Have just encountered a very strange phenomenon whilst trying to program a board. Its a custom board with m664 @ 20MHz. 100s in the field all have been fine til this one.
I can read the signature, the fuse bits, lock bits. I can even verify the flash and eeprom contents but for some strange reason a write action has no affect. I can't write flash, eeprom or any fuse bits and a chip erase also has no affect.
The heartbeat LED simply pauses briefly with every action to enter programming mode.
I believe all connections are good as I can talk to it. I believe my ISP MKII is good as I've successfully programmed another board before and after this problem.
I have just noticed the firmware also cannot write to its own eeprom when I try to change setting.
Anyone seen something remotely similar before? Know of any causes/solutions?