The SM2708 controller serves hot and fast

Flash-based portable media, such as SD cards, has a variety of uses in products, from content recorders to portable game consoles. Many computer systems (including computers and smartphones) also use them to increase available storage capacity. There are two main standards bodies in this segment – the SD Association (SDA) and the CompactFlash Association (CFA), with the SDA being responsible for Secure Digital cards (SD / microSD) and the CFA having the CompactFlash and CFast card markets.

The products currently serving this market segment are based on technology standards developed when SATA was the peak of internal memory performance, but in fact both standards bodies jumped to NVMe in late 2010. Replaceable memory cards based on PCIe / NVMe are slowly appearing on the market – CFexpress from CFA and SD Express from SDA. Both ADATA and Lexar have announced plans to launch their SD Express cards in the next few quarters. Cards from both manufacturers are based on the Silicon Motion SM2708 controller.

Silicon Motion sampled our reference design SM2708 (with a capacity of 250 GB) to go through our rigorous memory card evaluation package. This review discusses card performance in detail along with Realtek’s RTL9211DS card reader reference design. It also serves as an overview of what consumers can expect from SD Express cards that will hit the market in the next few quarters.

Let’s go: The rise of PCIe and NVMe

SD cards and CompactFlash cards have emerged as storage media for selecting content recording devices, such as digital cameras and camcorders. SD cards have also enjoyed support in the portable game console and as driver units for single-board computers. Currently popular SD and CF cards are based on standards developed when SATA ruled roost as the selection protocol for internal storage devices.

As PCIe-based NVMe stormed the SSD storage market, both SDA and CFA introduced PCIe-based interchangeable card standards. CFA actually jumped on PCIe quite early, with the 1 Gbps standard in 2011 – the XQD card format. However, the lack of backward compatibility with existing CompactFlash readers meant that the format never succeeded, even though it went into retail. While the original CF cards were based on PATA (predecessors of SATA), a faster version based on SATA was introduced in 2009 as CFast. The CFA announced plans for the CFexpress standard in 2016, and it was released in mid-2017. CFexpress cards retain the XQD form factor, and several CFexpress cards are already on the market.

SD cards are also popular in several market segments. To meet the needs of all serviced markets, the SD Association introduced the SD Express standard based on NVMe (SD 7.0) in 2018, with an upgrade to SD 8.0 2020. SD cards as well as card readers based on these new standards are touring various trade shows from 2019. However, none appeared on the retail market. That will change in the coming months, and ADATA and Lexar are announcing plans to launch their SD Express cards based on the Silicon Motion SM2708 card controller in the next few quarters.

What we are testing today: SD 7.1 reference design

Consumers also need SD Express cards and card readers to take full advantage of PCIe / NVMe in SD format. While SD Express cards are enabled by controllers like the SM2708, card reader platforms based on JMicron and Realtek have also appeared at various trade shows (such as the RTS5261 demonstrated at Computex 2019). Silicon Motion, in coordination with Realtek, sampled their 250GB SM2708 reference design along with a Realtek RTL9211DS and RTS5261-based card reader for this review.

The Silicon Motion SM2708 card controller supports both SD UHS-I and PCIe Gen 3.0 x2 on the upstream side. On the flash side, Toggle 3.0 and ONFI 4.1 NAND interfaces at 800 MT / s are supported. The controller is limited to two-channel operation with 8 options per channel, and both 3D TLC and QLC can be used.

Silicon Motion qualified the SM2708 to work with SanDisk / Kioxia 96L TLC (2-plane flash) and Micron B27B (96L) and B47R (176L) (both 4-plane flash). With 4 plane flashes, Silicon Motion claims to have write performance of more than 700 MB / sec with four NAND flashes. Writing performance is greatly affected by the number of flash tracks, NAND tPROG, and plane numbers.

The 250GB reference design, sampled for review, comes with a SanDisk / Kioxia BiCS 4 96L TLC flash in 2 planes.

The Realtek card reader is a two-chip solution used by RTL9211DS and RTS5261. The RTL9211DS is more commonly seen in storage bridges that allow the use of M.2 NVMe SSDs with a USB host. It has a USB 3.2 Gen 2 (10 Gbps) upstream interface and a PCIe 3.0 x2 downstream interface.

In the card reader, the functionality is the same – in SD Express mode, PCIe strips connect directly to SD card pins (with PCIe pins RTS5261 operating in bypass mode). However, in legacy mode, the UHS-I interface operation is implemented with RTS5261, and the RTL9211DS acts only as a component of the main interface. In this case, communication between RTL9211DS and RTS5261 is via firmware.

Standard SD Express

SDA standard SD Express aims to facilitate the production of interchangeable cards in inherited SD and microSD formats, while maintaining basic compatibility. Therefore, SD Express cards support both PCIe / NVMe interface as well as UHS-I interface.

The pin layout of SD Express cards is similar to UHS-II cards. Depending on the capabilities of the host, the card controller can switch between inherited SD UHS-I or NVMe modes. The card controller recognizes the host’s capabilities based on the supply voltage – PCIe / NVMe requires a 1.8 V power supply and uses both rows of pins on the card, while the legacy mode is the default and uses only the top row of pins to transmit data. It should be noted that SD 8.0 introduced in 2020 provides a third row of pins to support the second PCIe tape. Due to format limitations, microSD Express does not support the two-tape feature.

Although initialization can be in both ways, access via PCIe allows the card to present itself to the host as a standard NVMe device. This is especially important for our rating scheme, as the SMART approach via NVMe allows temperature monitoring and card status monitoring, among other features.

Test bench setting and evaluation methodology

Direct-connect storage devices (including SD Express cards) are rated using Quartz Canyon NUC (essentially, Xeon / ECC version of Ghost Canyon NUC) configured with 2x 16GB DDR4-2667 ECC SODIMMs and PCIe 3.0 x4 NVMe SSD IM2P33E8 1TB with ADATA.

The most attractive aspect of the Quartz Canyon NUC is the presence of two PCIe slots (electric, x16 and x4) for additional cards. In the absence of a discrete graphics processor – for which there is no need for a DAS test desk – both slots are available. In fact, we also added a spare SanDisk Extreme PRO M.2 NVMe SSD to the CPU directly plugged into the M.2 22110 slot on the motherboard to avoid bottlenecks in the DMI when evaluating Thunderbolt 3 devices. This still allows two additional cards running on x8 (x16 electric) and x4 (x4 electric). Since the Quartz Canyon NUC does not have the original USB 3.2 Gen 2×2 port, Silverstone’s optional SST-ECU06 card is installed in the x4 slot. All non-Thunderbolt devices were tested using the Type-C port provided by the SST-ECU06.

The specifications of the test table are summarized in the table below:

2021 AnandTech DAS test space configuration
System Intel Quartz Canyon NUC9vXQNX
CPU Intel Xeon E-2286M
Memory ADATA Industrial AD4B3200716G22
32 GB (2x 16 GB)
DDR4-3200 ECC @ 22-22-22-52
OS Drive ADATA Industrial IM2P33E8 NVMe 1TB
Secondary drive SanDisk Extreme PRO M.2 NVMe 3D SSD 1TB
Additional card SilverStone Tek SST-ECU06 USB 3.2 Gen 2×2 Type-C host
YOU Windows 10 Enterprise x64 (21H1)
Thanks to ADATA, Intel and SilverStone Tek for the building components

Testing hardware is only one segment of evaluation. In the last few years, typical workloads of memory cards with direct connection have also developed. High-speed 4K videos at 60 fps have become commonplace, and 8K videos are starting to appear. Game installation sizes have also grown steadily even on portable game consoles, thanks to high-resolution textures and artwork. With this in mind, our memory card evaluation scheme includes multiple workloads that are described in detail in the relevant sections.

  • Ready-to-use performance using CrystalDiskMark workloads and fio Sequential Access
  • Extended usage simulation using custom workloads for robocopy and real-world access traces using the PCMark 10 storage reference value
  • Evaluation of long-term effects of use using CrystalDiskMark and fio Sequential Access workloads
  • Performance renewal testing

The SM2708 reference design was evaluated in two modes – one using a Realtek card reader in SD Express mode and the other using a Lexar Professional Workflow SR2 SDHC / SDXC UHS -II USB 3.0 reader (used in our standard SD / uSD card evaluation) in UHS -And mode. The remaining sections in this review will cover card performance in both of these modes, along with some observations on power consumption and thermal temperature.

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Naveen Kumar

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