power to UNO - both wall wart and usb

New to Arduino and have not found any explicit answer to this issue.
The Uno can be powered from a computer via usb or via Wall-wart (~9v) according to the write up on the Uno

Can I plug both sources into the Uno at the same time?

What I want to do is use the wall wart for the normal power and from time to time attach a computer to get updates/status from the Uno.

Thanks in advance for any advice.

Yes. If the barrel jack is >~7.3V, the barrel jack voltage will be selected for power instead of USB for power.

HI My wall wart is ~9vdc. Just to be absolutely clear, plugging a usb cable into a computer will allow me to see what the Uno is reporting and then unplugging the usb cable is safe to do? Thanks and have a nice day.

Yes. Plugging in will likely reset the Uno unless you cut the Reset Enable trace on the board.


Thanks for your quick reply.

So, I can plug the usb cable in (with the Uno powered from a wall ward) but the Uno will likely reset itself. Maybe? So, if this happens, I will loose any stored variables.

But, if I try it, I will not hurt the computer or the Uno! Is this correct?

Have a nice day.

You will lose variables when it resets, yes. You can cut the Reset Enable trace and they will not get lost. The Uno/PC will not be hurt when you connect/disconnect from the PC.

HI Great. Thanks. I can try this safely and if need be, I will cut the Reset Enable trace. Have a nice day.