STM32 RMII Reference Clock

[mattr]

We have a current application in which our F7 is connected to an external Microchip PHY which provides a 50 MHz reference clock for RMII.  We are attempting to place the system into a low power mode in which we disable the PHY thus stopping the reference clock.  Is there a way to do this that will not cause the network stack on the MCU to fail?  We are using Zephyr as our RTOS which may or may not be relevant.

[T_Hamdi]

Hello ​@mattr 

If the PHY stops providing the 50 MHz RMII REF_CLK, the STM32 Ethernet MAC will lose its clock source. In that state, you cannot keep the network stack operating normally.

So the practical solution is:

1. shut down the network interface first
2. stop/disable the PHY
3. restore the PHY and clock later
4. reinitialize the interface

If your requirement is “PHY off, but network stack still alive and usable”

then the answer is no for RMII on STM32 F7, because the MAC depends on that clock.

Best regards,

hamdi