SD Express Speed Class –As introduced in SD 9.1 Specification

White Paper | October 2023

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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Telephone: +1 (925) 275-6615

The information contained in this whitepaper is provided
as is without any representations or warranties of any kind.
No responsibility is assumed by the SD Association for any
damages, or any infringements of patents or other rights of the
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to any third party.

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E-mail: help@sdcard.org

Specifications are subject to change without notice. Nonmetricweights and measurements are approximate. All data weredeemed correct at the time of creation. SD Association is notliable for errors or omissions. All brands, product, service names
and logos are trademarks and/or registered trademarks of theirrespective owners and are hereby recognized and acknowledged.

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PCI Express® is a registered trademark of PCI-SIG®.
NVM ExpressTM and NVMeTM are trademarks of NVM Express, Inc.

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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Executive Summary
The release of the SD Express card specifications (SD7.x and SD8.0) was a major evolutionary step for the
SD memory card standard, introducing a massive increase in its sequential performance capabilities.
Full-size SD and microSD form factors are the leading removable memory cards in the consumer andindustrial markets.
In addition to the technical specifications developed by the SD Association (SDA), the organization always
encourages adding features and markings to provide proper communication between application-hostvendors and their consumers who purchase cards in stores, ensuring the full functionality of the features
provided by host vendors.
An excellent example of feature marks are the Speed Classes introduced by SDA years ago, allowing
various video equipment vendors to inform consumers which card should be used to ensure proper
recording and flawless playback. Continuing this methodology, after introducing SD Express, it wasdetermined that it would be helpful to introduce the SD Express Speed Class to define performancelevels--features introduced in the SD9.1 specification.
This specification defines the access rules required to ensure the minimum defined performance of the
PCIeTM and NVMeTM interface, including multi-stream recording of up to eight streams.
Also, it introduces two features for Maximum Power and Thermal Management to ensure cards and hostdevices perform at their best while ensuring minimum access performance.
This whitepaper explains what SD Express memory cards are, what speed class is, why it is necessary, andthe speeds defined for SD Express Speed Class marks.

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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Introduction
What is an SD Express Card?
SD Express memory cards are the newest generation of SD memory cards supporting the PCIe andNVMe interface beside the legacy SD UHS-I interface on the same card.
The first SD Express cards were introduced with SD7.0 specifications for the full-size SD form factor
supporting PCIe Gen 3 x1 lane interface. SD7.1 added the same capability to the microSD form factor.
Then, SD8.0 introduced additional PCIe interfaces – PCIe Gen 4 x1, PCIe G3 x2 and PCIe Gen4 x2.
Throughout this whitepaper we will use the words “SD Express memory cards” for both form factors.

SD Memory Cards
Sequential Performance
3938B/s

SD Express – PCIe G4 x2 (SD8)
SD Express – PCIe G3 x2 (SD8)
microSD Express and SD Express G4 x1 (SD8)~x40

1969MB/s
985MB/s

SD Express & microSD Express – PCIe G3 x1 (SD7)~x20

SD UHS-III

~x10

PCIe
G3x1
G4x1

SD UHS-II

PCIe G4x1
PCIe G3x1

PCIe G3x2
PCIe G4x2

Bandwidth (MB/s)

SD UHS-I
0

500

1000

1500

2000

2500

3000

3500

4000

Figure 2-1 SD card speeds - from UHS-I to SD Express

SD Express is the most significant evolution for SD since it was introduced. This new generation ofcards answers new and evolving market needs to support increased performance requirements ofcontrollers, memories, and other application interfaces. SD Express can fully support almost any use
case demanding higher speed removable or semi-removable memory cards using the widely adopted
SSD technology.
The SD Express specifications give host devices flexibility, allowing the operation of the card throughthe PCIe interface or the SD interface, delivering backward compatibility with existing UHS-I hosts.
The PCIe interface may be accessed and operated directly from any PCIe / NVMe host using standard
PCIe/NVMe drivers because the SD Express card is introduced to the host as a standard NVMe memory
device. Implementing an SD Interface requires a hardware update of the host SD interface. SDA has
provided several whitepapers explaining how easy it is to add SD Express to virtually any host device.

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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What is Speed Class? Why do we need it? Who should care about this?
There are various applications
Products with Removable orConsumersin the market using SD memory
Semi-removable memory card
cards. The first group of• Consumers who own these products expect
applications saves offline content,
the promised features
Products may require
like pictures and documents.
minimum assured• Product manufacturers may specify what
Another group uses cards formemory accessreal-time recording or reading of
performance tofeaturesensure proper
content, such as video recorders/• While most important for continuous video
support of expected
players and gaming devices,
recording, it can also be important for otherfunctional operation
to name a few. The first typeapplications like – multi-stream recording,of application usually does not
VR Headset or Gaming and more.
require any assured performance;
however, the second requires
minimum assured performance
Figure 2 -2 Why is Speed Class important?
to ensure proper operation andminimum sequential performance levels for SD Express
playback. The SDA uses Speed Class levels to define amemory cards when a host device accesses the card throughcommon language between host device manufacturers,
the PCI/NVMe interface. Importantly, these classes support
card manufacturers, and consumers. Manufacturers markmulti-steam recording as defined by NVMe version 1.4.
those levels on the card by noting the guaranteed minimum
sequential performance supported by the card under predefined conditions.
As a continuation of the existing Video Speed Class for the SD
interface, a new set of SD Express Speed Classes was created
for the PCIe/NVMe interface. These speed classes defineThis specification involves various aspects of the card’s
characteristics, host device characteristics and the host’s card
access drivers. Anyone involved in these areas who plans oncomplying with this specification will want to understand thisspecification.

• RD/WR Block sizes and recording periods• Stream and Queue handling• Suspend/Resume conditions• Power management conditions• Thermal management• Various measurement conditions• ...

Figure 2 -3 SD Express Speed Classes Vendor Impact

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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SD Express Speed Class Goals and Implementation Examples
The SDA created several Speed Class specifications for realtime stream recording. These include Speed Class, UHS Speed
Class, and Video Speed Class. The SDA also standardized SD
Express card with PCIe bus and operated by NVMe protocol
for several GB/s data transmission.
Imaging equipment is required to support high-speed
stream recording exceeding 100MB/s for 4K/8K intra videos*or RAW contents. These requirements are often found in
professional camcorders or high-end digital single-lensreflex cameras (DSLR) that need to record video streams
with several hundreds of MB/s (Figure 3-1 (a)). Moreover,there is a demand to aggregate multiple streams captured
by surveillance or in-vehicle cameras onto one SD memory
card, considering data portability and/or captured data
management (Figure 3-1 (b)).

The current Video Speed Class specification guarantees
recording just one data stream and is limited to a minimumof rate. For example, V90 ensures a minimum of 90MB/s inreal-time recording. The SD Express Speed class defines a few
minimum assured performance levels using the PCIe/NVMe
interface, including multi-stream recording.
Real-time recording for high resolution and noncompressed video and/or multiple stream recording onto
one card interface are examples supporting the needfor SD Express Speed Class. It ensures specific minimum
performance levels when using the PCIe/NVMe interface of
SD Express memory cards.

Figure 3-1 SD Express Speed Class Application Examples(*) Intra video, is a compression technique performed relative to information contained only within the current
frame, and not relative to any other frame in the video sequence. Therefore, it is usually larger in size.

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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SD Express Speed Classes - Overview
SD Express Speed Classes 150, 300, 450, and 600 are the
newly defined classes. For example, SD Express Speed Class
600 means stream recording speed at a minimum of 600MB/s
is guaranteed when both the card and host device use the
PCIe bus and conform to the SD Express Speed Class accessrules defined in the specification. In addition, up to eightstreams can be recorded simultaneously. Multiple data
streams may be recorded as long as the sum of the recording
speed for these streams is within the supported speed class.
In addition, sequential performance is guaranteed even when
write and read operations are mixed. Figure 4-1 illustrates
examples of bandwidth allocation when a card supports
Class 600:(a) 8 writing streams at 75MB/s each(b) 1 writing stream at 530MB/s, and 7 writing streams at
10MB/s each(c) 2 writing streams at 100MB/s, 2 writing streams at 50MB/s,
2 reading streams at 100MB/s, and 2 reading streams at
50MB/s for each.

Similar to all prior Speed Classes, if the card supports a
specific SD Express Speed Class, it must also support all lower
classes. Therefore, when the card supports SD Express Speed
Class 600, it needs to also support SD Express Speed Classes
450, 300, and 150. Moreover, cards with SD Express Speed
Class must perform at the minimum speed for all PCIe bus
modes defined in any SD Express specification. For example,
if a card has PCIe Gen4 x1 mode and supports Class 300, thecard must also perform equally when used in PCIe Gen3 x1
mode, which is mandatory for all SD Express memory cards.
The SDA created the following SD Express Speed Classes tomeet the requirements of any SDXC or SDUC Host Device/Ancillary Products that require minimum writing performanceusing the PCIe/NVMe interface of SD Express for real-timevideo recording, to name one use. The SD Express Speed
Class Pictographs will help users identify the performancecapabilities of the SDXC/SDUC Memory Cards offering the SD
Express Bus and the performance requirements of SDXC/SDUC
Host/Ancillary Products equipped with an SD Express Bus.
SD Express Speed Classes(a)

SD Express Speed
Class Pictographs

SD Express Speed Class 150
SD Express Speed Class 300

Minimum Read/Write
Performance
150 Mbytes/Sec!

300 Mbytes/Sec

SD Express Speed Class 450

450 Mbytes/Sec

SD Express Speed Class 600

600 Mbytes/Sec

Table 4-1 SD Express Speed Classes marks and their related minimum assured performance(b)

Table 4-1 describes each SD Express Speed Class andminimum data transfer rate of the corresponding
SDXC/SDUC Memory Cards. The SD Express Speed Class
Pictographs indicated on a card or packaging or in a manual
shall be identical to the SD Express Speed Class Identifier stored
in the Identify Controller Data Structure of the SD Express card.

(c)Figure 4-1 Examples of Stream Bandwidth Allocationin SD Express Speed Class

7The SD Express Speed Class specification is based on the NVMe
v1.3 or later specification. This maintains compatibility withgeneral SD Express host devices, while allowing some vendorspecific features supporting SD Express Speed Class features.
SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
www.sdcard.org | ©2023 SD Association. All rights reservedNew Features for SD Express Speed Class
Thermal Management

The new features found in the SD Express Speed Class
specification 9.1 are:• Power Management• Thermal Management• Access Rule for Multi-stream Recording.
Power and Thermal Management features are introduced
to maintain faster recording speed with hundreds of MB/s.
The Access Rule for Multi-stream Recording feature ensuresefficient memory use when performing multi-stream recording.

Power Management
The NVMe specification has a thermal throttling function toprotect memory devices from breakdowns caused by heat.
This function allows the device to reduce the access speed
to the memory when the internal temperature reaches
a defined threshold. Therefore, it is essential to suppress
temperature increases in the card to maintain higher
recording speeds, especially when speeds exceed 100MB/s.

To resolve this issue, the SD 9.1 specification introduced
Thermal Management where the card indicates a group of
its specific thermal thresholds. The host device may then set
appropriate Thermal Management parameters for the card
according to the target class and the selected PCIe bus mode,much like an MP value for power management.

An SD Express card indicates several Maximum Power (MP)values supported by the card. The card consumes power up
to one of the MP values set by the host device. When thehost device initializes the card, it detects MP values from thecard and selects the largest MP value, allowing it to reach its
best performance.
Sometimes, a card’s best performance is not always required
for every action; therefore it is unnecessary to always specify
the largest MP value. The SD 9.1 specification also introduces a
new power management feature. The card suggests necessary
and sufficient MP values according to the targeted SD Express
Speed Class and the selected PCIe bus mode. Using thisfeature, the host device can select an appropriate MP value
depending on the card’s capabilities so that the card does notconsume excessive power for the targeted recording.

8Power saving and card temperature management are
essential to maintain targeted recording speed. There are
two internal card temperature thresholds related to thethermal throttling set by the host device. The first thresholdis for activating a light throttling, and the second is for aheavy one. The second threshold is critical for the speed classrecording because the card cannot continue to record due toheavy throttling. In addition, the first threshold is effective to
reduce temperatures through light throttling, not to exceed
the second threshold. These thresholds are key parameters
for controlling the card temperature to maintain thetarget recording speed. Since every card may offer uniqueperformance capabilities based on its structure or materials,
it is impossible for the host device to know the appropriate
values of every card in advance.

With this feature, the host can adequately control card’s
internal temperature to maintain the target recording speed
for each connected card.

SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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Access Rule for Multi-stream Recording
In order to minimize the overhead of programmingtime to NAND flash memory, conventional Speed Class
specifications regulate the stream recording rule thatstream data shall be written continuously from the starting
address of a specific memory unit that is fully vacant. This
is now called an SGS unit by SD Express Speed Class. Notethat SGS stands for Stream Granularity Size and is originallydefined in the NVMe specification.
For multi-stream recording, different stream data arerecognized by the Stream Identifier (SID). Considering theactual usage of a host device with multiple cameras, thedevice usually transmits a block of stream data individuallyregardless of the SID. In this case, programmed data in the
SGS unit is multiplexed in terms of the SID, and such an
SGS unit cannot be used for stream recording even if oneof the stream data is deleted, but this is not optimum forachieving efficient memory use.
SGS unit A

To resolve this issue, the SD 9.1 specification defines an accessrule that each SGS unit can be occupied by stream datawith only one SID. As shown in Figure 5-1, stream data with
SID=1 and SID=2 are written in SGS unit A and B respectively,
regardless of card’s receiving order. After deleting stream datawith SID=2, SGS unit B can be reused for recording upcoming
stream data because it is fully vacant. This rule ensures efficient
memory usage when the host device repeats recording anddeleting video streams in a card.
In addition, the Current Address Saving feature found inVideo Speed Class is also available in this specification. It
enables host resume recording from the middle of an SGS
unit for its effective use, even after a power cycle.

SGS unit B

1 1 1 1 1 1 2 2 2 2 2 2

Stream Identifier (SID)
Stream data with SID=1 and SID=2 arerecorded in SGS unit A and B respectively.

SGS unit A
Stream data with SID=2
are deleted.

SGS unit B

1 1 1 1 1 1

Stream Identifier (SID)
SGS unit B can be reused forstoring upcoming stream data.

Figure 5-1 Accessing Rule for Multi-Stream Recording

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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Summary
Any host/ancillary device planning to implement SD Express memory cards and utilize its
high-speed sequential access for applications that require a minimum assured read/write
performance level – the SD Express Speed Classes are important specifications to adopt.
Accessing the card using the specified access rules and handling the thermal and power
management wisely as the specification defines to meet the SD Express Speed Class in an
optimized manner.
Card vendors producing SD Express memory cards will want to support this specificationand communicate the minimum assured performance of an SD Express Class, allowing
consumers to know which card will best fit their product, whether that is a camera,drone, VR, gaming console, etc.
The SD Express Speed Class features defined in the SD 9.1 specification, is supported by
an updated SD Logo Guideline, providing all the required information necessary for any
company to support it.

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SD Express Speed Class – As introduced in SD 9.1 Specification White Paper
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