i have 2 circuit boards with the 16u2 cpu. I used a usbTiny device connected to the cpu's SPI interface to do 1)full chip erase, 2)unlock the lock bits, 3)program the LOW, HIGH, and EXTENDED fuse bits with different values. 4)Verified the new values. Did this on both boards.
then i took one of the boards and connected direct to computer USB, verified with Device Manager, ran Arduino IDE, and programmed the chip with an Arduino bootloader. success.
But the second board will not enumerate on the computer USB. Dunno why but now i guess i need to program the chip again by SPI with the Microchip USB firmware. But i can't find that firmware.
Anybody know where to get it?