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STM32F4 RMII ETH external clock source

Question asked by toth.janos on Sep 23, 2013
Latest reply on Sep 24, 2013 by waclawek.jan
Hy all,

I've a question about the RMII mode at STM32F4 mikrocontrollers. I checked the datasheet and the stm3240g-eval user guide.
So the eval board user guide say that at page 19:
"In RMII mode the 50 MHz clock must be provided to Ethernet PHY by an external oscillator. This oscillator (ref SM7745HEV-50.0M or equivalent) must be soldered on the U3 footprint (located under CN3) and jumper JP5 must be removed. This oscillator is not provided with the board."

But the stm32f4 datasheet show that the external 25MHz is enough for the ETH RMII mode via the MCO pin.

My question is what is the correct? The ETH RMII mode need an external 50MHz clock source or I can use the MCO pin, with only one 25MHz clock source for the mikrocontroller?