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I'm designing an Ethernet switch with KSZ8863RLL. It's a 3-way switch but only has 2 integrated PHYs and for the 3rd port it has an RMII interface.

Datasheet says that the 3rd port can work in 'MII MAC mode' or 'MII PHY mode'. On power on the default is 'MII PHY mode' (page 58 -> Register 53).

For External PHY I'm going to use KSZ8051RNL.

Which mode do I need to use to connect an external PHY?

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MII MAC Mode

This mode will be used when you want to connect MAC(Media Access Control) Devices which have inbuilt Ethernet Interface Control like Processor(say iMX28), Controller(say AT91SAM).These are capable to taking the data for processing and using it for different purposes.Other case is cascading i.e. connecting two or more KSZ8863RL together for increasing the number of Ports on your Switch.

MII PHY Mode

This mode will be used when you want to connect KSZ8863RL with external PHY(say KSZ8051RLN), in short the purpose of PHY is to convert the data coming from Processors or MCU to convert it in the format so that it can be transmitted over copper CAT5 cables and when the data is received from the CAT5, they convert it in the format that can be used by our Processors or MCUs.

Since you want to connect KSZ8051RNL which is a PHY, Physical Transceiver, you should use MII PHY Mode.

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The datasheet PDF for the switch IC is protected, so I'm just going to post an image an then say what I understand. The image is from page 30, the paragraph above Table 7. (Please feel free to edit and type it out, I'm just in a rush!)

Screenshot of KSZ8863RLL switch datasheet, page 30

From this I understand that the 'MII PHY' mode is intended for connecting to a PHY device like the one you plan to use.

If you ever want a 4-port switch, you could use two of the switch ICs and connect them back to back using the 'MII MAC' mode (I think on just one of them, the other in 'MII PHY').

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