Electronic – Problem with Ethernet PHY

ethernetphysignal integrity

One of my clients is experiencing problems with a SMSC LAN8710A PHY that is connected to a Xilinx FPGA. The Ethernet link works perfectly when the board is connected to my MacBook or to my office router, but fails with other partners such as USB-Ethernet adapters. In that case, the auto-negotiation succeeds but the link does not go up and the PHY reports symbol errors through its SYM_ERR_CNT register.

I'm suspecting a signal integrity issue on the board. Is there anything else that could explain this behavior? Any advice about how to debug issues like these?

Update: the symbol errors (there are usually one or two of them) seem to happen shortly after the link goes up. When auto-negotiation is OFF, the symbol error count remains at 1 or 2 and the link works fine. When ON, auto-negotiation re-starts every second or so and the symbol error counter gets incremented every time.

Best Answer

I can't comment yet so I'll post this as an answer.. I've had a similar problem in the past with a DP8384C and we were able to get the link to work by forcing the connection to 10mbps (on either end).

We had to re-spin the boards to fix it but it helped debug the issue. We basically rebalanced the differential pairs lengths from the PHY to the magjack and checked trace impedances, haven't had any issues on any designs since.