How to generate 50MHz clock for RMII Ethernet PHY?

[miles]

Posted on March 27, 2009 at 12:47

How to generate 50MHz clock for RMII Ethernet PHY?

[olivier239955]

Posted on May 17, 2011 at 13:05

Hello,

You can find attached the evaluation board user manual draft which includes the schematics.

Both MII & RMII modes are supported.

I hope this answers to your questions.

Best regards,

[miles]

Posted on May 17, 2011 at 13:05

I was just about to order PCBs for my first design with the STM32, and then I saw the data sheet for the STM32F107, with ethernet. I have (barely) enough free pins that I can reshuffle things and use RMII hooked up to a PHY. My plan is to still leave the serial and CAN interfaces in place, so I'm not out anything if the ethernet doesn't work, and I can use the F103 until the F107 is available.

I checked ST's website, and I don't see any example schematics or eval board schematics I can clone. They do have drawings at the end of the 'F107 data sheet, Appendix A, pages 83-85 and 87-88, that show various configurations. Right now I'm planning on using page 85, ''RMII with a 25MHz crystal'', but I'm unsure how the 50MHz clock is supposed to be generated by the STM32.

Can I use any timer pin? Or do I have to use the ''MCO'' (shown on the Figure 50 example on page 87)? The main reason I picked ''RMII with a 25MHz crystal'' is because it had a part-number for a PHY to use, which is one less thing for me to worry about. ''RMII with a 50MHz oscillator'' would be okay by me too, if anyone would like to suggest a PHY and oscillator part. Or else will the DP83848 work with a 50MHz oscillator vs. having the STM32 PLL it off of the 25MHz crystal? I don't mind the extra cost of an oscillator vs. a crystal if it adds robustness (crystal startup issues are sometimes problematic and a PITA to trackdown), but I really don't know if oscillators are any better than crystals.

Thanks,

Miles

[guyvo67]

Posted on May 17, 2011 at 13:05

When are those 105/107 types available ?

-G

[simonellwood9]

Posted on May 17, 2011 at 13:05

Well that evaluation board had a seperate xtal for the Phy. This means it did not answer the question.

I would suggest driving the stm32 and the phy from the same oscillator module. We have an ARM based project (not stm32) using that approach and it works fine.