What do the two caps connected to the crystal do?

What do the two caps connected to the crystal to ground do?

(C2 and C3 as seen here: http://www.arduino.cc/en/uploads/Main/Arduino-Diecimila-schematic.pdf

The schematic calls for two 22pF caps. If I wanted to breadboard/“one-off” an arduino clone, do I need to stick to that exact value?

They insure that the crystal ‘sees’ it’s proper capacitance load so as to accurately start and oscillate at it’s specified frequency. It’s not super critical as to value, the next size higher or lower should still function OK. Many people use 3 terminal resonators instead of crystals as they have their caps built in.

Lefty

More on crystal:cap. values here
=> http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1239377960

/david

Thanks - both posts were helpful.