Uno Board subjected to over voltage on DI input pin - Toast?

Im new to Arduino. I got a couple of Uno's for my explorer engineering students to do a project with. All boards worked fine out of the box, were successfully connected to the computer and uploaded with the students sketches.

All was well until one student accidentally connected a switch input to his board which was pulled up to 12V on the high side of the switch. No current limiting resistor was present in the circuit. The input was connected to pin D8 on the Uno.

We were watching the output from his sketch on the serial monitor at the moment of failure. When he flipped the miswired switch serial comm was immediately lost. It took us a few moments to realize what had happened. By then the AtMega chip was quite toasty.

I got some spare AtMega chips off Amazon that had the boot loader pre programmed. I put one of them in (that DIP28 socket is unusually tight!) and I expected to be off and running again, but it still does not work.

Here is the testing I have done so far;

  1. 5V reference - 4.96V
  2. 3.3V reference - 3.31V
  3. Windows PC recognizes the USB port and opens a com link
  4. I see the RX LED flash a little bit when I try to upload a sketch, but not nearly as much as usual
  5. I tried swapping the new ATmega with one out of the other Uno board (that wasnt damaged) just to ensure the new chip works. The new chip works fine in the known good board, and the known good chip has the same problem in the damaged board.

I have attached the error message I get when trying to upload a sketch.

My question is, is there anything else worth testing? Or just chuck it an get another one?

Remember, if you apply 12V to an input pin of the processor, it basically comes out of the Vcc pin of the processor and will power everything with 12V.

I would start with the loopback test. If that fails, the USB-to-Serial chip (16U2 on original Arduino Uno) is buggered as well.