Hello @lb2 ,
It is not recommended to have two MCUs communicate with a single PHY (such as the LAN8742) simultaneously using different interfaces (RMII and MII).
In fact, the LAN8742 PHY is typically configured to work with either RMII or MII, but not both simultaneously. The configuration is usually set during the initialization phase and is dependent on the hardware design and software configuration. RMII and MII have different clock and timing requirements. RMII typically uses a 50MHz clock, while MII uses a 25MHz clock. Managing these different clock domains simultaneously would be complex and could lead to timing issues. The MDIO/MDC interface is used to configure and manage the PHY. If two MCUs are trying to control the same PHY, there could be conflicts in the configuration and management of the PHY registers.
Regards
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