Silicon Motion announces SM2708 SD Express controller


Silicon Motion has announced its first SD card controller supporting NVMe-based SD Express interface. The new SM2708 controller is capable of sequential transfer speeds of 1700MB / s, well above 104MB / s, most devices and SD cards are limited to using the older but widely used UHS-I interface in charge.

In 2018, version 7.0 of the SD specification introduced the SD Express interface based on PCIe and NVMe as a new way forward for SD cards. The old UHS-II and UHS-III interfaces developed in versions 4 to 6 of the SD standard and capable of speeds of 156MB / s to 624MB / s have been abandoned in favor of a single lane of PCIe gen3 (~ 985MB / s). Last year, version 8.0 of the SD specification added support for PCIe gen4 speeds and a second PCIe lane, bringing the theoretical maximum transfer speed to nearly 4 GB / s.

Silicon Motion’s SM2708 is a two-lane controller, but still using PCIe gen3 speed, therefore, the maximum speed cannot quite reach 2Gb / s. This has the potential to bring SD card performance close to the levels of entry-level mainstream NVMe SSDs for laptops and desktops, competing with slightly outdated controller-based SSDs like the Phison E8T. or the Silicon Motion SM2263XT. The SM2708 controller uses two NAND channels instead of the four typically used by entry-level SSD controllers, but the SM2708 is capable of 1200 MT / s I / O speed which allows it to achieve good performance of the latest generations of NAND flash without the power and size penalties of a four-channel solution.

In 2019, Silicon Motion’s main competitor, Phison, announced its PS5017 SD Express Controller. This is based on the earlier SD 7.0 specification and therefore is a PCIe 3 x1 design and limited to around 870MB / s. In February 2021, Phison announced that she was about to start shipping cards based on this solution. Silicon Motion’s SM2708 controller might not take that long to transform into actual products, but they clearly missed the first round of the SD Express competition – although they may be able to skip Phison’s solution.

The challenge of poor adoption is at the root of all developments related to recent flash memory card standards. For years, storage technology has evolved much faster than camera technology. Storage technology companies are ready to deliver more advanced memory cards, but they can only be successful in the market if host devices are ready to use the best performance. We’ve seen a decade of failed successors to the old SD and CF standards that now seem woefully slow. UHS-II and UHS-III cards from SD, CFast and XQD and UFS from CF have all been demonstrated to be working technologies and all have eventually been commercialized to some extent, but with very limited success. The SD and CF worlds have converged on PCIe and NVMe as the way forward, embracing interfaces that already have a thriving ecosystem and long-term viability in other form factors. This makes it more likely that standards like SD Express will spread, but it may still be several years before PCIe interfaces are supported on all phones or more than a handful of high-end cameras.



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