Doubts about connecting STM32P157 with RTL8211F-CG (RGMII) based on STM MB1272C reference design

[PawelB]

Hello

I am currently working on the STM32P157 to RTL8211F-CG connection diagram using the RGMII interface. Since Realtek does not provide much documentation such as application notes, I used the MB1272C as a reference design. By the way, thanks for sharing. In the mentioned reference design, pin 36 (XTALL_IN) of the RTL8211F-CG is connected via a 22R resistor to port PB5 (ETH_CLK) of the STM32P157 processor.

However, in AN5031, on page 74 (Figure 47), ETH_CLK is connected to pin 37 (XTALL_OUT) of the RTL8211F(I)-CG. Also in RM0436 on page 503 in Table 49, ETH_CLK is as a clock output for the Ethernet PHY.

 

However, in the RTL8211F-CG note, pin 37 is described as XTAL_OUT/EXT_CLK, theoretically a clock input, but the pin is marked as output.

I am a bit confused,did I miss something? I do not know now whether I should connect the ETH_CLK output to XTALL_IN or XTALL_OUT, common sense tells me that the output is connected to the input, but...

Thanks for any sugestions.

[PatrickF · Accepted solution]

Hi,

I agree the possible connection of a 25MHz clock to RTL8211F is wrong on the MB1272 board (this option is anyway not used, R69 is not soldered).

ETH_CLK 25MHz is only used when you want to get rid of the 25MHz crystal.

Maybe you could refer to AN5031 which give slightly more details around RTL8211F clocking options.

Regards.

[PawelB]

Thank you, You cleared up my doubts.