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DP83848C autonegotiation problem with 10 Mbit/s

Other Parts Discussed in Thread: DP83848C

We're having a problem with our product that uses a DP83848C. The issue is that when the PHY has troubles getting the link up. The LEDs on the switch indicate that the link goes up, down, up, down. Every cycle takes around a second. There are three things that must occur for the problem to happen:

1. We have disabled support for 100 Mbit (full and half duplex) support in ANAR

2. In a different port in the switch, a device is transmitting multicast packets at 500 Hz.

3. It only happens for certain switches. We have tested this with three switches: N-TRON 105TX, Netgear FS105 and Netgear GS108T. The first two have the problem, but when the GS108T is connected, everything is fine.

If we use 100 Mbit/s, or unplug the multicast source, the link goes up without any problems.

To rule out board issues, we have reproduced the exact same problem with the STM3220G-EVAL board using the demo software from ST, and just tweaking ANAR.

Looking at the signalling with an oscilloscope, it seems as if the switch decides that the link is up and starts letting the multicast packets through, but the DP83848C does not think that the link is up and gets upset about the traffic and restarts autonegotiation. I realize that I'm speculating here.

Are you aware of any problems in this area? Is there anything we can try? Currently we are working around this issue by disabling autonegotiation and using 10 Mbit/s half duplex, but we'd much rather use full duplex.

/Tobias