Ethernet controller clock

[Pavel A.]

Sorry for a noob question...

The DS for STM32H7 (DS12117 Rev 7, 3.42) says that a PHY can be clocked using 25 MHz from the microcontroller. The PHY we want to use, KSZ8873, has its own connection for 25/50 MHz oscillator. We'd prefer to put only one oscillator on the H7.

I don't understand from the KSZ document, can it support clock from the microcontroller? If yes, is MII (vs. RMII) interface required? Is separate crystal for the PHY a much better option?

Thanks in advance

-- pa

[Tesla DeLorean]

http://ww1.microchip.com/downloads/en/DeviceDoc/00002348A.pdf

You can provide a TCXO clock source into X1 of the KS8873, where X2 is NC (No Connect)

The source could come from the PA8 MCO pin of the H7 feeding thru HSE

Or you could feed the TCXO output, through a buffer, to OSC_IN(STM32) and X1(KS8873)

fHSE max for the H7 is 50 MHz

[Pavel A.]

Thank you Clive. the 2nd variant can work for me.

-- pa

[Tesla DeLorean · Accepted solution]

Ok

Other note would be to not use the STM32 PLL as a source.

One could use 50 MHz HSE and output via MCO at DIV1 or DIV2

[Pavel A.]

This looks good too. Thanks Clive.

-- pa

[Piranha]

Look at sections 3.1.3 and 3.3.9, especially TABLE 3-5. You can also use 25 MHz crystal or oscillator for HSE, output HSE on MCO and input on PHY's X1. PHY has it's own internal PLL, which will generate 50 MHz and other necessary clocks from your 25 MHz clock. The 25/50 MHz input clock is selected via SPIQ input strap.