Crystal Oscillator for STM32F407 connected to LAN8720

[lealoureiro]

I'm planning to build a custom board with STM32F407 chip for a system where I also need Ethernet connectivity. 

I'm planning to connect this board to a LAN8720 with breakout board from Smart Electronics to have Ethernet connectivity (two independent boards connected via PHY interface).

But I'm a bit in doubt regarding which Oscillator to couple with STM32F407. 

I read that for RMII we need a 25MHz oscillator to achieve the 50MHz required for RMII circuit. But I also notice that breakout board already its own 50MHz crystal oscillator. 

I also notice that some of these boards do not include this extra crystal oscillator, not the case with the board I have.

In this case I still need the use a 25MHz crystal oscillator with STM32F407 or can I use the "standard" 16MHz crystal that also comes with a lot of available boards (DevEBox, DiyMoore, etc).

Thanks in advance!

Cheers,

Leandro

 [1]: https://i.stack.imgur.com/dN0Bb.jpg

[Tesla DeLorean]

>>In this case I still need the use a 25MHz crystal oscillator with STM32F407 or can I use the "standard" 16MHz crystal that also comes with a lot of available boards (DevEBox, DiyMore, etc).

Not sure 16 MHz is a "standard", ST typically uses 8 MHz (sourced from ST-LINK) on DISCO/NUCLEO boards, and a 25 MHz HSE on EVAL boards.

Generating clocks from HSI or PLL is problematic from a stability and jitter perspective.

Using 25 MHz HSE, setting HSE_VALUE to 25000000, provides the most flexibility

[lealoureiro]

Maybe I got confused with HSI when I was saying "standard 16MHz" crystal. Forget about that, I'm still learning a lot regarding STM32 arch :)

I understand now using 25MHz crystal would be the most flexible.

But is possible to use the external crystal on LAN8720 board just for the PHY comunications?