STM32H743BIT6: same clock source for both MCU and ethernet PHY?

[Meghna]

Hello, I am using STM32H743BIT6 as a MAC for DP83TC814R Ethernet PHY, I have one query regarding oscillator, I am using Ethernet in RMII slave mode which requires 50MHz reference clock, can i use the same clock which i am using for MCU HSE or should i use different one for EThernet PHY, But how the clocks can be synchronised in that case?

[MHoll.2]

Yes you can use the same clock (50MHz is the max clock allowed on OSC_IN), the synch is via the RMII_REF_CLK input pin of the STM32H7 (50MHz from PHY).

[Meghna]

Could you please elaborate on the pin numbers & names at both MAC & PHY side

MAC part number : STM32H743BIT6

PHY part number : DP83TC814R

[MHoll.2]

I will no do your work, please read the Datasheet / Reference manual for the STM32H74x and the DP83TC814.

Some small hint: The STM32 MAC with RMII interface needs the same clock as the PHY, see chapter 58.6.3 of the Reference manual (rm0433).

[Meghna]

I got your point but now if we are giving the output of 50MHz oscillator to XI pin of PHY and ref_clk pin of STM(MAC), there is one more OSC_IN pin(which is used for HSE operations), then same will be given to this pin as well?

[MHoll.2]

Yes, if You whant to use the same clock as CPU clock.

[Meghna]

The clock path is very not clear, this is how we have implemented in our design, please guide us here, 

But from the reference manual it shows from ETH transceiver REF CLK is input to STM32 RMII_ETH_Refclk, but is this the clk going to XI pin of ETH transciver, how the synchronisation will be?

[Meghna]

Hello, Can we use OSC50M-3.3I/S3 for HSE operations of STM32H743BIT6?

[MHoll.2]

Ok:

1) 50MHz Clock has to go to PH0_OSC_IN (not PH1_OSC_OUT).

2) Aside from the point 1) the clock is OK (the DP83TC814R use XI as reference clock), from the Datasheet:

3) You have tu use an Oscillator capable of driving the 3 Inputs. OSC50M-3.3I/S3 should be OK (max. 15pF)

[Meghna]

Is this ok, We are planning to use ETH transceiver in RMII slave mode?