Debugging PHY interface

I am debugging a prototype PCB that includes a Micrel KS8721BL PHY connected to the Ethernet MAC of an Atmel AT91SAM9 CPU.

Frame reception appears to work, but *all* transmitted frames are lost.

If I connect the board to an Ethernet switch, the activity LED blinks, but the switch doesn't attempt to forward the frame to the other port(s).

I have checked all the obvious stuff:

- MLT-3 waveform on TX+ and TX- looks good.

- MII signals: TX_EN, TX_D0[3:0] look right

- 25MHz clocks to MAC and PHY are present

- Link is up and FLP pulses during autonegotiation advertise the correct options

The interesting bit was when I connected the board to an old NetGear DS104 hub (which is based on the AC206 hub controller IC from Altima):

About half of the transmitted frames cause collisions. These can be seen via the collision LED on the hub and also the evidence of a 'frame' coming from the hub to my board *during* the transmission of the outgoing frame. (By 'during' I mean its start delayed by about 1 microsecond but cuts off at the same time as the outgoing frame. I detected this via the MII COL signal coming from my board's PHY.

On further examination of RX_D0[3:0] during this incoming 'frame', I see that the PHY is sending my board a 0x55 followed by a long series of repeated


What I'd like to know is what this incoming frame is supposed to be, and why the hub would be sending it anyway.

Can anybody help?

Thanks, Mark


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