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I am creating a breakout board for the CM4 which should support Gigabit Ethernet and POE. I embedded a magjack onto the breakout board.

I am struggling with the connection stability, however: the CM4 successfully connects and I can measure speeds of up to 1 Gbps both ways, with no packet loss, however after a minute or so the connection drops completely.

I can see console messages saying the eth0 interface has dropped.

My guess is a hardware issue, but I don't know what could it be: either PCB trace design, wrong impedance, or potentially overheating.

In theory, my design matches the manufacturer's requirements for 50 Ω differential impedance.

I did notice the board getting very warm (I'm measuring approx 40 ºC), could that be causing issues?

Finally, when it comes to POE, how should the magjack be grounded? Using the Ethernet-side AC GND, not the CM4 side DC GND, right?

Here is the cut out of my Ethernet PCB traces.

PCB design

Any help would be greatly appreciated!

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