I've read numerous posts on crystal load capacitance "“ mainly questions about how to calculate values.
But why these capacitors have to be there, doesn't seem to be covered. It's mainly written off as "good practise", and that's that. This use of the "good practise" term may also deter further questions. The "I must be too stupid by asking this question" soft of thing.
After reading a number of informative pages, I've concluded that the only reason for adding load capacitors is pure and simple frequency trimming. It has been mentioned here and there that the load capacitors have some (very little) influence on the clock frequency, but I was surprised to see that there is apparently no other reason for adding them. A couple of examples:
Now for my own experience "“ which was about 10 years ago. The first time I used an ATmega128, it was on a sort of piggyback CPU board I'd made. And as the datasheet said I should, I added the mysterious "load capacitors". I'm not longer sure what the value was, but I think it was 12 pF. The crystal frequency was 14.7456 MHz.
The clock wouldn't run at all. No chance. Nothing. Then in desparation, I removed those cursed capacitors. And - VOILA - there be clock cycles! Ok, there might have been something wrong with one of the capacitors, but I don't think I tried to measure that. I was just happy to see oscillation. And it was probably late in the evening...
Since then I've not even bothered adding the load capacitors on later circuit boards. The frequency accuracy is (for my purposes) absolutely adequate without them.
Some application notes say something like "Start with 1 - 2 pF, and then increase...". Why not just start with zero? I've never ever had an AVR that wouldn't run at a stable frequency without load capacitors. So my suggestion is: Use them for trimming (if necessary), or don't use them at all.
My purpose with this thread is not to pick on those who by habit just add 2 x 22 pF, but rather to de-mystify load capacitors for those who are as confused as I have been. If anyone has more scientific reasons for adding load capacitors, please share your knowledge. I love learning new stuff. :wink: