How should I connect two MII devices back to back?

Question

I have researched a bit on this topic. Each time I found different opinions and examples.

Earlier, I've made a successful connection between a RMII featured micro-chip and an ethernet PHY, again RMII featured LAN8720A.

Now I want to connect an RTL8306 and an RTL8201 using their MII interfaces.

My question is about the connection between data pins.

Should we connect TX pins to RX pins, or can they also be connected TX to TX and RX to RX?

For example, this suggests a cross connection.

I would like to know the reality behind this.

Answer 1

Just look at the data directions on the pins.

Generally, you only connect outputs to inputs, so...

The short answer for these two chips is that RX connects to RX and TX connects to TX. Also the RTL8306 must be operated in MAC mode and not operated in PHY mode on any MII interfaces connecting these two chips.

On the RTL8201...

• The TXD pins are inputs.
• The RXD pins are outputs.
• Both TXC and RXC are outputs.

On the RTL8306 the pins can be switched between MAC mode and PHY mode...

• In MAC mode the RXD pins are input and the TXD pins are output.
• In PHY mode the RXD pins are output and the TXD pins are input.

Note that the TXC and RXC are outputs on the RTL8201 (the PHY drives both RX and TX clocks on MII). In that case you must use the RTL8306 in MAC mode so that its RXC and TXC are inputs.

If the RTL8306 MII port is in MAC mode, then connect RX pins on RTL8306 to RX pins on RTL8201 and connect the TX pins on RTL8306 to TX pins on RTL8201.