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UHS Speed Class
UHS-I symbol
The Ultra-High Speed (UHS) designation is available on some SDHC and SDXC cards.
[44] The following ultra-high speeds are specified:
- UHS-I cards, specified in SD Version 3.01,[41] support a clock frequency of 100 MHz (a quadrupling of the original "Default Speed"), which in four-bit transfer mode could transfer 50 MB/s. UHS-I cards declared as UHS104 also support a clock frequency of 208 MHz, which could transfer 104 MB/s. UHS-I is the only class for which products are currently available.[45]
- Double data rate operation at 50 MHz (DDR50) is also specified in Version 3.01, and is mandatory for microSDHC and microSDXC cards labeled as UHS-I. In this mode, four bits are transferred when the clock signal rises and another four bits when it falls, transferring an entire byte on each full clock cycle.
- UHS-II cards, defined in Version 4.0, further raise the data transfer rate to a theoretical maximum of 312 MB/s.[46][47]
UHS memory cards work best with UHS host devices. The combination lets the user record HD resolution videos to
tapeless camcorders while performing other functions. It is also suitable for real-time broadcasts and capturing large HD videos.
Cards that comply with UHS show UHS-I or UHS-II on the label, and report this capability to the host device. Use of UHS requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation and select the 4-bit transfer mode.