Two Nanos one one breadboard with power pins in parallel.


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,


I think that your current solution is the best one, where both boards are always powered together. This will prevent signal levels outside the allowed range of an unpowered board..

If you want to power both boards from a single external source, put a diode before each Vin pin, to prevent current rushing out of Vin.

Thanks, so I won't damage the regulator by drawing about 50mA back through it's input?



Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Showing your power connections and inter connections between the two controllers

Thanks.. Tom... :slight_smile:

No problem, here you go.

I’ll also have a few lines between digital pins to communicate simple information by having one MCU pull the line low and the other reading it, but I haven’t finalised that part yet so I haven’t shown it. It should be faster than I2C as it’s only one pin toggle per message and I don’t need to send anything more complicated than “pass”, “fail”, “intermittent”, “clear”, and mutually exclusive states can share a line.

I understand that connecting an OUTPUT, HIGH to an OUTPUT, LOW would cause a large current flow if there were no resistors, but I’ll have an INPUT_PULLUP on the receive end and signal data with OUTPUT, LOW so there shouldn’t be any issue there.

Would I be better using the 5V pins rather than Vin? This seems like it would still power both boards from one USB connection, but it would avoid abusing the Vin regulator as I seem to be doing at the moment.