Bus Speed (Default Speed/High Speed/UHS/SD Express)

Bus Speed

The initial SD bus speed of 12.5MB/s is the Default Mode and was defined by SD1.0. Then a 25MB/s High Speed Mode was defined by SD1.1 to support digital cameras. As higher performance levels were needed to support new and faster devices, the SD Association introduced faster speed bus interfaces: UHS-I, UHS-II, UHS-III and SD Express. All of these faster interfaces are available for SDHC, SDXC and SDUC memory cards.

UHS-I provides faster bus speed using just one row of pins. And UHS-II, UHS-III and SD Express have ability to provide even faster speeds than UHS-I by using two lanes for data transfer via two rows of pins. The 2nd row of pins interface offers the Low Voltage Differential Signaling (LVDS) technology.

Using two lanes for data transfer, cards with UHS-II or UHS-III achieves Full Duplex (default mode) by assigning one lane for a downstream direction (transferring data from the host to card) and the second lane is used for an upstream direction (transferring data from the card to host). Packets can be transferred in both directions at the same time. Half Duplex switches both lanes to the same direction during data transfer to make faster data transfers. For example, 156MB/s in Full Duplex can be switched to 312MB/s in Half Duplex for UHS-II. To makes PHY design simple, UHS-III Card does not offer Half Duplex and delivers 624MB/s bus speeds at Full Duplex.

SD Express offers the fastest data transfer rates up to 3940MB/s using PCIe Gen.4 interface and NVMe application protocol.

Bus Interface Card Type Bus Mark Bus Speed Spec Version
Default Speed SD, SDHC, SDXC and SDUC --- 12.5MB/s 1.01
High Speed SD, SDHC, SDXC and SDUC --- 25MB/s 1.10
(SDR50, DDR50)

104MB/s (SDR104)

UHS- II SDHC, SDXC and SDUC 156MB/s Full Duplex
312MB/s Half Duplex
UHS- III SDHC, SDXC and SDUC 312MB/s Full Duplex
624MB/s Full Duplex
SD Express SDHC, SDXC and SDUC 985MB/s
PCIe Gen.3 × 1Lane
PCIe Gen.4 × 1 Lane
PCIe Gen.3 × 2 Lane
PCIe Gen.4 × 2 Lane

Maximum speed differs from the bus I/F speed. It varies depending upon the card performance. The average speed that a device writes to an SD memory card may vary depending upon the device and the operation it is performing. The speed may also depend on how other data is stored on the SD memory card.