Sometimes the smallest things cause the worst of all troubles...

When building Thor IV I had to deal with various forms of misbehaviour after system power down:

  • BIOS settings being reset randomly
  • RTC jumping to the year 2300
  • system requiring multiple POSTs to detect video

Most of them were gone with the final parts constellation (AORUS Xtreme X570 and RTX 2080SUPER) but after a couple of months in deceptive peace the BIOS issues returned.

I was already about to resign on this matter when I stumbled upon a post on Reddit about the DisplayPort Pin 20 Issue which I had not heard of yet.

Unfortunately I don't have the original link anymore but there are several good articles on the matter online.

Without rehasing it all again here is the TL;DR:

  • DP Pin 20 (DP_PWR) is specified to supply power from one side at 3.3V for active in-cable components like powered adaptors
  • cheap cables violate the VESA specification and simply route that pin through both ends
  • this can cause massive electrical issues when such a cable is used to connect a monitor to a graphics card as both may try to supply power

In my case (having exactly such a cable) my monitor was 'backdriving' my system, feeding power through my graphics card over PCIe to the motherboard, which clearly didn't like it.

Replacing the cable with a proper one that does not connect DP_PWR fixed my BIOS reset issue.