Although I have had more than my share of dead MEGA's from debugwire mishaps, this is a new one for me.
After exiting a debugwire session (which appeared to end smoothly), I went back to the STK500 to program some fuses. Although the STK500 would connect, the signature read from the device was 0xFF,0xFF,0xFF. The fuses, flash, and EEPROM would program/verify just fine. But signature was wrong. After checking the usual suspects like ISP clock rate, etc, the signature still wrong.
How can the signature get erased ? Are the signature bytes hard coded or is it in a "protected" flash or eeprom area ? Also, it appears as if the OSC calibration value is also erased (0xFF).
I know that the ultimate resolution will be to replace the part. Has anyone seen this before ?