The Media Independent Interface is a synchronous 4-bit wide nibble data interface that connects the PHY to the MAC. The MII is fully compliant with IEEE 802.3-2002 clause 22.

The MII signals are summarized in .Table 2-14

Reference the waveforms below to verify the expected MAC data and clock signals for 100BASE-Tx MII Mode. Table 2-24 displays specs taken from the data sheet shown in the waveforms. MII signaling needs to be 2.5MHz if PHY is not linked up or linked up at 10Mbps, and needs to be at 25MHz if linked at 100Mbps. Note that both TX_CLK and RX_CLK are outputs of the PHY.

If a MAC bus (TX or RX) is suspected to be problematic, probe the lines at the receiver side of the trace, making sure that the receiver's setup and hold times are met, along with VIH/VIL. Typical symptoms of violating these specifications is packet errors at the MAC while the PHY is indicating clean traffic (Reg 0x15).