My apologies in advance if this has already been asked
I have a MCU (esp8266 or esp32) which will primarily live off mains power with transformer 110VAC -> DC 3.3V
Occasionally USB is plugged into the device for programming and diagnostic. Expect a CP210x, FTDI or similar connected to the esp.
I'd prefer not to 'cut' the USB +5V power line to the device, or in other words, I'd like both power sources to be able to coexist. What would you recommend?
Additional consideration: some of the boards considered (like Wemos D1) already have the CP210x hard wired to power the device
The design will have a 5V -> 3.3V regulator onhand for powering from USB.
I am interested in powering the device from USB for the programming and diagnostic portion. I'd prefer not to have to unplug the mains -> 3.3V supply at that time, and would also like the option of powering the device solely from USB occasionally.