What is the SD Express?

The SD Express incorporates the PCIe^® and NVMe™ interfaces into SD memory cards alongside legacy SD interface. In anticipation of the next generation of high-performance mobile computing requirements, adding PCIe technology and the advanced NVMe protocol delivers faster access to data files than today’s SD platform, achieving speeds of up to 985 MB/s. An upper-layer protocol allows advanced memory access mechanism, enabling a new world of opportunities for the popular SD memory card. This new protocol allows SD Express memory cards to serve as removable Solid State Drives (SSD). A short video released by SDA describes shortly the features of SD Express.

These speeds are essential for high-resolution content applications such as super-slow-motion video, RAW continuous-burst-mode photography, 8K video capture and playback, 360-degree video, speed-hungry applications running on cards and mobile computing devices, ever-evolving gaming systems, multi-channel IoT devices and numerous automotive storage needs, to name a few.

In addition to the many performance advancements, SD Express maintains the long-standing advantage of SD’s backward compatibility and interoperability. The PCIe interface was added alongside the existing SD (UHS-I) interface, allowing a card with SD Express to operate interchangeably in new SD Express capable hosts as well as in billions of existing SD host products in the market today.

SD Express, part of the SD 7.0 specification, defines a full-size form factor using two power supplies: a traditional 3.3 volt and 1.8 volt. SD Express also allows an optional 1.2v supply for future form factors that would require additional pin #18. The future optional 1.2 volt will allow additional power/performance optimization. Cards using SD Express may consume up to maximum of 1.80 Watts from the host, representing the accumulated wattage from the two power supplies.

The figure below shows the bit rate performance levels of SD Express:

SDA’s white paper, SD Express Cards with PCIe^® and NVMe™ Interfaces, offers more technical details, along with some of the benefits that card and system manufacturers may get by SD Express adoption. The following SD protocol features are supported partially by the PCIe/NVMe interface, assuming use of NVMe version 1.3. In the future, NVMe may fully support these features:

• Password Lock/Unlock: The card may be password locked/unlocked only through the legacy SD interface. A locked card does not allow access to the memory, either through the SD or through the PCIe/NVMe interface.
• Write Protect features: The card may be set to write-protected mode through the legacy SD interface. A write-protected card does not allow any write operation to it, either through its SD interface or through the PCIe/NVMe interface.

The following traditional SD features are not supported through the PCIe interface:

• SD CPRM Security: If the SD Express host access card includes CPRM encrypted files through the PCIe interface, it will read them as encrypted data, yielding the same result as inserting the card to any SD host that does not support CPRM.
• Speed Class, UHS Speed Class and Video Speed Class are not supported through the PCIe interface.

By relying on successful protocols already in the marketplace, the industry can leverage existing PCIe and NVMe designed blocks as well as existing test equipment, saving on development processes. When companies use existing building blocks of existing designs, this translates to cost savings as well as an improved bottom line.

SD Express is eventually combining the best and most popular protocols for removable memory devices into a single, reliable SD card form factor, as shown in the below figure:

With the existing support of major operating systems and the popularity of drivers supporting PCIe and NVMe, the market adoption of SD Express should be seamless.

With the SD 7.0 specification, the SDA extended the limits of traditional SD memory cards’ capacity to meet the ever-growing needs of users everywhere. In addition to SD Express, the SDA also created SD Ultra Capacity (SDUC) to extend the potential maximum SD memory card capacity from 2TB to up to 128TB. SD Express may be implemented on SDHC, SDXC or SDUC memory cards.

The Takeaway

While the sequential and random performance of the existing SD interface may be good enough for most of today’s uses, new applications will require ever-higher performance levels – which may be achieved with the newly released SD Express, using its added higher performance PCIe and NVMe interfaces.

With SD memory cards used for higher capacity as well as for speed-intensive and/or multi-channel applications, there is a growing need for a big jump in random and sequential performance levels as well as more modern protocols. The SD Express feature demonstrates new opportunities for SD memory cards and opens a whole new world of uses while keeping backward compatibility to billions of existing SD hosts, creating benefits for both consumer and industrial users.

Yosi Pinto is Director of standards in the CTO office at Western Digital (former SanDisk) and chairman of the board at SD Association. Pinto can be reached at Yosi.Pinto@wdcsandisk.com.

PCI Express^® is a registered trademark of PCI-SIG^®.

NVM Express™ and NVMe™ are trademarks of NVM Express, Inc.

¹ Source: ABI Research Edge Analysis in IoT, 2017, as presented by Chris Bergey in Computex 2018

© SD Association. All rights reserved. SD, SDHC, miniSDHC, microSDHC, SDXC and microSDXC Logos are trademarks of SD-3C LLC

A SHORT VIDEO TO INTRODUCE THE CAPABILITIES