AVR-Chat

At 09:46 AM 3/14/2004 +0000, Brian Fairchild wrote:

>They are not equal on the STK300 and there is no reason they should be equal.

Sorry, that's wrong. 

The two caps absolutely should be equal, unless you have a good reason for them not to be.  The two caps are in series, from the crystal's point of view, and in parallel with the parasitics on the board.

The formulae from the various crystal manufacturers to determine the crystal cap values all reflect this, and they all result in equal values. 

Even when you are tuning the crystal to a specific frequency, your goal is to arrive at equal values, but being a few pF off balance isn't as important as having the crystal on frequency, in those applications.