DP83869HM: Link up/down issue with some link partners

Hi,

I experiece some link stability issues with the DP83869HM with some link partners.

The DP83869HM is operated in 1000-Base-X media converter mode.

I have seen this on our hardware design where the copper side of the DP83869HM is capacitively coupled to a microchip KSZ9893R switch chip with 100nF caps. SGMII is connected to a SFP slot with ~10mm impedance controlled differential pair traces w/o any vias. Meanwhile I purchased a DP83869EVM kit and I am also able to reproduce the same issue with the DP83869EVM kit and a PC:

After powering on the board in 1000-Base-X media converter Mode with copper side connected to PC and no SFP inserted the copper the link sometimes comes up, but sometimes it always toggles with link up for ~1 second and link down for ~1.5 seconds. Link up and down can be seen at the LINK led of the EVM board and also on the remote network card. Remote side is a Linux PC with Realtek R8169 network card. Cable length between the ports is 2m, directly connected between PC and DP83869EVM.

When I insert s SFP module with a remote fiber connection link up/down toggle can be seen on the copper side and also on the SGMII side of the DP83869HM.

How can I fix this to get a stable link?

Thanks,

Tom