I looked at the powering schematic for arduinoi nano. Docs say that if i need to use external power i must connect it to +5V pin which is used as +5V output pin when powering from USB. The power selection is made by a diode.
What would happen if i have both powers connected and external power goes very low, say, +3V. I think condition when Udiff between Uusb and Uext is higher than Vf of the diode will (may) create a short cicuit. Am i right about it?
Also, if i need to power peripherals (modules) only when external power is present (need over 1A) should i use ADC to check external power rail going separately from external PS and use MOSFETS as switched to provide power to other parts of circuitry?