Well, obviously that voltage will kill the avr on the board I designed.
I have a simple circuit based on the Arduino Leonardo, with two shift registers to read 25 push buttons.
When my boards arrived from the fab house I was excited, and connected everything but when plugged, the TX/RX leds were dim, and a couple of minutes later it started to smoke.
While troubleshooting I removed the dead avr and leaved the traces free and clean. I connected again the USB and the RX led turned on! And it is not connected to anything. So after a lot of hours I set the multimeter to AC and read 6V AC in the LED. So then I placed the probes on the USB power lines and I read 10V AC.
Seems that the trouble is that there's 10V AC coming from my USB port. I know that's not good, but aside, how can I protect the circuit against it?
Attached is my sch, layout and photo.
Thanks in advance
*edit: added images