I have a project which needs two use two Nano boards as I need one code loop to run uninterrupted, and in under 100us for intermittent fault detection, and another board to send data to an LCD to display the results. Because of the time taken to talk to the LCD this cannot happen in the code loop that's doing the intermittent testing, and I'd rather not interrupt that.
I have put both Nanos into a breadboard (facing opposite directions so I can access the USB connectors) and have connected each one so that it's Vin pin connects to a positive rail of the breadboard and it's gnd pin connects to a negative rail. Both sets of power rails on the breadboard are connected together so that I can supply one source of unregulated power and have both Nanos receive it at Vin.
The issue comes when I connect USB to one of the boards, and both of them power on, presumably because current is flowing back out through the Vin pin of the USB connected board and into the Vin of the other. I understand that the second board is probably not receiving an adequate power supply as the voltage arriving at it's Vin is below the regulator dropout voltage, but will it cause damage if I allow it to receive power while programming the other? I can pull the jumper between the rails to solve this, but it would be nice to know.
I've already had it connected like this for about an hour before it struck me as odd that both boards had their power LEDs on, I presume the 40mA current limit may not apply to the power pins as the board uses more than that at idle, and they are not Atmega pins but rather supporting components. Is the regulator OK with this?
Thanks for your help,