Debugging a "hot CPU" boot failure issue. A custom motherboard design would only boot when the CPU was cold, like ICE cold (put it in a freezer, or hit it with cold spray).
Turns out the bias current node (external RBIAS resistor sets bias current) for PCIe was routed too close to an inductor for a power rail. When the CPU was warm, the power rail pulled more current, causing the inductor to ring more, causing the crosstalk on the bias net to screw up the PCIe subsystem and hang the CPU.
Found the issue accidentally on a layout change. Had to prove it by drilling out the via and re-routing the signal with wire.
Turns out the bias current node (external RBIAS resistor sets bias current) for PCIe was routed too close to an inductor for a power rail. When the CPU was warm, the power rail pulled more current, causing the inductor to ring more, causing the crosstalk on the bias net to screw up the PCIe subsystem and hang the CPU.
Found the issue accidentally on a layout change. Had to prove it by drilling out the via and re-routing the signal with wire.