First my english is no so well,but i'll try my best to make you understand my problem.
Dear all, I was learning the arduino uno rev3 schemtic, and i read about the overview about Uno 3 from the mainpage ,it says that "The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically."
and now i was confused by a question. Think about this ,if thers is no external power supply to the board, and when Uno connected to USB,now the board should be powered by USB, and USBVCC is connected to a PMOSFET FDN340P which is a delpletion mode and should be supplied a negative volt to its G pin to get on,but from the schemtic we can see the gate pin was connected to an OMP LMV358 and when there is no external power supply the OMP's output should be 0? But if the pmosfet is not on how can USBVCC pass through it to supply a 5V to rest part of the board.