The SD Express specification represents a quantum leap in possibilities for the integration and use of removable storage in high-performance electronic devices and products. By having performance levels, such as transfer speeds up to 4GB/sec thanks to the use of PCIe® and NVMe® architectures, the SD Association (SDA) is ensuring that full-size SD and microSD form factors will continue to be the leading removable memory cards for consumer, industrial and other professional markets well into the future.
New SD Express Innovations
The SDA recently introduced several innovations and new features, including doubling the microSD Express memory card speed up to 2GB/s, using PCIe Gen4 interface, as defined in the microSD Addendum version 8 specification. In addition, the SD 9.1 specification defines new SD Express Speed Classes over the PCIe/NVMe interface, supporting multi-stream recording along with new associated features that allows optimized Power and Thermal Management control while assuring the minimum assure speed class performance.
New Speed Classes
The consumer market has found the speed classes as useful means for product manufacturers to tell consumers which speed class is required to allow proper operation of their features and at the same time, help consumers identify the right card to use for their device. It also allows host manufacturers to optimize their hosts by using the defined access methods to the card. As NAND technology advanced, the access methods have changed and the last speed class spec defined is the known Video Speed Classes used over the SD UHS-I and UHS-II interfaces.
With the introduction of SD Express cards and its PCIe/NVMe interface, there was a need to adapt the existing Video Speed Classes to operate over NVMe. SD9.1 has introduced new SD Express Speed Classes 150, 300, 450, and 600.
These SD Express Speed Classes were created to meet the requirements of any applicable host device or products that require minimum writing performance using the PCIe/NVMe interface offered by SD Express such as with real-time video recording. The table below describes each SD Express Speed Class and minimum data transfer rate of the corresponding SDXC/SDUC or microSDXC/microSDUC memory cards. The SD Express Speed Class Pictographs indicated will be used on an SD Express memory card, packaging and in a device manual to clearly indicate its Speed Class. Consumers only need to match the host device with the right card to enjoy the best possible experience.
| SD Express Speed Classes | SD Express Speed Class Pictographs | Minimum Read/Write Performance |
| SD Express Speed Class 150 |  | 150 Mbytes/Sec |
| SD Express Speed Class 300 |  | 300 Mbytes/Sec |
| SD Express Speed Class 450 |  | 450 Mbytes/Sec |
| SD Express Speed Class 600 |  | 600 Mbytes/Sec |
Implementing an SD Express 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. There’s also a whitepaper providing more details about the new features and SD Express Speed Classes defined by SD 9.1.
Card vendors producing SD Express memory cards will want to support this specification and communicate the minimum assured performance of an SD Express Class, allowing customers to know which card will best fit their camera, drone, VR, gaming console, etc.
Host vendors producing SD Express interface products who care about having an assured minimum performance level by a card will want to support this specification by meeting the card access methods defined by the standard and then be able to publish what is the minimum required SD Express speed class to assure their product’s operation.
Multi-Stream Recording
The market has seen demand for imaging equipment that supports high-speed video stream recording as well as the ability to aggregate multiple streams from surveillance or in-vehicle cameras, for example, onto one SD memory card.
The ability to record multiple streams gives new options for obtaining the best video output with different settings. Multiple data streams may be recorded if 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.
Maximum Power
Power and Thermal Management are new features of SD9.1 that maintain faster recording speed with hundreds of MB/s. The NVMe specification has a thermal throttling function to protect memory devices from breakdowns caused by heat. To maintain higher recording speeds, especially when speeds exceed 100MB/s, it is essential to suppress temperature increases.
The SD 9.1 specification introduces a new power management feature where the card suggests necessary and sufficient Maximum Power (MP) values according to the targeted SD Express Speed Class and the selected PCIe bus mode. Using this feature, the host device can select an appropriate MP value depending on the card’s capabilities so that the card does not consume excessive power for the targeted recording.
Thermal Management
Power saving and card temperature management are essential to maintaining targeted recording speed, but because individual cards may have unique performance capabilities due to their structure or materials, it is impossible for the host device to know the appropriate values of every card in advance. To resolve this issue, the SD 9.1 specification introduced Thermal Management where the card indicates a group of its specific thermal thresholds, which allows the host device to then set appropriate Thermal Management parameters for the card according to the target class and the selected PCIe bus mode. This feature empowers the host to adequately control the card’s internal temperature to maintain the target recording speed class.
Conclusion
The SDA delivers a powerful combination of new SD Express capabilities. The SDA is continuing to evolve the SD Express features – beside the enhancement of the microSD Express PCIe interface to support up to nearly 2GB/s, the SDA also adopted the Video Speed Class into its PCIe/NVMe interface by adding a new set of SD Express Speed classes allowing assurance of minimum performance levels. SDA is committed to SD Express cards standard evolution and believes it’s the best fit for the next generation of consumer and industrial product.
Yosi Pinto is chairman of the SD Association. Pinto can be reached at Yosi.Pinto@wdc.com.
