Media Independent Interface - Reduced Media Independent Interface

Reduced Media Independent Interface

Reduced Media Independent Interface (RMII) is a standard which was developed to reduce the number of signals required to connect a PHY to a MAC. Four things were changed compared to the MII standard to achieve this:

  • The two clocks TXCLK and RXCLK are replaced by a single clock. This clock is an input to the PHY rather than an output, which allows the clock signal to be shared among all PHYs in a multiport device, such as a switch.
  • The clock frequency is doubled from 25 MHz to 50 MHz, while the data paths are narrowed to 2 bits rather than 4 bits.
  • RXDV and CRS signals are multiplexed to one signal.
  • The COL signal is removed.

These changes means that RMII uses about half the number of signals compared to MII. The high pin count of MII is more of a burden on microcontrollers with built-in MAC, FPGA's, multiport switches or repeaters, and PC motherboard chipsets than it is for a separate single port Ethernet MAC which partially explains why the older MII standard was more wasteful of pins.

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