AVR-Chat

> I've always used 15pF without problems on all boards.

Sigh..

Where do you get these crystals with the appx 6pF loading spec?
Normally such crystals are rather expensive and hard to find. 

Have you checked your oscillator margin?

Do you <KNOW> that you don't have problems, or things just sort of 
seem to be working ok, more or less?



Unfortunately, there is a bit of design work required to get it right.

First, the crystal determines the cap values, in it's loading cap 
spec. If your crystal dosen't have a loading cap spec, then it's a 
series mode rock, and you shouldn't use it in a parallel mode circuit 
like this. It won't operate at the nameplate frequency since the 
parallel and series resonant points cannot be at the same frequency, 
and it may not even start up reliably in this mode.

Second, from the crystal's point of view, the caps are in series, so 
they need to be almost twice the value specified by the crystal. 
Almost, because there is always some amount of stray capacitance on 
the board. You can guess at 5pF and start there. CL = (2*Cs)-5 

Third, you need to check drive power, it is possible to damage or 
even shatter a crystal with too much drive.  This involves a 
measurement of drive current and crystal impedance, see data sheet. 
Power is of course I^2 * R where R is the crystal impedance at 
resonance. You can add a small value resistor in series with the 
drive pin, and measure voltage across it.

Fourth, it's always a good idea to do a margin check, which involves 
adding series resistance between the micro's output pin and the 
crystal. You adjust this value upward till the oscillator just starts.
This value should be at least three times the crystal's ESR value, 
and higher ratios are better.

With the AVR in particular, there is one more consideration, the 
CKOPT fuse should be programmed. The default (unprogrammed) state 
enables a low power "vittoz" mode oscillator that typically operates 
at about 1/2VCC amplitude, and will cause significant problems unless 
the crystal is specifically designed for this mode.