Arduino Uno SMD not recognized by Windows 7

I have an Arduino UNO SMD that showed up as a COM port, as expected.

Last night, after a reboot, it stopped being recognized as a viable COM port and appeared as an unknown device.

This happens with two different boards, with numerous restarts in between, different USB ports, and using external power. Uninstalling and reinstalling the driver does nothing to help.

Any ideas why this may be happening, or advice on how to make it work properly?


Hey, I think I had this happen to me too. Have you tried other computers? If the arduino cannot be recognized by more than one computer, the board itself is probably bad (as opposed to wonky software on your computer). Later I figured out that the program on the ATmega8U2 had become corrupted. If this chip doesn't ring a bell, it's the one that lets the main microcontroller on your board (ATmega328) communicate to and get programmed by your computer through USB. After fighting through some stuff, I figured out I needed to put the firmware back on the 8U2. After this was completed, the computer recognized the board and I had a working Uno again!

I couldn't find many good resources on reprogramming the 8U2, so after attempting it successfully, I blogged about it. You can find it here:

Hopefully that will help you out, if this is the same problem you're having!

Another way that could work is located here: This way, though, you don't need a programmer, but you may need to make some temporary board modifications.