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How to generate 50MHz clock for RMII Ethernet PHY?

miles
Associate II
Posted on March 27, 2009 at 12:47

How to generate 50MHz clock for RMII Ethernet PHY?

4 REPLIES 4
miles
Associate II
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

olivier239955
Associate II
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,

simonellwood9
Associate II
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.

guyvo67
Associate II
Posted on May 17, 2011 at 13:05

When are those 105/107 types available ?

-G