PC Engines system board schematics
PC Engines hardware may be the most "open source" hardware available on the market. Not only you are getting full access to the source code of the bios, but also you get full access to the hardware schematics. When something breaks, which almost never happens, you can read the schematics and figure it out for yourself.
APU4 board schematics
APU3 board schematics
APU2 board schematics
APU1 board schematics
You can find the latest version of the bios source code here: https://github.com/pcengines/coreboot