Serial Console

[pfeerick]

You almost always need ground to be common (i.e. all grounds connected together) in electrical circuits, as it's the reference that makes other voltages 'positive'. And even when you don't connect the 5v of the usb uart or serial console cable, you may have fun with the tx line of the usb uart/serial console cable supplying just enough power to make your device not want to boot properly... I used to just leave the tx connection off if I was only wanting to monitor (i.e. receive) or capture boot logs, or only connect a few seconds after powering up the board.