Content originally posted in LPCWare by hlsa on Tue May 13 23:06:02 MST 2014
Hello Philippe,
I cannot give you a definite advice (and I assume no one can, since this topic is too complex). However, I can give you three hints:
1. Have a look at AN-1469 from TI, especially at chapter 5.2 (Max Trace length). This application note is for a TI Phy, but the recommendations should apply to other devices as well.
2. Several years ago I made my first ethernet device, where I had a similar problem. I put the phy far away from the MAC, but close to the RJ45 connector. This was a catastrophe. The connection between the MAC and phy is very critical. (In addition, I placed the connection traces in parallel to a motor power connection. Bad mistake.)
Since this time, the first thing I do in a new device, is making sure that the phy and MAC are placed next to the RJ45 connector. I am doing this step before the mechanical design.
If I need to chose between long traces from MAC to phy and from phy to RJ45, I would chose the latter one.
3. I do have a prototype of a new device, where the PHY is put on an add-on card. However, the distance between MAC and Phy is about 2 inches and the connector between the main print and the add-on card is full with GND pins. Everything works fine.
Best regards,
Holger