Next Gen NVMe SD Card Review: The SM2708 Controller Serves it Hot and Fast

Miscellaneous Aspects and Concluding Remarks

The appearance of PCIe / NVMe in the memory cards space enables us to bring over some of our direct-attached storage evaluation metrics. Two important aspects are power consumption and thermal characteristics.

Power Consumption

Bus-powered devices can configure themselves to operate within the power delivery constraints of the host port. While Thunderbolt ports are guaranteed to supply up to 15W for client devices, USB 2.0 ports are guaranteed to deliver only 2.5W (500mA @ 5V). In this context, it is interesting to have a fine-grained look at the power consumption profile of the memory card / card reader combination. Using the Plugable USBC-TKEY, the bus power consumption of the drives was tracked while processing the complete test suite. The graphs below plot the instantaneous bus power consumption against time, while singling out the maximum and minimum power consumption numbers.

Power Consumption Profile [Card + Reader]
TOP: SM2708 SD 7.1 250GBSM2708 UHS-I 250GB	BOTTOM: SM2708 UHS-I 250GBSM2708 UHS-I 250GBSM2708 SD 7.1 250GB

The Realtek card reader and the SM2708 reference design combination has a peak power consumption of 4.58W, with the average being around 3.25W. On the other hand, in UHS-I mode, the Lexar card reader and the reference design operate at less than 1.67W, with the average being around 1.25W. The extra performance in SD Express mode comes at the cost of power consumption, but that is no surprise.

Thermal Profile

The performance consistency evaluation of the reference design in SD Express mode revealed the internal temperature of the SD card to be around 100C when subject to heavy stress. In order to rule out any errors in the S.M.A.R.T polling, we used a FLIR One Pro thermal imager to capture the thermal profile of the card while being subjected to the performance consistency test.

To our dismay, we found the surface temperature to be as high as 96C - something entirely unsafe for handling with bare hands. It does look like safety standards allow for such surface temperatures, provided the device is under 50mm x 50mm in size. Unfortunately, it is a sad reality that the SD card and microSD card form-factors do not allow for the thermal design necessary to sustain extended operation with NVMe-class speeds. In any case, Silicon Motion's customers can tweak the firmware's configurable parameters to achieve a different balance between performance and temperature. The SM2708 might also held back a bit by its 28nm fabrication node. Once SD Express achieves scale of deployment, it is likely that Silicon Motion can choose more advanced nodes to drive down the power consumption and stress temperatures.

Final Words

Reviewing the Silicon Motion SM2708 reference design has helped us greatly in setting the consumer expectations for SD Express using NVMe right. The advertised numbers can only be achieved in burst mode, and sustained writes may be affected by a combination of SLC cache sizes and thermals. Performance similar to that of a high-end SATA SSD or a PCIe 3.0 x2 NVMe SSD can be expected, though the nature and number of flash pieces inside the card can heavily influence the performance.

Silicon Motion has a couple of design wins ready to ship, with ADATA and Lexar in the final stages of bringing their SD Express products into the market.

Overall, consumers have been waiting for SD Express for quite some time. While CFexpress has taken the early lead with camera / camcorder makers, SD cards have a much wider range of markets to service and target. We are bullish on the prospects for SD Express in verticals such as portable game consoles and SBCs, but it might take a few years for mid-range cameras to start supporting it.

In the meanwhile, products such as the Silicon Motion SM2708 and Realtek RTL9211DS / RTS5261 can prime the market for the new standard on the PC side. A lot is dependent on the vendors not charging too much of a premium for SD Express over the standard UHS-II cards / readers.