4.3
as optional, 9.5
overview, 9.6.5
stuffed bits. See bit stuffing
subclasses
device subclass codes, 9.2.3, 9.6.1
interface subclass codes, 9.2.3, 9.6.3
SubClass field, 9.2.3
substrate materials
plug contact materials, 6.5.4.3
plug shell materials, 6.5.4.2
receptacle contact materials, 6.5.3.3
receptacle shell materials, 6.5.3.2
successful transfers, 8.6.2, 10.3.4
supply current, 7.3.2 Table 7-5
supply voltage
DC electrical characteristics, 7.3.2 Table 7-5
oscillators, 7.1.11
surge limiting, 7.2.4.1
Suspend bus state
global suspend, 7.1.7.4.1
overview, 7.1.7.4
power control during suspend/resume, 7.2.3
reset signaling, 7.1.7.3
resume signaling, 7.1.7.5
selective suspend, 7.1.7.4.2
Suspend Delay state, 11.4, 11.4.2
suspended devices
global suspend, 7.1.7.4.1
hub support for suspend signaling, 11.9
power control during suspend/resume, 7.2.3
power-on and connection events, 7.1.7.1
remote wakeup, 9.2.5.2, 10.2.7, 10.5.4.6
resume signaling, 7.1.7.5
selective suspend, 7.1.7.4.2
single-ended transmissions, 11.6
Suspend bus state, 7.1.7.4
Suspended device state, 9.1.1.6
suspended hubs
hub reset behavior, 11.10.1
resume signaling and, 11.1.2.2
suspended ports
C_PORT_SUSPEND, 11.16.2.6.2.3
getting port status, 11.16.2.6.1
port status change bits, 11.16.2.6.2
PORT_SUSPEND, 11.16.2.6.1.3, 11.16.2.9

Universal Serial Bus Specification Revision 1.1

Suspended state, 9.1.1 Table 9-1, 9.1.1.6, 11.5,
11.5.1.8
Suspend state, 11.6.1, 11.6.1.2
switching thresholds for single-ended receivers, 7.1.4
SYNC fieldin data signaling, 7.1.7.2
in electrical specifications overview, 4.2.1
overview, 8.2
SynchFrame() request, SYNCH_FRAME, 9.4, 9.4.11,
11.16.1
synchronization. See also synchronization types
clock synchronization, 5.10.3
data-per-time synchronization, 5.10.7
data toggle synchronization, 8.4.3, 8.6:8.6.5
endpoint synchronization frame, 9.4.11
frame timer synchronization, 11.2, 11.2.1
jitter, 2.0 glossary (See also jitter)physical and virtual devices, 5.10.4.3
SYNC field, 8.2
sync pattern, 7.1.10
transmitter and receiver synchronization in
isochronous transfers, 5.10
synchronization types
adaptive, 5.10.4.1.3
asynchronous, 5.10.4.1.1
defined, 2.0 glossary, 5.10.4
overview, 5.10.4.1
synchronous, 5.10.4.1.2
synchronous data connectivity, 5.10.4.3.2
synchronous data devices, 5.10.4
synchronous endpoints, 5.10.4.1.2, 5.10.4.3
synchronous RA, 2.0 glossary, 5.10.4.3
synchronous SRC, 2.0 glossarysync pattern, 7.1.9, 7.1.10
system configuration. See configuration
System Programming Interface, defined, 2.0 glossary
system software. See USB System SoftwareTtaxonomy of application space, 3.2
TDM, defined, 2.0 glossary
telephone interconnects, 1.1
temperature
data-rate inaccuracies and, 7.1.11
ranges for cables, 6.6.4
termination
blunt cut and prepared termination, 6.4.2, 6.4.3
DC electrical characteristics, 7.3.2 Table 7-5
defined, 2.0 glossary
detachable cable assemblies, 6.4.1
electrical specifications overview, 4.2.1
full-speed captive cable assemblies, 6.4.2
low-speed captive cable assemblies, 6.4.3
signal termination, 7.1.5
USB topology rules, 6.4.4
termination data, 6.5.2
test criteria for electrical, mechanical andenvironmental compliance, 6.7
Test for Flammability of Plastic Materials for Parts in
Devices and Appliances, 6.7.1
thermal shock standards, 6.7 Table 6-7
Thevenin resistance, 7.1.5

tiered star topology, 5.2.3
Time Division Multiplexing (TDM), 2.0 glossary
timed states
Disconnected state, 11.5.1.3
Resuming state, 11.5.1.9
timeout
bus transaction timeout, 5.10.7
defined, 2.0 glossary
detecting timeout conditions, 10.2.6
timeout intervals in error detection, 8.7.3
timing. See also cable delay; propagation delay;
skew; synchronization; timing waveforms
bus timing/electrical characteristics, 7.3.2
bus transaction time calculations, 5.9.3
bus turn-around timing, 8.7.2
clock model, 5.10.2
clock synchronization, 5.10.3
completion times for hub requests, 11.16.1
current frame timer, 11.2.1
data source signaling, 7.1.13
device event timings, 7.3.2 Table 7-11
frame interval adjustments, 7.1.12
frame timer, 11.2.1
hub event timings, 7.3.2 Table 7-10
hub frame timer, 11.2:11.2.2
hub signaling timings, 7.1.14
isochronous transfer feedback, 5.10.4.2
isochronous transfer importance, 5.10
next frame timer, 11.2.1
in non-USB isochronous application, 5.10.1
port disconnect timer, 11.5.2
power-on and connection events timing, 7.1.7.1
remote wakeup timing relationships, 11.9
request processing timing, 9.2.6.1
Resetting state and Resuming state intervals,
11.5.1.9
SE0 for EOP width timing, 7.1.13.2
skew accumulating between host and hub, 11.2.2
SOF PID timing information, 8.4.2
SOF tokens as clocks, 5.10.5
synchronization types, 5.10.4.1
timing waveforms, 7.3.3
differential data jitter, 7.3.3 Figure 7-39
differential-to-EOP transition skew and EOP width,
7.3.3 Figure 7-40
hub differential delay, differential jitter, and SOP
distortion, 7.3.3 Figure 7-42
hub EOP delay and EOP skew, 7.3.3 Figure 7-43
receiver jitter tolerance, 7.3.3 Figure 7-41
toggle mode. See data toggle
token packets
in bulk transfers, 8.5.1
bus protocol overview, 4.4
CRCs, 8.3.5.1
defined, 2.0 glossary
in isochronous transfers, 8.5.4
overview, 8.4.1
packet field formats, 8.3:8.3.5.2
token PIDs, 8.3.1 Table 8-1. See also IN PID; OUT
PID; SETUP PID; SOF PID

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Universal Serial Bus Specification Revision 1.1

topology
bus topology, 4.1, 4.1.1, 5.2:5.2.5
hub tiers defined, 2.0 glossarytransaction completion prediction, 11.3.3
transaction list
defined, 5.9.1.4
HCD role in, 5.9.1.3
Host Controller and, 5.9.1.5
transactions
allocating bandwidth for, 5.9.1:5.9.1.5, 10.3.2
buffer size calculations, 5.9.4
bus protocol overview, 4.4
defined, 2.0 glossary
error detection and recovery, 8.7:8.7.4
organization within IRPs, 5.9.2
timeout, 5.10.7
tracking transactions, 5.9.2
transaction completion prediction, 11.3.3
transaction formats
bulk transfers, 5.8.4, 8.5.1
control transfers, 5.5.4, 8.5.2:8.5.2.3
interrupt transactions, 8.5.3
isochronous transfers, 5.6.3, 5.10.6, 5.10.7,
8.5.4
overview, 8.5transaction list, 5.9.1.3, 5.9.1.4, 5.9.1.5
transaction schedule, 4.4transaction time calculations, 5.9.3
transfer management, 5.9.1:5.9.1.5
allocating bandwidth, overview, 4.7.5, 5.9.1.1
client software, 5.9.1.1
HCD, 5.9.1.3
Host Controller, 5.9.1.5
illustrated, 5.9.1
transaction list, 5.9.1.4
USB driver, 5.9.1.2
USB System, 10.3.2
transfers, 5.0. See also transactions; names of
specific transfer types (i.e., bulk transfers)
bulk transfers, 2.0 glossary, 4.7.2, 5.8:5.8.5, 8.5.1
bus access for transfers, 5.9:5.9.5
bus bandwidth reclamation, 5.9.5
calculating buffer sizes in functions andsoftware, 5.9.4
calculating bus transaction times, 5.9.3
transaction tracking, 5.9.2
transfer management, 5.9.1:5.9.1.5
bus protocol overview, 4.4
bus topology, 5.2:5.2.5
communication flow, 4.1, 5.3:5.3.2.2
control transfers, 4.7.1, 5.5:5.5.5, 8.5.2:8.5.2.4
data prebuffering, 5.10.5
data signaling overview, 7.1.7.2
defined, 2.0 glossary
error detection and recovery, 8.7:8.7.4
Host Controller responsibilities, 4.9, 10.1.3
hub connectivity and, 11.1.2.1
implementer viewpoints, 5.1
interrupt transfers, 2.0 glossary, 4.7.3, 5.7:5.7.5,
8.5.3
isochronous transfers, 2.0 glossary, 4.7.4,
5.6:5.6.5, 5.10:5.10.8, 8.5.4

308transfers (continued)
operations overview, 9.2.4
organization of transactions within frames, 5.9.2
overview, 5.0
period transfers, 5.6.4
power management, 9.2.5
request processing, 9.2.6:9.2.6.5
standard device requests, 9.4:9.4.11
time limits for completing, 9.2.6.4, 9.2.6.5
transaction formats, 8.5:8.5.4
transfer types, 4.7:4.7.5, 5.4:5.8.5
USB device requests, 9.3:9.3.5
USBD role in, 10.1.1, 10.5.3:10.5.3.2.4
USB System role in, 10.3.3
transfer types. See also transactions; transfers;
names of specific transfer types (i.e., bulk
transfers)
allocating USB bandwidth, 4.7.5
bulk transfers, 2.0 glossary, 4.7.2, 5.8
in calculating transaction times, 5.9.3
control transfers, 4.7.1, 5.5
interrupt transfers, 2.0 glossary, 4.7.3, 5.7
isochronous transfers, 2.0 glossary, 4.7.4, 5.6
for message pipes, 5.3.2.2
overview, 4.7:4.7.5, 5.4
pipes and, 4.4
for stream pipes, 5.3.2.1
transfer types defined, 2.0 glossarytransmit phase of signaling, 7.1.1
Transmit state, 11.5, 11.5.1.7
transmitters
Active state, 11.6.2.2
Generate End of Packet Towards Upstream Port
state (GEOPTU), 11.6.2.5
Inactive state, 11.6.2.1
RepeatingSE0 state, 11.6.2.3
SendJ state, 11.6.2.4
Send Resume state (Sresume), 11.6.2.6transmitter data jitter, 7.1.13.1
transmitter sequence bits, 8.6, 8.6.2
transmitter state descriptions, 11.6.2
transmitter state machine, 11.6, 11.6.2
transmitter state machine, 11.6, 11.6.2
truncated packets, 11.3.2
turn-around times
defined, 2.0 glossary
error detection, 8.7.2
overview, 7.1.18
turning power on for ports, 11.11
twisted data pair in cables, 6.6.1
Tx_active signal/event, 11.6.1
Tx_resume signal/event, 11.6.1

U
UEOP signal/event, 11.7
UL listing for cables, 6.6.5
UL STD-94, 6.7.1
UL Subject-444, 6.6.5, 6.7.1
unacceptable cables, 6.4.4

Universal Serial Bus Specification Revision 1.1

underplatingplug contact materials, 6.5.4.3
plug shell materials, 6.5.4.2
receptacle contact materials, 6.5.3.3
receptacle shell materials, 6.5.3.2
Underwriter's Laboratory, Inc., 6.6.5, 6.7.1
The Unicode Standard, Worldwide Character
Encoding, 9.6.5
UNICODE string descriptors, 9.6.5
unique addresses
assigning after dynamic insertion or removal, 4.6.3
device initialization, 10.5.1.1
operations overview, 9.2.2
SetAddress() request, 9.4.6
time limits for completing addressing, 9.2.6.3
Universal Serial Bus
architectural extensions, 4.10
bus protocol, 4.4
clock model, 5.10, 5.10.2
components, 5.1
configuration, 4.6:4.6.3, 10.3.1
data flow and transfers, 4.7:4.7.5, 5.1:5.10.8
description, 4.1:4.1.1.2
feature list, 3.3
goals, 3.1
host hardware and software, 4.9, 10.2:10.6
hubs, 11.1:11.16
mechanical and electrical specifications, 6.1:6.9,
7.1:7.1.19, 7.3:7.3.3
motivation for development, 1.1
physical interface, 4.2:4.2.2
power distribution, 4.3:4.3.2, 7.2:7.2.4.2
protocol layer, 8.1:8.7
range of USB data traffic workloads, 3.2
robustness and error detection/recovery, 4.5:4.5.2
USB device framework, 9.1:9.7.3
USB devices, 4.8:4.8.2.2
USB schedule, 4.1
Universal Serial Bus Driver. See USBD (USB Driver)
Universal Serial Bus Resources, 2.0 glossary
up counters in hub timing, 11.2.1
upgrade paths, 3.3
upstream plugs, 6.2
upstream ports and hubs
defined, 4.8.2.1
full-speed drivers and, 7.1.2.1
full-speed port transceiver, 7.1.7
hub architecture, 11.1.1
hub EOP delay and EOP skew, 7.3.3 Figure 7-43
input capacitance, 7.1.6
jitter, 7.3.2 Table 7-7
low-speed source electrical characteristics, 7.3.2
Table 7-7
pull-up and pull-down resistors, 11.12.1
receivers, 11.6.1:11.6.1.6
reset on upstream port, 11.10.1
signaling delays, 7.1.14
transmitters, 11.6.2:11.6.2.6
upstream connectivity defined, 11.1.2.1
upstream defined, 2.0 glossary
upstream hub delay, 7.3.3 Figure 7-42
USB. See Universal Serial Bus

USB Bus Interface layer
in bus topology, 5.2.2
detailed communication flow illustrated, 5.3
Host Controller implementation, 10.1.1
illustrated, 5.1
interlayer communications model, 10.1.1
USBD (USB Driver). See also USBDI (USB DriverInterface)in bus topology, 5.2.1
command mechanisms, 10.5.1:10.5.2.12
as component of USB System, 10.1.1
configuration and, 10.3.1
control mechanisms, 10.1.2
data transfer mechanisms, 10.1.3
defined, 2.0 glossary, 5.3, 10.5
driver characteristics, 7.1.1
full-and low-speed drivers, 7.1.1.1, 7.1.1.2, 7.1.2.1
HCD interaction with, 10.4
hub drivers, 10.3.1
initialization, 10.5.1.1
overview, 10.5.1
passing preboot control to operating system, 10.5.5
pipe mechanisms, 5.9.1.2, 10.5.1:10.5.3.2.4
request data format mechanisms, 10.3.4
service capabilities, 10.5.1.3
software interface overview, 10.3
in transfer management, 5.9.1, 5.9.1.2
USB System and, 10.5.4:10.5.4.6
USB device framework, 9, 9.7.2
descriptors, 9.5:9.7.3
device class definitions, 9.7:9.7.3
generic USB device operations, 9.2:9.2.7
address assignment, 9.2.2
configuration, 9.2.3
data transfer, 9.2.4
dynamic attachment and removal, 9.2.1
power management, 9.2.5
request error, 9.2.7
request processing, 9.2.6:9.2.6.5
standard descriptor definitions, 9.6:9.6.4
standard device requests, 9.4:9.4.11
USB device requests, 9.3:9.3.5
USB device states, 9.1:9.1.2
USB Device layer
detailed communication flow illustrated, 5.3
illustrated, 5.1
interlayer communications model, 10.1.1
USB devices. See devices
USBDI (USB Driver Interface)adding devices, 10.5.2.5
alternate interface mechanisms, 10.5.2.10
getting descriptors, 10.5.2.3
granting Master Client status, 10.5.3.2.4, 10.5.4.2
removing devices, 10.5.2.6
role in request data format, 10.3.4
sending class commands, 10.5.2.8
sending vendor commands, 10.5.2.9
setting descriptors, 10.5.2.12
software interface overview, 10.3
USB host. See hostUSB host controller. See Host Controller
USB Icon, 6.5, 6.5.1

309

Universal Serial Bus Specification Revision 1.1

USB interconnect model, 4.1, 5.10.4.3
USB Logical Devices. See logical devices
USB Physical Devices. See physical devices
USB schedule, 4.1
USB Specification Release Number, 9.6.1
USB System. See also HCD; host software; USBD
allocating bandwidth, 10.3.2
buffers and, 10.2.9
data transfer role, 10.3.3
HCD component, 10.1.1
Host Controller interaction, 10.1.1
host software component, 10.1.1
power management, 10.5.4.3
remote wakeup, 10.2.7, 10.5.4.6
software interface overview, 10.3
state handling, 10.2.1
status and activity monitoring, 10.1.4
USBD component, 10.1.1
USB System Software
asynchronous data transfers, 4.9
bus enumeration, 4.9
in bus topology, 5.2.1
in communication flow, 5.3
detecting hub and port status changes, 11.13.2
as implementation focus area, 5.1
interrupt transfer support, 5.7.3
isochronous transfer support, 4.9
power management, 4.9role, 4.9

Vvariable-length data stages, 8.5.2.2
variable-sized data payloads, 5.3.2
VBus leads
bypassing, 7.2.4.1
cable electrical characteristics, 7.3.2 Table 7-9
detachable cables, 6.4.1
in electrical specifications overview, 4.2.1
full-speed captive cable assemblies, 6.4.2
low-speed captive cable assemblies, 6.4.3
standardized contact terminating assignments,
6.5.2
upstream port power supply and, 7.2.1
Vendor IDs in device descriptors, 9.6.1
vendor information in device characteristics, 4.8.1
vendor-specific descriptors, 9.5
vendor-specific requests, 10.5.2.9
V/I characteristics of full-speed connections, 7.1.1.1
virtual devices, 2.0 glossary, 5.10.4.3
visible device states, 9.1.1
visual inspection standards, 6.7 Table 6-7
voltageaverage voltage on D+/D- lines, 7.1.2
cross-over voltage in signaling, 7.1.2
droop, 7.2.3, 7.2.4.1
flyback voltage, 7.2.4.2
full-speed connections, 7.1.1.1
open-circuit voltage, 7.1.1
ratings for cables, 6.6.3
reduction due to cable resistive effects, 7.2.3
supply voltage, 7.3.2 Table 7-5
voltage drop budget, 7.2.2

310

voltage (continued)
voltage drops, 7.2.1.1
voltage drop topology, 7.2.2
zero impedance voltage sources, 7.1.1

W
Wait for End of Packet from Upstream Port state(WFEOPFU), 11.7, 11.7.2
Wait for End of Packet (WFEOP) state, 11.7, 11.7.4
Wait for Start of Packet from Upstream Port state(WFSOPFU), 11.7, 11.7.1
Wait for Start of Packet (WFSOP) state, 11.7, 11.7.3
wander, defined, 11.2.2
waveforms
differential data jitter, 7.3.3 Figure 7-39
differential-to-EOP transition skew and EOP width,
7.3.3 Figure 7-40
full-speed driver signal waveforms, 7.1.1.1
hub differential delay, differential jitter, and SOP
distortion, 7.3.3 Figure 7-42
hub EOP delay and EOP skew, 7.3.3 Figure 7-43
maximum input waveforms for signaling, 7.1.1
receiver jitter tolerance, 7.3.3 Figure 7-41
WFEOPFU state, 11.5.1.6, 11.7, 11.7.2
WFEOP state, 11.7, 11.7.4
WFSOPFU state, 11.7, 11.7.1
WFSOP state, 11.7, 11.7.3
wHubChange field, 11.16.2.5
wHubCharacteristics field
hub descriptor, 11.15.2.1
multiple gangs and, 11.11.1
over-current reporting, 11.13.5
power switching settings, 11.11
wHubStatus field, 11.16.2.5
wIndex field
hub class requests, 11.16.2
overview, 9.3.4
Setup data format, 9.3
standard device requests, 9.4
wire gauge in cables, 6.6.2
wire insulation in cables, 6.6.2
wiring assignments for conductors, 6.5.2
wLANGID[] field (string descriptors), 9.6.5
wLength field
hub class requests, 11.16.2
overview, 9.3.5
Setup data format, 9.3
standard device requests, 9.4
wMaxPacketSize fieldbulk transfers and, 5.8.3
control transfer packet size, 5.5.3
endpoint descriptors, 9.6.4
hub descriptors, 11.15.1
interrupt transfer packet size, 5.7.3
variable-length data stages, 8.5.2.2
words, defined, 2.0 glossary
working space, location and length of, 10.3.4
worst case bit stuffing, 5.9.3
wPortChange field, 11.16.2.6, 11.16.2.6.2
wPortStatus field, 11.16.2.6, 11.16.2.6.1
wTotalLength field (configuration descriptors), 9.6.2

Universal Serial Bus Specification Revision 1.1

wValue field
hub class requests, 11.16.2
overview, 9.3.3
Setup data format, 9.3
standard device requests, 9.4

Zzero impedance voltage sources, 7.1.1

311