SD Card data rate and frequency

Question

We are designing a SD card interface with a processor.The processor supports SDR104 with clock frequency of 192MHz. Along similar lines,the UHS modes and corresponding clock frequencies mentioned are, SDR25 - 48 MHz ; SDR50 - 96 MHz ; DDR50 - 48 MHz ;

Don't we need at-least 208 MHz clock frequency to transfer 4 bit of data in SDR104 Mode(at 104MB/s) or about 50 and 100 MHz for SDR25/DDR50 and SDR50 modes? If the maximum achievable clock frequency is 192 MHz,are we still operating in SDR104 mode?

Please help me to understand these data rates and clock frequencies.

Answer 1

SDR104 operating mode specifies various things, including

• maximum power drawn by the card
• communication voltage
• maximum allowed clock speed

You can operate the interface at a lower clock speed, if you wish to do so, but you still need to meet the other requirements of SDR104 mode.

If you decide that 50MB/s is enough, you can design your system to meet the relaxed requirements of SDR50 mode. If you decide that you require 105MB/s, you need to choose the entirely different UHS-II mode.