At [reprap.org] and on the BOM it calls out 2 x 15 pF caps but printed on the board it says 22 pF. Does anyone know why the discrepancy and which one is correct? I am referring to C1 and C2.

Thanks,

Evan

Good spot, best person to answer this would be Zach, i'm guessing its depending on the type of crystal used.

These are decoupling capacitors for the oscillator, you can use anything below 33pF down to 10pf.
15pF may have been the first choice, with what he had at hand, but 22pf is more common to use to decouple the oscillator

No they not decoupling they are load capacitors for the crystal. Their value should be twice the value shown on the crystal's data sheet minus the capacitance of the micro pins and PCB tracks. Other values will work but the frequency will be slightly out.

However, in this application I think the error will be too small to notice. Very important if you are making a real time clock for example, and possibly an issue with on board Ethernet PHYs, etc.

[www.hydraraptor.blogspot.com]

I emailed Zach and the word is that they switched from 22 pF to 15 pF