i have an Arduino Mega2560 board.
i must build an external board with other TTL logics, so i have a unique 5V power supply, for either my board either Arduino Mega.
can i connect my power supply directly to 5V pin of Arduino Mega board, without pass through external power connector X1 ?
i must use usb connector to communicate with PC while the board is working, what's happen to usb supply ?
usb ground jumper must be closed or leave open ?
ATmega2560 have 40mA max current output per pin, but how many outputs pins can give this current simultaneously ?
i think the problem is the control of mosfet T1, it depend on state of VIN, that is 0V when the board is powered from +5V pin directly.
i read the thread you link, so there is also another problem due to voltage regulator that in this condition, is reverse powered.
i think that with 2 little modification on the board the 2 problem can be solved (i will test ASAP).
1- a diode with anode to +5V and catode to VIN to protect the voltage regulator IC1 from reverse powering.
2- remove resistor RN5D so also with 4,4V ( 5V-Vak of new diode) can trigger the comparator IC7B
modify the board in this way, let all functionality as before, it can be powered either from usb, X1 connector, and +5V, but when the last 2 method are used, usb power will remain isolated from board.