What have I broken?

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,


formant84: I've traced the connections

So you removed all of the components and did continuity testing?

Testing while traces while components are still connected isn't a valid test--especially if something failed as a short.

Yeah i see what you're saying, but given that the usb data lines aren't connected at the socket end there's no closed loop there so i would expect the readings i get to be valid, and if not surely i'd get some nonsensical readings and not exactly the readings i'd expect to get from solid connections? Either way i think when i get a chance i might try soldering some thin copper wire to the data lines on the chip and running them to a separate socket w/ resistors and diodes to see if that does any good.

Have you tried a different usb cable?

Yeah, and my other uno works seamlessly with the same cable and computer.

It's obvious that the board healed. I knew a guy that had a Volkswagen that would do that.

Try reloading the bootloader and start from scratch.

Bob Macklin K5MYJ Seattle, Wa

Tried reloading the bootloader, and reloading the firmware for the 8u2. Both the chips seem to be working fine, its just when i plug it into any computer nothing shows up at all so must be some kind of problem with the usb connection.

Try booting the computer with the unit plugged in. Is the driver already installed?

In my case when I plug it into a Win2K computer it is seemingly ignored. But when I plug it into a WinXP computer I get "UNKNOWN DEVICE"

There were some sketches / hex files floating around that reprogrammed the 8u2 to act as a HID device (mouse / keyboard). As a test, try loading one of those. Maybe as a standard HID device, the Windows logs will give you more details about whether or not it is even trying to enumerate the device.

It sounds like the USB PHY in the 8u2 has failed.