While working on a schematic for a small development board, I was curious how much work it would be to support an external voltage source (ie. bench-top power supply) in addition to the ICSP header and USB port.
For two sources this is pretty straightforward - I would normally run VBUS from the USB port to the emitter on a BC807 PNP transistor with the base driven by the ICSP power pin (self-power dominant configuration).
I've never worked with more than two sources, so this is a bit of a puzzler to me. Ideally the user would be able to (mistakenly) wire VDD to multiple sources without damaging the board.
(Be kind, first post!)