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STM32f217VET ethernet project via MII

Question asked by Pablo Cottens on Nov 25, 2017
Latest reply on Nov 30, 2017 by Pablo Cottens

Hello I am using this microcontroller for a project with ethernet. From what I've been reading everyone recommends that I use MII instead of RMII since RMII incurs in a lot of packet loss. And reading the errata I noticed that there is a problem with the PLL (MCO) because of jitter and the workaround for this problem in MII mode  is to use a 25 MHz external crystal to generate the HSE clock and output the clock signal on the MCO pin to clock the PHY.


Couldn't I simply use a separate 25MHz oscillator especifically for the PHY? Or does it have to synced with the microcontroller?? According to this reddit thread Simply using a separate oscillator instead of the MCO pin would suffice (one oscillator for the microcontroller and another for the PHY instead of connecting MCO to the PHY). Is this correct? 


What I want to do is use the internal oscillator of the microcontroller and the PHY .. something like the images attached ... where z-phy is the connections to the phy and z-stm32 the connections to the microcontroller. 


From what I've been reading this is not good, since I'd need a stable clock for the microcontroller as well ... meaning that the only modification to my schematic would be to add an oscillator for the microcontroller .. am I right?