1

I'm drawing the layout for an RMII interface, but I have two doubts:

1 - Do RMII interface traces need length matching?

2 - Do the traces of the RMII interface have to be impedance controlled?

I ask because searching on the internet you find conflicting information, in some guides it is written that you don't need a doubled impedance, while others indicate that 50 ohms are necessary.

Doing or not doing controlled impedance isn't a big deal, but length matching is, particularly for the clock signal, which in some guides states that it "must be as short and linear as possible", while others say that must be in the range +- 10mm with the other signals.

So I don't know whether to do layout A or layout B for the clock signal, but the same goes for the other signals of the RMII interface as well. enter image description here

Federico Massimi
  • 735
  • 1
  • 10
  • What do the data sheets of your devices say? How those chips send and receive data will define how much length mismatch is allowed. – Justme Apr 30 '23 at 14:55

2 Answers2

0

There are different specification revisions for RMII and to my knowledge, there are no clear requirements other than the timing requirements in the datasheet of the PHY.

I recently implemented such an interface without impedance and length matching and I ran into signal integrity problems. I was able to overcome these problems by reducing the MPU output pin driver strength. So it also heavily depends on the signal drivers.

The short answer is: Impedance and length control are not required per se, but it should cause no harm to do it.

Stefan Wyss
  • 6,471
  • 1
  • 11
  • 22
0

Not really at 50MHz the velocity on most PCBs is about 15cm/ns or 300cm, so if you can get your traces matched to the clock under 300 cm (maybe 10% less for riding edges) then the design will work. Matching doesn't really hurt though, except for the time that it takes to do it.

Impedance control doesn't really matter if your loads aren't matched, what you do need to be worried about is capacitance and keeping capacititance low.

If your PHY on the same board as the controller this probably isn't a big problem or if you are doing a short run. If your traces are a few inches long then you should probably calculate the capacitance with a PCB calculator and see how it's affecting the rise time. Just follow good routing practices don't use too many vias, and don't split the ground plane.

Voltage Spike
  • 75,799
  • 36
  • 80
  • 208