4.2 Reduced Media Independent Interface (RMII)

The integrated Reduced Media Independent Interface (RMII) provides a common low pin count interface between the physical layer and MAC layer devices, adhering to the RMII Specification Revision 1.2. RMII reduces the pin count of MII while retaining a serial management interface (MDIO/MDC) identical to MII. PLCA is fully supported through the RMII.

The RMII has the following characteristics:

  • A single 50 MHz clock reference is used for both transmit and receive
  • It provides independent 2-bit (di‑bit) wide transmit and receive data paths

The RMII includes the following interface signals:

  • Transmit Data - TXD[1:0]
  • Transmit Enable - TXEN
  • Receive Data - RXD[1:0]
  • Receive Error - RXER
  • Carrier Sense / Data Valid - CRSDV

For timing information, refer to the RMII Timing section. Refer to the RMII Specification Revision 1.2 for additional information.

When operating in RMII mode, the LAN8670/1 REFCLKIN pin must be connected to a 50 MHz reference clock source. This clock source may be driven by the MAC or from a common oscillator driving both the MAC and LAN8670/1. When a common oscillator is utilized, it is recommended to match the length of the PCB traces from the source to the LAN8670/1 and MAC to within 0.5 inches (13 mm). Care should be taken when laying out the board to prevent the LAN8670/1 clock from becoming a victim of crosstalk as any noise introduced to the clock input of the LAN8670/1 could become radiated onto the network.

The CRSDV pin combines both carrier sense and receive data valid. On carrier sense, the CRSDV pin is asserted asynchronously to the 50 MHz reference clock. Thereafter, CRSDV is only negated synchronously to the reference clock. Carrier sense is indicated by the assertion of CRSDV during which the first di‑bit of a data nibble is presented on RXD[1:0]. Similarly, receive data valid is indicated by the assertion of CRSDV when the second di‑bit of a data nibble is presented on RXD[1:0]. See the RMII Specification Revision 1.2.

A collision is signaled on RMII by the assertion of carrier sense when the MAC is asserting TXEN. A half-duplex RMII MAC must be able to properly derive the collision indication from CRSDV and TXEN.

Note: The connection of a 10 kΩ pull-down resistor on TXEN is recommended to prevent unintended transmission if the MAC does not actively pull-down or drive this pin low at all times during its reset and initialization.