SD Card Bus Speed Specifications

In what has accidentally turned into a three-part series, I'm going to share one final article on decoding the wide variety of SD Cards—what do all the markings on an SD Card mean? 

I first wrote an article on the New SD Express Speed Classes, which discussed the concept of speed classes in general, and the new speed classes created for SD Express cards, in particular.

Second, I wrote an article on SD Card capacity, as expressed in the various card types:  SD, SDHC, SDXC, and SDUC.  See: SDHC, SDXC, SDUC, Oh My!

It's hard to believe, but all these various compliance criteria, and their associated markings on an SD Card, are still not enough to decode an SD Card's capabilities.  So, for part three, I'll discuss SD Card Bus Speeds.

The original bus speed of the SD Card specification was 12.5MB/s.  As the speed of the card media and host devices continued to increase, the SD Association (SDA) introduced faster bus technologies to keep up with it.  The bus speed tells you the theoretical maximum transfer speed of which the host or card are capable.  The following table from the SDA illustrates this nicely:

A few things to keep in mind:

1. "UHS" here stands for "Ultra High Speed"
2. The "Express" in "SD Express" refers to its association with, and leveraging of, PCI Express bus technology
3. SD Card capacities are orthogonal to the bus interface.  UHS-I, UHS-II, UHS-III and SD Express bus technologies can host cards of any of the modern capacity types:  SDHC, SDXC, or SDUC.
4. UHS-III has not been widely adopted in the industry, and so you are not likely to find UHS-III cards for sale
5. A UHS-III reader is backwards compatible with UHS-II and UHS-I cards, and a UHS-II reader is backwards compatible with UHS-I cards
6. SD Express is only backwards compatible with UHS-I, not with UHS-II or UHS-III.  The reason has to do with the different ways that SD Express and UHS-II/UHS-III define the signals on the second row of pins on the card/connector.  The net effect of this is that UHS-II cards inserted into an SD Express card reader will perform at UHS-I speed.

So, if you want to ensure you're getting the best possible speed out of your equipment, you need to match the bus speed of your host card reader with the capability of the card.  Putting a UHS-I card into a UHS-II reader isn't going to get you anything beyond UHS-I performance.  Conversely, putting a UHS-II card into a reader that only supports UHS-I will limit that UHS-II card to UHS-I performance.  Lastly, for an SD Express card reader, any card other than an SD Express card is going to perform at UHS-I performance levels.

Finally, recall that Speed Class is distinct from the speed of the bus technologies we're discussing in this article.  The speed class is the minimum sustained write speed of the card media.  Between two cards with the same bus speed, the card with the higher speed class is going to perform better.

Real-Life Example

Here is a real-life SD Card from SanDisk:

Let's decode this card's capabilities:

• Capacity = 128GB
• Type = SDXC
• Bus interface = UHS-II (denoted by both the "II" marking and "300MB/s"
• Speed classes: (minimum sustained write speed)
• speed class = 10 (10 MB/s)
• UHS speed class = U3 (30MB/s)
• video speed class = V90 (90MB/s)

Conclusion

This concludes our interlude into the world of SD Card performance classifications.  It is surprisingly complex, but considering the outsized-role that SD Cards play in our technology landscape, these concepts are important to master!