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Z80 PIO USER=S MANUALTABLE OF CONTENTS
Chapter 1. Introduction
Page
1.0
Introduction..........................................
3
1.1
Features..............................................
3
Chapter 2. Architecture
2.0
Overview ............................................

4

Chapter 3. Pin Description
3.0
Pin Description....................................

6

Chapter 4. Programming the PlO
4.0
Reset...................................................
9
4.1
Loading the interrupt Vector................. 9
4.2
Selecting an Operating Mode............... 10
4.3
Setting the Interrupt Control Word........ 11
Chapter 5. Timing
5.0
Output Mode (Mode 0).......................... 12
5.1
Input Mode (Mode 1)..........................… 13
5.2
Bidirectional Mode (Mode 2)...............… 13
5.3
Control Mode (Mode 3)......................... 14
Chapter 6. Interrupt Servicing
6.0Interrupt servicing................................. 16
Chapter 7. Applications
7.0
Extending the Interrupt Daisy Chain....... 18
7.1
I/O Device Interface............................... 18
7.2
Control interface.................................... 19
Chapter 8. Programming Summary
8.0
Load Interrupt Vector............................. 22
8.1
Set Mode............................................... 22
8.2
Set Interrupt Control.............................. 22

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Z80 PIO USER=S MANUAL

List of figures
Figure 2 -1
Figure 2 -2
Figure 3 -1
Figure 3 -2
Figure 5 -1
Figure 5 -2
Figure 5 -3
Figure 5 -4
Figure 6 -1
Figure 6 -2
Figure 6 -3
Figure 7 -1
Figure 7 -2
Figure 7 -3

Page
PIO Block Diagram................................ 5
Port I/O Block Diagram.......................... 5
PIO Pin Functions...............................… 8
PIO 44 - Pin PLCC Pin Arrangements.…
Mode 0 (Output) Timing......................… 12
Mode 1 (Input) Timing.........................… 13
Port A, Mode 2 (Bidirectional) Timing..…
Control Mode (Mode 3) Timing............…
Interrupt Acknowledge Timing.............…16
Return From Interrupt Cycle................…17
Daisy Chain Interrupt Service..............…
Method of extending the Interrupt
Priority Daisy Chain.............................…
Example of I/O Interface......................…19
Control Mode Application.....................… 21

8

14
15

17
18

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CHAPTER 1
1.0

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INTRODUCTIONINTRODUCTION

The Z8O Parallel I/O (PlO) Circuit is a programmable, two port device which provides a TTL
compatible interface between peripheral devices and the Z80-GPU. The CPU can configure
the Z8O-PIO to interface with a wide range of peripheral devices with no other external
logic required, Typical peripheral devices that are fully compatible with the Z80-PIO include
most keyboards, paper tape readers and punches, printers, PROM programmers, etc. The
Z8O-PIO is packaged in a 40-pin DIP, or a 44-pin PLCC, or a 44-pin OFP. NMOS andCMOS versions are also available. Major features of the Z80-PlO include.
One of the unique features of the Z80-PlO that separates it from other interface controllers
is that all data transfer between the peripheral device and the CPU is accomplished under
total interrupt control. The interrupt logic of the PIO permits full usage of the efficientinterrupt capabilities of the Z80-CPU during I/0 transfers. All logic necessary to implement a
fully nested interrupt structure is included in the PIO so that additional circuits are notrequired. Another unique feature of the PlO is that it can be programmed to interrupt the
CPU on the occurrence of specified status conditions in the peripheral device. For example,
the PlO can be programmed to interrupt if any specified peripheral alarm conditions should
occur. This interrupt capability reduces the amount of time that the processor must spend inpolling peripheral status.

1.1

FEATURES•

Two Independent 8-Bit Bidirectional Peripheral interface Ports with >Handshake=
Data Transfer Control.

•Interrupt Driven ‘Handshake’ for Fast Response.

•

Any One of Four Distinct Modes of Operation May be Selected for a Port
including.
Byte Output
Byte Input
Byte Bidirectional Bus (Available on Port A Only)
Bit Control Mode.
All with Interrupt Controlled Handshake•

Daisy Chain Priority Interrupt Logic included to Provide for Automatic interrupt
Vectoring Without External Logic.

•

Eight Outputs are Capable of Driving Darlington Transistors•

All Inputs and Outputs Fully TTL Compatible•

Single 5V Supply and Single Phase Clock are Required.

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Z80 PIO USER=S MANUAL

CHAPTER 2
2.0

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PIO ARCHITECTURE

OVERVIEW

A block diagram of the Z80-PIO is shown in Figure 2-1.The internal structure of the Z8OPIO consists of a Z80-CPU bus interface, internal control logic, Port A I/O logic, Port B I/O
logic and interrupt control logic. The CPU bus interface logic allows the PlO to interface
directly to the Z8O-CPU with no other external Logic. However, address decoders and/orline buffers may be required for large systems. The internal control logic synchronizes the
CPU data bus to the peripheral device interfaces (Port A and Port B). The two I/O ports (Aand B) are virtually identical and are used to interface directly to peripheral devices.
The Port I/O logic is composed of 6 registers with „hand-shake“ control logic as shown in
Figure 2-2. The registers include an 8-bit data input register, an 8-bit data output register, a2-bit mode control register, an 8-bit mask register an 8-bit input/output select register and a
2-bit mask control register.
The 2-bit mode control register is loaded by the CPU to select the desired operating
mode(byte output, byte input, byte bidirectional bus, or bit control mode). All data transfer
between the peripheral device and the CPU is achieved through the data input and data
output registers. Data may be written into the output register by the CPU or read back to
the CPU from the input register at any time. The handshake lines associated with each port
are used to control the data transfer between the PIO and the peripheral device.
The 8-bit mask register and the 8-bit input/output select register are used only in the bit
control mode. In this mode, any of the eight peripheral data or control bus pins can be
programmed to be an input or an output as specified by the select register. The mask
register is used in this mode in conjunction with a special interrupt feature. This featureallows an interrupt to be generated when any or all of the unmasked pins reach a specifiedstate (either High or Low). The 2-bit mask control register specifies the active state desired(High or Low) and if the interrupt should be generated when all unmasked pins are active(AND condition) or when any unmasked pin is active (OR condition). This feature reduces
the requirement for CPU status checking of the peripheral by allowing an interrupt to be
automatically generated on specific peripheral status conditions. For example, in a systemwith three alarm conditions an interrupt may be generated if any one occurs or if all threeoccur.
The interrupt control logic section handles all CPU interrupt protocol for nested priority
interrupt structures. The priority of any device is determined by its physical location in adaisy chain configuration. Two lines are provided in each PlO to form this daisy chain. The
device closest to the CPU has the highest priority. Within a PlO port A interrupts havehigher priority then those of Port B. In the byte input, byte output or bidirectional modes, aninterrupt can be generated whenever a new byte transfer is requested by the peripheral. In
the bit Control mode an interrupt can be generated when the peripheral status matches a
programmed value. The PIO provides for complete control of nested interrupts. That is,lower priority devices may not interrupt higher priority devices that have not had theirinterrupt service routine completed by the CPU. Higher priority devices may interrupt theservicing of lower priority devices.
When an interrupt is accepted by the CPU in Mode 2, the interrupting device must provide
an 8-bit interrupt vector for the CPU. This vector is used to form a pointer to a location in
the computer memory where the address of the interrupt service routine is located. The 8bit vector from the interrupting device forms the least significant eight bits of the indirect

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Z80 PIO USER=S MANUAL

pointer while the I Register in the CPU provides the most significant eight bits of thepointer. Each port (A and B) has an independent interrupt vector. The least significant bit of
the vector is automatically set to a 0 within the PlO since the pointer must point to two
adjacent memory Isolations for a complete 16-bit address.
The PlO decodes the RETI (Return from interrupt) instruction directly from the CPU data
bus so that each PlO in the system knows at all times whether it is being serviced by the
CPU interrupt service routine without any other communication with the CPU.

Input/Output
Select Reg(8 Bits)
Mode
Control
Reg(2 Bits)

Output Enable

Data
OutputReg(8 Bits)

Internal Bus

8 Bit
Peripheral
Data Or
Control Bus

Mask
Control
Reg(2 Bits)

Mask
Reg(8 Bits)

Data
Input
(8 Bits)
Input Data

ReadyInterrupt
Requests

HandshakeControl
Logic

Figure 2-2. Port I/O Block Diagram/StobeHandshakeLines

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CHAPTER 3

Page 6 of 22

PIN DESCRIPTION

3.0 PIN DESCRIPTION
A diagram of the Z8O-PIO pin configuration is shown in Figure 3-1. This section describes thefunction of each pin.
D7 - DO Z80-CPU Data Bus (bidirectional, tri-state) This bus is used to transfer all data andcommands between the Z80-CPU and the Z80-PIO. DO is the least significant bit of the bus.
B/A Sel Port B or A Select (input, active High). This pin defines which port will be accessed during adata transfer between The Z80-CPU and the Z8O-PIO. A Low level on this pin selects Port A while aHigh level selects Port B. Often, Address bit A0 from the CPU will be used for this selection function.
C/D Sel Control or data Select (input, active High). This pin defines the type of data transfer to be
performed between the CPU and the PlO. A High level on this pin during a CPU write to the PIO
causes the Z80 data bus to be interpreted as a command for the port selected by the B/A Select line.
A Low level on this pin means that the Z80 data bus is being used to transfer data between the CPU
and the PlO Often Address bit Al from the CPU will be used for this function.
/CE Chip Enable (input, active Low). A Low level on this pin enables the PIO accept command ordata inputs from the CPU during a write cycle or to transmit data to the CPU during a read cycle. Thissignal is generally a decode of four I/O port numbers that encompass Ports A and B, data, andcontrol.
Φ System Clock (input). The Z80-PIO uses the standard Z80 system clock to synchronize certain
signals internally. This is a single phase clock.
/M1 Machine Cycle One Signal from CPU (input, active Low). This signal from the CPU is used as async pulse to control several internal PIO operations. When /M1 is active and the /RD signal isactive, the Z80-CPU is fetching an instruction from memory. Conversely, when /M1 is active and/IORQ is active, the CPU is acknowledging an interrupt. In addition, the /Ml signal has two otherfunctions within the Z80-PIO.
1.

/M1 synchronizes the PIO interrupt logic.

2.

When /M1 occurs with out an active /RD or /IORQ signal, the PIO logic entersa reset state.

/IORQ Input/Output Request from Z80-CPU (input, active Low). The /IORQ signal is used inconjunction with the B/A Select, C/D Select, ICE, and /RD signals to transfer commands and data
between the Z80-CPU and the Z80-PlO. When /CE, /RD, and /IORQ are active, the port addressedby B/A will transfer data to the CPU (a read operation). Conversely, when /CE and /IORQ are active
but /RD is not active, then the port addressed by B/A will be written into from the CPU with either
data or control information as specified by the C/D Select signal. Also, if /IORQ and /M1 are active
simultaneously, the CPU is acknowledging an interrupt and the interrupting port will automaticallyplace its interrupt vector on the CPU data bus if it is the highest priority device requesting an interrupt.
/RD Reed Cycle Status from the Z80-CPU (input, active Low). It /RD is active a MEMORY READ or
I/O READ operation is in progress. The /RD signal is used with A/B Select, C/D Select, /CE and/IORQ signals to transfer data from the Z80-PIO to the Z80-CPU.
IEl Interrupt Enable In (lnput, active High). This signal is used to form a priority interrupt daisy chain
when more than one interrupt driven device is being used. A High level on this pin indicates that noother device of higher priority are being serviced by a CPU interrupt service routine.
IEO Interrupt Enable Out (Output. active High). The IEO signal is the other signal required to form adaisy chain priority scheme. It is High only if IEI is High and the CPU is not servicing an interrupt
from this PlO. Thus, this signal blocks lower priority devices from interrupting while a higher priority

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device is being serviced by its CPU interrupt service routine.
/INT Interrupt Request (output, open-drain, active Low). When /INT is active, the Z80-PIO isrequesting an interrupt from the Z80-CPU.
A7-AO Port A Bus (bidirectional, tri-state). This 8-bit bus is used to transfer data and/or status orcontrol information between Port A of the Z80-PIO and a peripheral device. A0 is the least significant
bit of the Port A data bus.
/ASTDB Port A Strobe Pulse from Peripheral Device (input, active Low) The meaning of this signal
depends on the mode of operation selected for Port A as follows:1.

Output mode: The positive edge of this strode is issued by the peripheral to acknowledge thereceipt of data made available by the PlO.

2.

Input mode: The strobe is issued by the peripheral to load data from the peripheral into thePort A input register. Data is loaded into the PIO when this signal is active.

3.

Bidirectional mode: When this signal is active data from the Part A output register is gatedonto Port A bidirectional data bus. The positive edge of the strobe acknowledges the receiptof the data.

4.

Control mode: The strobe is inhibited internally.

ARDY Register A Ready (output, active High). The meaning of this signal depends on the mode ofoperation selected for Port A as follows:1.

Output mode: This signal goes active to indicate that the Port A output register has been
loaded and the peripheral data bus is stable and ready for transfer to the peripheral device.

2.

Input mode: This signal is active when the Port input register is empty and is ready to accept
data from the peripheral device.

3.

Bidirectional mode: This signal is active when data is available in the Port A output registerfor transfer to the peripheral device. In this mode, data is slot placed on the Port A data bus
unless ASTB is active.

4.

Control mode: This signal is disabled and forced to a Low state.

B7- B0 Port B Bus (bidirectional, tri-state). This 8-bit bus is used to transfer data and/or status orcontrol information between Port B of the PlO and a peripheral device. The Port B data bus iscapable of supplying 1.5 m @ 1.5V to drive Darlington transistors. B0 is the least significant bit of thebus.
/BSTB Port B Strobe Pulse from Peripheral device (input. active Low). The meaning of this signal issimilar to that at /ASTB with the following exception:
In the Port A bidirectional mode, this signal strobes data from the peripheral device into the Port A
input register.
BRDY Register B Ready (output, active High). The meaning of this signal is similar to that of A
Ready with the following exception:
In the Port A bidirectional mode this signal is High when the Port A input register is empty and readyto accept data from the peripheral device.

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Z80 PIO USER=S MANUAL

20

D3
D4

10

39

9

38
3

D5
D6

8
7

2

D7

Z80-PIO
18

6

Port A/B SEL

16

5Control/Data SEL
PIOControl

13
12

1
40

D2
CPU
Data
Bus

15
14

19

D0
D1

27

4
37

/Chip Enable
/M1

36

/IORQ

11

GND

A5
A7
ARDY
/ASTB
B0
B2

34

B3
B4
B5
B6
B7

25

/INT
Interrupt
Control

21
17

23

BRDY
/BSTB

24
22

INT Enable In
INT Enable Out

Figure 3-1. PIO Pin Functions

6
B//A

7

PA7

8

PA6

9

PA5

10

PA4

11

N/C

12

GND

13

PA3

14

PA2

15

PA1

16

PA0

5

4

3

2

1

44

43

42

41

40

Z80-PIO

17
18

19

20

21

22 23

24

25

26

27

39

/RD

38

PB7

37

PB6

36

PB5

35

PB4

34

PB3

33

PB2

32

PB1

31

PB0

30

+5V

29

CLK

28

Figure 3-2. PIO 44-Pin PLCC Pin Assignments

Port A
I/O

A6

B1

32
33

26

A3
A4

29
31

+5V

A2

28
30

35

/RD

A0
A1

Port B
I/O

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Z80 PIO USER=S MANUALPROGRAMMING THE PIORESETThe Z8O-PIO automatically enters a reset state when power is applied. The reset state
performs the following functions:
1

Both port mask registers are reset to inhibit All port data bits.

2.

Port data bus lines are set to a high-impedance state and the Ready „handshake“signals are inactive (Low) Mode 1 is automatically selected.

3.

The vector address registers are not reset.

4.

Both port interrupt enable flip-flops are reset.

5.

Both port output registers are reset.

In addition to the automatic power-on reset, the PIO can be reset by applying an /M1 signal
without the presence of a /RD or /IORQ signal. If no /RD or /IORQ is detected during /M1,
the PlO will enter the reset state immediately after the /M1 signal goes inactive. The
purpose of this reset is to allow a single external gate to generate a reset without a power
down sequence. This approach was required due to the 40-pin packaging limitation.
Once the PlO has entered the internal reset state, it is held there until the PIO receives acontrol word from the CPU.

4.1

LOADING THE INTERRUPT VECTOR

The PIO has been designed to operate with the Z80-CPU using the Mode 2 interrupt
response. This mode requires that an interrupt vector be supplied by the interrupting
device. This vector is used by the CPU to form the address for the interrupt service routineof that port. This vector is placed on the Z80 data bus during an interrupt acknowledge
cycle by the highest priority device requesting service at that time. (Refer to the Z80-CPU
User’s Manual Section for details on how an interrupt is serviced by the CPU). The desiredinterrupt vector is loaded into the PIO by writing a control word to the desired port of the
PIO with the following format:
D7

D6

D5

D4

D3

D2

D1

D0

V7

V6

V5

V4

V3

V2

V1

0

0 in D0 Signifies This Control Word
is an Interrupt Vector
D0 is used in this case as a flag bit which when low, causes V7 through V1 to be loaded
into the vector register. At interrupt acknowledge time, the vector of the interrupting port willappear on the Z80 data bus exactly as shown in the format above.

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Z80 PIO USER=S MANUALSELECTING AN OPERATING MODE

Port A of the PlO may be operated in any of four distinct modes: Mode 0 (output mode),
Mode 1 (input mode), Mode 2 (bidirectional mode), and Mode 3 (control mode) Note thatthe mode numbers have been selected for mnemonic significance; ie., 0 = Out, 1 = In, 2
Bidirectional. Port B can operate in any of these modes except Mode 2.
The mode of operation most be established by writing a control word to the PIO in thefollowing format:
D7

D6

D5

D4

D3

D2

D1

D0

M1

M0

X

X

1

1

1

1

Mode Word

X=Unused
Bit

Signifies Modes Word To
Be Set

Bits D7 and D0 from the binary code for the desired mode according to the following table:
D7

D6

Mode

0
0
1
1

0
1
0
1

0 (output)
1 (input)
2 (bidirectional)
3 (control)

Bits D5 and D4 are ignored. Bits D3-D0 must be set to 1111 to indicate „Set Mode“.
Selecting Mode 0 enables any data written to the port output register by the CPU to beenabled onto the port data bus. The contents of the output register may be changed at any
time by the CPU simply by writing a new data word to the port. Also the current contents of
the output register may be read back to the Z80-CPU at any time through the execution of
an input instruction.
With Mode 0 active, a data write from the CPU causes the Ready handshake line of thatport to go High to notify the peripheral that data is available. This signal remains High until
a strobe is received from the peripheral. The raising edge of the strobe generates aninterrupt (if it has been enabled) and causes the Ready line to go inactive. This very simple
handshake is similar to that used in many peripheral devices.
Selecting Mode 1 puts the port into the input Mode. To start handshake operation, the CPU
merely performs an input read operation from the port. This activates the Ready line to theperipheral to signify that data should be loaded Into the empty input register. The peripheraldevice then strobes data into the port input register using the strode line. Again, the risingedge of the strobe causes an interrupt request (it has been enabled) and deactivates theReady signal. Data may be strobed into the input register regardless of the state of the
Ready signal if care is taken to prevent a data overrun condition.
Mode 2 is a bidirectional data transfer mode which uses all four handshake lines.
Therefore, only Port A may be used for Mode 2 operation. Mode 2 operation uses the PortA handshake signals for output control and the Port B handshake signals for input control.
Thus, both APDY and BRDY may be active simultaneously. The only operational difference
between Mode 0 and the output portion of Mode 2 is that data from the Port A output
register is allowed on to the port data bus only when /ASTB is active in order to achieve abidirectional capability.

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Z80 PIO USER=S MANUAL

Mode 3 operation is intended for status and control applications and does not utilize thehandshake signals. When Mode 3 is selected, the next control word sent to the PlO must
define which of the port data bus lines are to be inputs and which are outputs. The formatof the control word Is shown below:
D7

D6

D5

D4

D3

D2

D1

D0

I/O7

I/O6

I/O5

I/O4

I/O3

I/O2

I/O1

I/O0

If any Bit is set to a one, then the corresponding data bus line will be used as an input.
Conversely, if the bit is reset, the line will be used as an output.
During Mode 3 operation, the strode signal is ignored and the Ready line is held Low. Data
may be written to a port or read from a port by the Z80-CPU at any time during Mode 3
operation. When reading a port, the data returned to the CPU will be composed of input
data from port data bus lines assigned as inputs plus port output register data from thoselines assigned as outputs.

4.3

SETTING THE INTERRUPT CONTROL WORD

The interrupt control word for each port has the following format
D7

D6

D5

D4

D3

D2

D1

D0

Enable
Int.

AND/
OR

High/
Low

Mask
Follows

0

1

1

1

Used in Mode 3 Only

Signifies Interrupt Control Word

If bit D7 = 1, the interrupt enable flip-flop of the port is set and the port may generate aninterrupt. If bit D7 = 0, the enable flag is reset and interrupts may not be generated If aninterrupt is pending when the enable flag is set, it will then be enabled onto the CPUinterrupt request line, bits D6, D5, and D4 are used only with Mode 3 operation. However,setting bit D4 of the interrupt control word during any mode of operation will cause any
pending interrupt to be reset. These three bits are used to allow for interrupt operation inMode 3 when any group of the I/0 lines go to certain defined states. Bit D6 (AND/OR)
defines the logical operation to be performed in port monitoring. If bit D6 = 1 an AND
function is specified and if D6=0, an OR function is specified. For example if the AND
function is specified, all bits must go to a specified state before an interrupt will be
generated while the OR function will generate an interrupt if any specified bit goes to theactive state.
Bit D5 defines the active polarity of the port data bus line to be monitored. If bit D5 = 1, theport data lines are monitored for a high state while if D6 = 0, they will be monitored for a
low state.
If bit D4 = 1, the next control word sent to the PlO must define a mask as follows:
D7

D6

D5

D4

D3

D2

D1

D0

MB7

MB6

MB5

MB4

MB3

MB2

MB1

MB0

Only those port lines whose mask bit is zero will be monitored for generating an interrupt ishigh, the forced state of Ready will prevent input register data from changing while the CPU
is reading the PIO. Ready will go High again after the trailing edge of the /IORQ as

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previously described.

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CHAPTER 5
5.0

Page 13 of 22

TIMINGOUTPUT MODE (MODE 0)

Figure 5-1 illustrates the timing associated with Mode 0 operation. An output cycle isalways started by the execution of an output instruction by the CPU. A /WR* pulse isgenerated by the PIO during a CPU I/O write operation and is used to latch the datafrom the CPU data bus into the addressed ports (A or B) output register.
The rising edge of the /WR* pulse then raises the Ready flag after the next
falling edge of Φ to indicate that data is available for the peripheral device. In most
systems, the rising edge of the Ready signal can be used as a latching signal in the
peripheral device if desired.
The Ready signal will remain active until: (1) a positive edge is received fromthe strobe line indicating that the peripheral has taken the data, or (2) if alreadyactive, Ready will be forced low one and one half Φ cycles after the leading edge of/IORQ if the ports output register is written into.
Ready will return high on the first falling edge of Φ after the trailing edge of/IORQ. This guarantees that Ready is low when port data is changing.
The Ready signal will not go inactive until a falling edge occurs on the clock(Φ) line. The purpose of delaying the negative transition of the Ready signal until
after a negative clock transition is that it allows for a very simple generation scheme
for the strobe pulse. By merely connecting the Ready line to the strobe line, a strobe
with a duration of one clock period will be generated with no other logic required.
The positive edge of the strode pulse automatically generates an /INT
request if the interrupt enable flip-flop has been set and this device is the highest
priority device requesting an interrupt.
If the PIO is not in a reset state, the output register may be loaded before
Mode 0 is selected. This allows the port output lines to become active in a user
defined state.

/WR*

Port Output(8 Bits)

Ready/Strobe/INT/WR* = /RD . /CE . /C/D . /IORQ

Figure 5-1. Mode 0 (Output) Timing

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5.1

Z80 PIO USER=S MANUAL

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INPUT MODE (MODE 1)

Figure 5-2 illustrates the timing of an input cycle. The peripheral initiates thiscycle using the strobe line after the CPU has performed a data read. A low level on
this line loads data into the port input register and the rising edge of the strobe line
activates the interrupt request line (/INT) if interrupt enable is set and this is thehighest priority requesting device.
The next falling edge of the clock line (Φ) will then reset the Ready line to an
inactive state signifying that the input register is full and further loading must be
inhibited until the CPU reads the data. The CPU will in the course of its interrupt
service routine read the data from the interrupting port. When this occurs, thepositive edge from the CPU /RD signal will raise the Ready line with the next low
going transition of Φ indicating that new data can be loaded info the PIO. If alreadyactive, Ready will be forced low one and one-half Φ periods following the leadingedge of /IORQ during a read of a PIO port. If the user strobes data into the PIO only
when Ready.

/StrobePort Input(8 Bits)

Sample

Ready
/INT
/RD*
/RD* = AD . CE . /C/D . IORQ

Figure 5-2. Mode 1 (Input) Timing

5.2

BIDIRECTIONAL MODE (MODE 2)This mode is merely a combination of Mode 0 and Mode 1 using all four
handshake lines. Since it requires all four lines, it is available only on Port A. When
this mode is used on Port A, Port B must be set to the Bit Control Mode, The sameinterrupt vector will be returned for a Mode 3 interrupt on Port B and an input
transfer interrupt during Mode 2 operation of Port A. Ambiguity is avoided if Port B is
operated in a polled mode and the Port B mask register is set to inhibit all bits.
Figure 5-3 illustrates the timing for this mode. It is almost identical to that

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previously described for Mode 0 and Mode 1 with the Port A handshake lines used
for output control and the Port B lines used for input control. The difference between
the two modes is that, in Mode 2, data is allowed out onto the bus only when the A
strobe is Low. The rising edge of this strobe can be used to latch the data into theperipheral since the data will remain stable until after this edge. The input portion atMode 2 operates identically to Mode 1. Note that both Port A and Port B must havetheir interrupts enabled to achieve an interrupt driven bidirectional transfer.
The peripheral must not gate data onto a port data bus while /ASTB is active.
Bus connection is avoided if the peripheral uses /BSTB to gate input data onto thebus. The PIO uses the /BSTB low level to latch this data. The PlO has been
designed with a zero hold time requirement for the data when latching in this modeso that this simple gating structure can be used by the peripheral. That is, the data
can be disabled from the bus immediately after the strobe rising edge.

/WR

ARDY
/ASTB
Port A
Data Bus/INT

Data Out

Data In
Sample/BSTB
BRDY/WR* = /RD . CE . /C/D . IORQ

Figure 5-3. Port A, Mode 2 (Bidirectional) Timing

5.3

CONTROL MODE (MODE 3)

The control mode does not utilise the handshake signals and a normal port
write or port read can be executed at any time. When writing, the data will belatched into output registers with the same timing as Mode 0. ARDY will be forced
low whenever Port A is operated in Mode 3. BRDY will be held low whenever Port B
is operated in Mode 3 unless Port A is in Mode 2. In the latter case, the state of
BRDY will not be affected.
When reading the PlO, the data returned to the CPU will be composed of
output register data from those port data lines assigned as outputs and input
register data from those port data lines assigned as Inputs. The input register willcontain data which was present immediately prior to the falling edge of /RD. See
Figure 5-4.

ZILOG

Z80 PIO USER=S MANUAL

Page 16 of 22

An interrupt will be generated if interrupts from the port are enabled and thedata on the port data lines satisfies the logical equation defined by the 8-bit mask
and 2-bit mask control registers. Another interrupt will not be generated until achange occurs in the status of the logical equation. A Mode 3 interrupt will be
generated only if the result of a Mode 3 logical operation changes from false to true.
For example, assume that the Mode 3 logical equation is an OR function. An
unmasked port data line becomes active and an interrupt is requested. If a second
unmasked port data line becomes active concurrently with the first, a new interrupt
will not be requested since a change in the result of the Mode 3 logical operation
has not occurred.
If the result of a logical operation becomes true immediately prior to or during/M1 an interrupt will be requested after the trailing edge of /M1.

T1

Port
Data Bus

T2

Data Word 1

Tw*

T3

Data Word 2/INT
Data Match
Occurs Here
/IORQ
/RD

D7-D0

Data In*Timing Diagram Refers to Bit Mode Read.

Data Word 1 Placed On Bus

Figure 5-4. Control Mode (Mode 3) Timing

ZILOGCHAPTER 6
6.0

Page 17 of 22

Z80 PIO USER=S MANUALINTERRUPT SERVICINGINTERRUPT SERVICING

Some time after an interrupt is requested by thin PlO, the CPU will send out an interrupt
acknowledge (/M1 and /IORQ). During this time the interrupt logic of the PIO will determine
the highest priority port which is requesting an interrupt. (This is simply the device with its
Interrupt Enable input high and its Interrupt Enable Output low). To insure that the daisy
chain enable lines stabilize, devices are inhibited from changing their interrupt request
status when /M1 is active. The highest priority device places the contents of its interrupt
vector register onto the Z80 data bus during interrupt acknowledge.
Figure 8-1 illustrates the timing associated with interrupt requests. During /M1 time, no newinterrupt requests can be generated. This gives time for the Int Enable signals to ripple
through up to four PIO circuits. The PIO, With IEI high and lEO Low during /INTA, will place
the 8-bit interrupt vector of the appropriate port on the data bus at this time.
If an interrupt requested by the PlO is acknowledged, the requesting port is under service.
lEO of this port will remain low until a return from interrupt instruction (RETI) is executed
while IEl of the part is high. If an interrupt request is not acknowledged, lEO will be forcedhigh for one /M1 cycle after the PlO decodes the opcode >ED’. This action guarantees thatthe 2-byte RETI instruction is decoded by the proper PlO port
(Figure 6-2).
Figure 6-3 illustrates a typical rested interrupt sequence that could occur with four ports
connected in the daisy chain. In this sequence Port 2A requests and is granted an interrupt.
While this port is being serviced, a higher priority port (1B) requests and is granted aninterrupt. The service routine for the higher priority port is completed and a RETI instruction
is executed to indicate to the port that its routine is complete. At this time the service
routine of the lower priority port is completed.

Last T
State

/INT

/IORQ

T1

T2

Tw*

Tw*

T3

SampleINT/IORQ and /M1 Indicate
Interrupt Acknowledge (/INTA)

/M1
IEO
IEI

Figure 6-1. Interrupt Acknowledge Timing

ZILOG

Page 18 of 22

Z80 PIO USER=S MANUAL
T1

T2

T3

T4

T1

T2

T3

/M1

/RD

D7 - D0

ED

4D

IEI

IEO

Figure 6-2. Return From Interrupt Cycle

T4

T1

ZILOGCHAPTER 7
7.0

Page 19 of 22

Z80 PIO USER=S MANUALAPPLICATIONSEXTENDING THE INTERRUPT DAISY CHAIN

Without any external logic, a maximum of four Z80-PIO devices may be daisy chained intoa priority interrupt structure. This limitation is required so that the interrupt enable status(IEO) ripples through the entire chain between the beginning of /M1 and the beginning of/IORQ during an interrupt acknowledge cycle. Since the interrupt enable status cannotchange during /M1, the vector address returned to the CPU is assured to be from thehighest priority device which requested an interrupt.
If more than four PlO devices must be accommodated, a „Iook-ahead“ structure may be
used as shown in Figure 7-1. With this technique, more than thirty PlO’s may be chained
together using standard TTL logic.

+5V

PIO

PIO

PIO

PIO

PIO

PIO

PIO

PIO

IEI IEO

IEI IEO

IEI IEO

IEI IEO

IEI IEO

IEI IEO

IEI IEO

IEI IEO

Z80CPU

Data Bus

Figure 7-1. A Method of Extending the Interrupt Priority Daisy Chain

7.1

I/O DEVICE INTERFACEIn this example the Z80-PIO is connected to an I/O terminal device which communicatesover an 8-bit parallel bidirectional data bus as illustrated in Figure 7-2. Mode 2 operation(bidirectional) is selected by sending the following control word to Port A:
D7

D6

D5

D4

D3

D2

D1

D0

1

0

X

X

1

1

1

1

Mode Control

Next, the proper interrupt vector is loaded (refer to CPU Manual for details on the operationof the interrupt).

ZILOG

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Z80 PIO USER=S MANUAL
D7

D6

D5

D4

D3

D2

D1

D0

V7

V6

V5

V4

V3

V2

V1

0

Interrupts are then enabled by the rising edge of the first /M1 after the interrupt mode word
is set unless that /M1 defines an interrupt acknowledge cycle. If a mask follows the interrupt
mode word, interrupts are enabled by the rising edge of the first /M1 following the setting of
the mask.
Data can now be transferred between the peripheral and the CPU. The timing for thistransfer is as described is Section 5.0.
ARDY
/ASTB
BRDY
/BSTB

D
S
T
B

Data Bus

Z80CPU

Z80PIO/IORQ/M1Port Data Bus

D
R
Q

D
R
C
V

D
A
V

/INT
C/D

B/A

I/O
Terminal/CE

Address
Bus
Decoder

Address Bus

Figure 7-2. Example of I/O Interface

7.2

CONTROL INTERFACE

A typical control mode application is illustrated in Figure 7-3. Suppose an industrial process
is to be monitored. The occurrence of any abnormal operating condition is to be reported toa Z80-CPU based control system. The process control and status word has the following
format:
D7

D6

D5

D4

D3

D2

D1

D0

Special
Test

Turn
on
Power

Power
FailureAlarm

Halt
Process
ing

Temp
Alarm

Turn
Htrs
On

Pressureon
SystemPressureAlarm

The PIO may be used as follows. First Port A is set for Mode 3 operation by writing thefollowing control word to Port A.
D7

D6

D5

D4

D3

D2

D1

D0

1

1

X

X

1

1

1

1

ZILOG

Page 21 of 22

Z80 PIO USER=S MANUAL

Whenever Mode 3 is selected, the next control word sent to the port must be an I/O selectword. In this example we wish to select port data lines A5, A3. and A0 as inputs and so thefollowing control word is written:
D7

D6

D5

D4

D3

D2

D1

D0

0

0

1

0

1

0

0

1

Next the desired interrupt vector must be Loaded (refer to the CPU manual for details):
D7

D6

D5

D4

D3

D2

D1

D0

V7

V6

V5

V4

V3

V2

V1

0

An interrupt control word is next sent to the port:
D7

D6

D5

D4

D3

D2

D1

D0

1

0

1

0

0

1

1

1

EnableInterrupts

OR
Logic

ActiveHigh

Mask
Follows

Interrupt Control

The mask word following the interrupt mode word is:
D7

D6

D5

D4

D3

D2

D1

D0

1

1

0

1

0

1

1

0

Now, if a sensor puts a high level on line A5, A3 or A0, an interrupt request will be
generated. The mask word may select any combination of inputs or outputs to cause aninterrupt. For example, it the mask word above had been:
D7

D6

D5

D4

D3

D2

D1

D0

0

1

0

1

0

1

1

0

then an interrupt request would also occur if bit A7 (Special Test) of the output register wasset.
Assume that the following port assignments are to be used.
E0H = Port A Data
E1H = Port B Data
E2H = Port A Control
E3H = Port B Control
All port numbers are in hexadecimal notation. This particular assignment of port numbers isconvenient since A0 of the address bus can be used as the Port A/B Select and A1 of theaddress bus can be used as the Control/Data Select. The Chip Enable would be thedecode of CPU address bits A7 through A2 (111000). Note that if only a few peripheraldevices are being used, a Chip Enable decode may not be required since a higher orderaddress bit could be used directly.

ZILOGCHAPTER 8
8.0

PROGRAMMING SUMMARY

LOAD INTERRUPT VECTOR
V7

8.1

Page 22 of 22

Z80 PIO USER=S MANUAL

V6

V5

V4

V3

V2

V1

0

M1 M0

X

X

1

1

1

1

SET MODE

M1

M0

Mode

0
0
1
1

0
1
0
1

OutputInput
Bidirectional
Bit Control

When selecting Mode 3, the next word must set the =I/O Register:

8.2

SET INTERRUPT CONTROL
Enable
Int

AND/
OR

High/
Low

Mask
Follows

0

1

1

1

Used inMode 3 Only

If the "mask follows" bit is high, the next control word written to the port must be the
mask:
MB7

MB6

MB5

MB4

MB3

MB2

MB1

MB0

MB = 0, Monitor bit
MB = 1, Mask bit from being monitored
Also, the interrupt enable flip-flop of a port may be set or reset without modifying therest of the interrupt control word by using the following command:
Enable
Int.

X

X

X

0

0

1

1