unebonnevie 
Hi,
I accidentally programmed my ATMEGA644's fuses to have LOW FUSE: 0xC1 and HIGH FUSE: 0xDD. From https://www.engbedded.com/fusecalc/ , I see that the ATMEGA644 now has "Ext Clock; Start-up time: 6 CK + 0 ms; [CKSEL=0000 SUT=00]" What does this mean? I think if I can feed the external clock with correct freq, I can revive the ATMEGA644?
Thanks for any tips!
Correct.
Correct frequency can be almost any frequency. The only caveat is that the ISP clock MUST be no faster than 1/4 of that external clock frequency. Often, you can use the CKOUT pin of another MCU to make that signal, though you MAY have to enable it in software.
Jim
I use a 74hc14n as a ring oscillator to jump start my chips. 3 inverters 4 resistors and you're done.
Or an HC inverter with a stabilizing (feedback) resistor and a crystal. don't bother with load caps because it does not even have to be really close to the printed crystal frequency. Really easy to make on a spring breadboard (and the spring contacts give you close to the capacitance you need, anyway). To get really fancy, add a second inverter to clean up the oscillator output and give it full logic levels.
Jim.
unebonnevie The only caveat is that the ISP clock MUST be no faster than 1/4 of that external clock frequency
Thanks! I think AVRDude has such an option. AVRDude option below:
-i delay
-i delay
For bitbang-type programmers, delay for approximately delay microseconds between each bit state change. If the host system is very fast, or the target runs off a slow clock (like a 32 kHz crystal, or the 128 kHz internal RC oscillator), this can become necessary to satisfy the requirement that the ISP clock frequency must not be higher than 1/4 of the CPU clock frequency. This is implemented as a spin-loop delay to allow even for very short delays. On Unix-style operating systems, the spin loop is initially calibrated against a system timer, so the number of microseconds might be rather realistic, assuming a constant system load while AVRDUDE is running. On Win32 operating systems, a preconfigured number of cycles per microsecond is assumed that might be off a bit for very fast or very slow machines.
