So a while back I killed my first Arduino Uno. I think I might have shorted the 5v and ground, though not exactly sure. I assumed I'd killed the 8u2 chip because the board powered on and carried on running my last sketch but it didn't show up on the computer at all. I bought a new Uno, found that my original 328 chip was fine which was nice and thought no more about it.
Then last week I came across the old board again and decided to have another look with my increased knowledge. Lo and behold, the 8u2 is fine! I can fully communicate with it and the 328 using a USBasp programmer via the ISP headers. The power circuitry seems to be fine as it's powering the board, 328 is getting 5v etc. I've traced the connections from the USB socket to the 8u2 with a multimeter and they all check out. the diodes to ground on the USB data lines seem to be okay (at least they only conduct in one direction, I assume this means fine). The connections from the 8u2 to its timing crystal are also okay.
So the question is...what is broken? i know there's plenty of other parts on the board, but i would have thought that if the USB data lines, the power connections and the connection from the crystal to the 8u2 are all fine then it should at least show up when I connect it to the computer? I've used USBasp to upload a fresh copy of the Uno USB to serial firmware in case it was somehow corrupted but that's had no effect. Is it possible the USB interface to the chip itself is damaged? I wouldn't have thought so just from shorting the power supply. I can't see any visible signs of damage to any of the components or tracks!
So What Have I Broken...?
Thanks in advance,