I want to connect USB to my ATmega32U4 microctroller, which is powered by an external power supply. I was thinking it might be possible that the USB +5V/GND would be different than the +5V/GND I am getting from external supply. From some reading, this seems unlikely if the external power is a wall wart, but would be possible if the power source was a battery or some non-earth grounded power supply.
The ATmega32U4 has a UGND pin for "USB Pads Ground". Can I connect the USB cords GND wire to this and keep the other GND pins connected to the external power supply? Should I even worry about this if I assume I will never use a battery? (I am planning on always using a wall wart)