MMCplus : MMC on steroids

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)

SD Ver 1.1 : The faster SD

Whereas the SD 1.01 standard specifies a maximum clock rate of 25 MHz, the newer SD 1.1 version doubles the clock rate to 50 MHz. Using 4-bit mode data transfer, this increases the theoretical data transfer rate of the newer SDs to 25MBps or 166X.

There are sites which state that the SD 1.1 standard allows for an optional 8-bit data bus width. If this is so, this can only be possible by adding more pin connectors like what was done with the 13-pin MMCplus (original MMC had only 7). The current 9-pin SD bus currently uses 4 pins for data, 3 for power/ground and a line each for clock and command/response signals.

As of this writing, the fastest SD card available is 133X~150X and comes in a 9-pin package.