Bus Speed (Default Speed/ High Speed/ UHS)

Bus Speed

SD bus speed of 12.5MB/s Default Mode was defined by SD1.0 and 25MB/s High Speed Mode was defined by SD1.1 to support digital camera. Its high performance function for continuous shooting required higher write speed memory card where SD Association introduced higher speed bus interfaces for SDHC and SDXC memory cards named UHS-I, UHS-II and UHS-III.

UHS-I can provide ultimate bus speed using 1st Row pins. UHS-II and UHS-III have ability to provide much higher bus speed than UHS-I using 1st Row and 2nd Row pins. 2nd Row interface is supported with Low Voltage Differential Signaling (LVDS) technology.
UHS-II and UHS-III has two lanes. Full Duplex (default mode) assigns a lane to downstream direction (host to card) and the other lane to upstream direction (card to host) and then packets can be transferred both directions at the same time. Half Duplex switches lanes to the same direction during data transfer to make data transfer rate twice (156MB/s in Full Duplex can be switched to 312MB/s in Half Duplex). However, UHS-III Card does not support Half Duplex for 312MB/s Full Duplex and 624MB/s Full Duplex to make PHY design simple.

Bus Interface Card Type Bus Mark Bus Speed Spec Version
Default Speed SD, SDHC and SDXC --- 12.5MB/s 1.01
High Speed SD, SDHC and SDXC --- 25MB/s 1.10
(SDR50, DDR50)
104MB/s (SDR104)
UHS-II SDHC and SDXC 156MB/s Full Duplex
312MB/s Half Duplex
UHS-III SDHC and SDXC 312MB/s Full Duplex
624MB/s Full Duplex

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.