Silicon Motion has announced their first SD Card controller to support the NVMe-based SD Express interface. The new SM2708 controller is capable of sequential transfer speeds of 1700 MB/s, vastly higher than the 104 MB/s most SD devices and cards are limited to using the older but widely-supported UHS-I interface.

In 2018, version 7.0 of the SD specification introduced the PCIe and NVMe-based SD Express interface as the new way forward for SD cards. The older UHS-II and UHS-III interfaces developed in versions 4 through 6 of the SD standard and capable of speeds from 156 MB/s to 624 MB/s were abandoned in favor of a single lane of PCIe gen3 (~985 MB/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 up to almost 4 GB/s.

Silicon Motion's SM2708 is a two-lane controller, but still using PCIe gen3 speed, hence the top speed can't quite reach 2 GB/s. This has the potential to bring SD card performance up to near the levels of entry-level consumer NVMe SSDs for laptops and desktops—competitive with SSDs based on slightly-outdated controllers like the Phison E8T or Silicon Motion SM2263XT.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 a 1200 MT/s IO speed that allows it to get good performance out of recent NAND flash generations without the power and size penalties of a four-channel solution.

In 2019, Silicon Motion's primary competitor Phison announced their PS5017 SD Express controller. This is based on the earlier SD 7.0 specification and thus is a PCIe 3 x1 design and limited to about 870 MB/s. In February 2021 Phison announced they were about to start shipping cards based on this solution. Silicon Motion's SM2708 controller might not take that long to turn into actual products, but they clearly have missed out on the first round of SD Express competition—though they may be able to leapfrog Phison's solution.

Underlying all the developments related to recent flash memory card standards has been the challenge of poor adoption. For years, storage tech has been advancing much faster than camera tech. Storage technology companies stand ready to supply more advanced memory cards, but they cannot succeed in the market unless there are host devices ready to use the higher performance. We've seen a decade of failed successors to the old SD and CF standards that now seem pitifully slow. SD's UHS-II and UHS-III, CF's CFast and XQD, and UFS cards have all been demonstrated as working technologies and all eventually made it to market to some extent, but with very limited success. The SD and CF worlds have converged on PCIe and NVMe as the way forward, adopting interfaces that already have a thriving ecosystem and long-term viability in other form factors. That makes it more likely that standards like SD Express will actually catch on, but it may still be several years before PCIe-based interfaces are supported on any phones or more than a handful of high-end cameras.

Source: Silicon Motion



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  • oRAirwolf - Thursday, April 8, 2021 - link

    So is the sd controller inside the sd card or is it on the host device like a laptop or external card reader? Reply
  • ZeDestructor - Thursday, April 8, 2021 - link

    in the SD card Reply
  • oRAirwolf - Saturday, April 10, 2021 - link

    Thanks for the reply. Those must be insanely small compared to a m.2 2280 controller. Reply
  • Oxford Guy - Thursday, April 8, 2021 - link

    Sometimes it's better to hang on to older standards for a longer amount of time. For example, the original compact disc format could have been replaced with a 20-bit audio standard at 48 KHz. That would have been basically enough to fully satisfy the capabilities of human hearing. Additionally, the disc could have been made larger so double LPs could fit completely. Finally, the new discs could have required a protective shell, like DVD-RAM's.

    So, the original flawed standard could have been replaced with a large improvement. But, would that have been best for consumers? The first-generation standard players wouldn't have been able to play the new discs. The biggest issue with rapid standard replacement is that it leads to a lot of fragmentation as well as 'unnecessary' expenditure.

    Personally, though, I would have been very pleased to see the replacement standard I just described bump off the original compact disc — the sooner the better. Making the discs without a protective shell was a particularly bone-headed move, one that was far worse (truly egregious) in terms of using optical discs for commercial video games. Cartridges were far superior for children, versus delicate opticals with no cases.
  • Drkrieger01 - Thursday, April 8, 2021 - link

    Correct me if I'm wrong, but aren't optical storage formats substantially slower than flash? And capacity limited now compared to what can be stored in micro-SD format? We have micro SD cards in the size ranges of 512GB now, IIRC even dual layer Blu-ray can't hit that. Even then, seek time versus flash is no contest. Reply
  • Small Bison - Thursday, April 8, 2021 - link

    SD Express cards can be read by older readers, though, just not at the newer speeds. It's probably more a case of manufacturers going "I don't know how to market this as a whiz-bang feature, so I won't implement it" Reply
  • rpg1966 - Thursday, April 8, 2021 - link

    There's a vanishingly small world of use-cases where more than 16 bits would be required or useful. 96dB is already "more than enough", even more so considering the 20-30dB noise floor in a reasonably-quiet listening environment. Listening to 120dB of dynamic range on top of a 20dB noise floor wouldn't be a great idea. Reply
  • Tomatotech - Thursday, April 8, 2021 - link

    Finally. Buying a SD or CF card has been a minefield of different standards and speeds, as well as a swirling morass of fake cards and fake capacities.

    Looking forward to a MicroSD card around the size of my fingernail, 1mm thick, with 1TB of nvme storage at 2GB/sec for around £/$100.

    Strange to think user-level storage is increasingly becoming a solved problem. Many non-technical people literally have no idea how much storage is available / remaining on their laptops / phones / smartwatches / ipads / cloud accounts. They just know it's 'enough'. Thanks to the wonders of cloud storage and tiered storage and download-on-demand storage.
  • Linustechtips12#6900xt - Thursday, April 8, 2021 - link

    so an SD CARD is better than a sata or a decent pcie 3.0 ssd, WOW Reply
  • DanNeely - Thursday, April 8, 2021 - link

    A non-terrible PCIe3 SSD will crush this all around. It's only got the equivalent of 2 PCIe3 lanes worth of bandwidth.

    In addition, nvme sd card's are dramless without being able to use system memory as cache, and will have a very low number (almost certainly just 1) of flash stacks; which will hurt them badly for random IO. For general use I wouldn't be surprised if that makes a decent SATA SSD faster.

    OTOH the NVME SD Card should be faster for sequential read/write than a SATA SSD. Which is all that really matters, the intended use case for these cards are super high end cameras recording 4k or higher video, or bursts of super high resolution images. (eg sport photography where you need both crazy megapixels so your target looks good after you massively crop because you need to take a wide view to make sure you've got the athlete in the shot and need to take a ton of pictures really fast because you can't time the catch/etc perfectly by hand.)

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