I have a clone Arduino MEGA board which uses an ATmega8u2 as the USB to serial link. It works fine.
What I did was to change the low fuse of the 8u2 from 0xFF to 0xF7 (i.e. setting CSEL3 to 0) to set the crystal oscillator from normal to full swing mode.
This caused the 8u2 to stop working (the oscillator stopped). I could not even reset the fuse to 0xFF.
In order to unbrick the chip, I had to "borrow" the 2560 crystal oscillator and jump X2 of the MEGA to X1 of the 8u2 to give it a clock and allow re-flashing.
After setting the low fuse back to 0xFF, it worked again.
Any ideas as to why the full swing crystal setting would STOP the oscillator? I use full swing on everything else (that supports full swing) without any trouble at all.
Any info will be appreciated.