I previously set up an ATTiny84 with a 20 MHz crystal and 2x 20pF caps. I could get it to run reliably without CKDIV8 enabled.
I'm now transplanting those caps and the crystal into my ATMega324P project, and while I can run with the external crystal, it doesn't want to play nice without CKDIV8. (i.e. it gets unpredictable, and I don't even care to try and analyze what the heck is going on)
The odd thing is, it has no problem running off the internal oscillator without CKDIV8. Therefore, with the internal osc I can get 8MHz, but with an external 20MHz crystal, I need CKDIV8 so I only get 2.5MHz.
My guess at the moment is that some electrical characteristic of this larger chip requires a different capacitance on the oscillator. I'm curious as to why this might be, and if anyone else has had a similar experience.
The 324P is a real powerhouse, so I'd love to get it running smooth with the 20MHz crystal. Thanks in advance!
EDIT: I have startup set to 258 CK + 64 ms