I have a m644p in my STK500 and have used it with a 20MHz crystal and it worked just fine.
2 days ago I experimented with some code that was written for the internal 8MHz oscillator so I just changed the fuses in the m644p.
Last night I wanted to go back to the 20MHz signal and the processor just didn't run. Changing the jumper to use the internal clock source (3.686MHz) and the processor runs. Changing the jumper back to use the crystal and - nothing.
I then had a look at the signal on the OSCSEL jumper and I can see a 20MHz signal, but it has 0.2V swing. Even worse, it is superimposed by a 3.686kHz signal, with about 0.5Vpp. On the XTAL1 jumper I measure a 20MHz signal modulated on the 3.686kHz signal with 2.7Vpp and a 1.3V offset. The amplitude of the 20MHz seems to be modulated with another frequency I can't measure and goes from 0Vpp to 1.5Vpp. AFAICT my processor doesn't even pick up the 3.686kHz (which is suspicously a 1000th of the internal clock).
The signals are the same, with and without processor (suggested in another thread). The STK500 is basically new and worked fine before I changed the fuses.
Does anybody have any idea what went wrong? And more importantly, what I can do about it?