The MMC specification up to version 3.x stipulated a clock frequency of 0-20 MHz and a 1-bit data bus. Since the original version of SD was already supporting 4-bit data bus at 25 MHz, the best MMC performance was nowhere near that of SD. The adoption of the MMC Version 4.x standard has addressed these limitations and gave birth to a new breed of cards called the MMCplus.
MMCplus are designed to operate at a clock rate of up to 52 MHz and an 8-bit data bus. To support the wider bus width, the MMCplus was fitted with 13 pins, 6 more than that of the original MMC. To maintain backward-compatibility with the earlier devices, the 13 pins are so laid out so that a pin coincides with each of the 7 pins of the original MMC.
Based on the specifications, the maximum theoretical data transfer rate of MMCplus is 416Mbps or 52MBps or 347X.
Pin layout of MMC (left) and MMCplus (right)
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