Reduced Media Independent Interface (RMII) is a standard that addresses the connection of Ethernet physical layer transceivers to Ethernet switches. It reduces the number of signals/pins required for connecting to the PHY from 16 (for an MII-compliant interface) to between 6 and 10. RMII is capable of supporting 10 and 100 Mbit/s; gigabit interfaces need a wider interface.

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  • Reduced Media Independent Interface (RMII) is a standard that addresses the connection of Ethernet physical layer transceivers to Ethernet switches. It reduces the number of signals/pins required for connecting to the PHY from 16 (for an MII-compliant interface) to between 6 and 10. RMII is capable of supporting 10 and 100 Mbit/s; gigabit interfaces need a wider interface. An Ethernet interface normally consists of 4 major parts: The MAC (Media Access Controller), the PHY (PHYsical Interface or transceiver), the magnetics, and the connector. Connectors with integrated magnetics are available. The MAC handles the high level portions of the Ethernet protocol (framing, error detection, when to transmit, etc) and the PHY handles the low level logic (4B/5B encoding/decoding, SERDES, and NRZI encoding/decoding) and analog portions. RMII is one of the possible interfaces between the MAC and PHY; others include MII and SNI, with additional wider interfaces for gigabit and faster Ethernet links. One or more MAC interfaces may be on the same chip and in some cases the chip may have many other functions. One or more PHY interfaces may be on the same chip, particularly in Ethernet switches. Some MAC and PHY ICs support both MII and RMII. Usually, the MAC and PHY are on the same board for 10/100 Ethernet though for gigabit and higher pluggable PHY modules may be used to allow the use of different media including twisted pair and optical fiber. Older coaxial Ethernet interfaces sometimes used an AUI interface between the MAC and transceiver which was often an external box (thicknet required an external transceiver). By comparison, the MII interface requires two additional data lines in each direction, RX_DV and CRS are separate rather than multiplexed, a separate TX_CLK and RX_CLK are used instead of a shared reference clock, and a collision signal is added for a total of 7 additional lines. The added pin count is more of a burden on microcontrollers with built in MAC, FPGA's, multitport 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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  • Reduced Media Independent Interface (RMII) is a standard that addresses the connection of Ethernet physical layer transceivers to Ethernet switches. It reduces the number of signals/pins required for connecting to the PHY from 16 (for an MII-compliant interface) to between 6 and 10. RMII is capable of supporting 10 and 100 Mbit/s; gigabit interfaces need a wider interface.
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  • Reduced Media Independent Interface
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