BenF:
If you’re using a pair of 18pF capacitors with a recommended crystal Cl of 18pF, you’re out of range. In such cases it may work on some AVR’s, but marginally so.
BenF, yes that's what I have on the breadboard setup. OTOH, the target board came with the crystal and caps, don't have xtal specs, so have to assume they did it right. Appreciate your input, I'm not terribly familiar with crystal oscillators, so I learned something!
In what way might things be marginal? I did happen to check my clock frequency on the breadboard as a little experiment recently, and it was roughly within 0.002%. The crystal I'm using is Digi-Key part number 887-1244-ND, along with 18pF caps as I said.
The ATmega328P datasheet gives "initial guidelines" of 12-22pF for C1 and C2 when operating between 8 and 16MHz, so I thought I was in the ballpark. Of course they don't know what crystal people might select. Maybe they should have refrained from giving that advice?