BeagleBone - Why MII over RMII?

[Andrew Bradford]

I'm just curious why the SMSC Ethernet PHY is connected to the AM3359
over MII rather than RMII on the BeagleBone. Can anyone provide
insight on this design decision?

The SMSC PHY supports RMII, there's even a physically smaller version
(SMSC LAN8720A) that only supports RMII and uses the same Linux kernel
driver. What's the advantage of using MII? Just simply the way the
mux works out (but I only see conflicts with GPMC pins going with RMII
instead of MII)?

I'm working on specifying an AM335x based design and would like to
learn.

Thanks!

[Gerald Coley]

Because we had issues getting RMII to work reliably and we had better results with the MII, so that is the route I went. I was running out of time to meet our deadline, The current design supports RMII, so if you want to try it, you can make the changes on the board to support it.

 

Gerald

 

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[Andrew Bradford]

Can you provide more info about what kind of issues you ran into?

The board I'm working to specify will have both Ethernet interfaces
used on the AM335x and RMII would be nice, if for nothing else to
limit the pin count. But I don't think going to MII would noticeably
impact my peripheral choices.

If you're willing to share your experience, I'd greatly appreciate it.

Thanks!

[Gerald Coley]

We had issues with low throughput when running data in both directions at the same time.Single directions were very good. MII gave us better performance. I don't hink it was really related to RMII vs MII as we also changed PHY when we moved to MII, but I had no time to go back and try to figure out for sure what it was. Also, MII has a slower clock rate that RMII, so it could also have been layout related.

 

Gerald