The photo attached is from the Atmel Mega168 Explained demo board's schematic.
There is a Mega32U4-MU on the board that is a "Board Controller" and provides Debugging for the Mega168.
This Board Controller can turn On and Off Power to the Mega168 through a two Mosfet High Side Driver, (NFet driving a PFet).
The Board controller supplies the "Power Enable" signal.
VccP5V0 is 5 volts from the USB connector.
Vcc_Target is the switched power that powers the Mega168, the micro featured on this Xplained board.
IF one was to want to supply power to the Mega 168 from another power supply, by feeding one's V+ to the Vcc_Target rail, while the USB was NOT connected, would it damage the PFet in the High Side Driver? (i.e. One is applying +5 V to the PFet's output, while its input and gate are floating, (not connected))
Obviously the Board Controller would not be powered up, but that isn't an issue in this instance.
I'm not very knowledgeable in P-Channel anythings, bi-polar transistors or PFets, and thought I might get some good guidance from those more familiar with these transistors.
The schematic does show J106. Unfortunately J106 is a very, very small PCB trace, and not a Header that one can jump or un-jump with a connector shorting block.
The PFet's data sheet is here.
Any guidance would be appreciated.