We put 25MHz HSE to STM32F4, then forward same clock from MCO to micrel phy and then use micrels 50MHz for RMII.
On Jul 2, 2013 8:50 PM, "Jeff Barlow" <
address@hidden> wrote:
On 7/2/2013 4:50 AM, Claudius Zingerli wrote:
In the final design, we plan use a 3-port switch from Micrel. It can be
clocked from 25MHz or 50MHz, but the board I'm using to develop the
software has a DP83848 that does need a 50MHz clock source for RMII.
STM32F4 /might/ be able to handle 50MHz as a main clock source.
I see. For a one-off dev board I think it's always less frustrating to just use a separate 50MHz oscillator. The older PHY chips can be really fussy about clock jitter. I think once you get away from that DP83848 you'll find things less painful.
I was just suggesting using the Micrel RMII clock output to feed the RMII clock input on the MCU. I've never tried feeding a 50MHz clock into the HSE on a STM32F407 but it strikes me as risky. I do seem to recall that some of the PHYs also have a direct 25MHz output that would work for that. Don't know if that includes your switch chip, however.
--
Later,
Jeff
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