AVR-Chat

--- In AVR-Chat@yahoogroups.com, "David VanHorn" <microbrix@...> wrote:

> The chip capacitance, like the
> loading caps, are in series,
> from the crystal's point of
> view, so 5pF is reasonable.

Then, your algorithm should be to subtract 5 pF before you double.  
Once you've doubled, you're looking at capacitance to ground.

> The problem with more loose
> methods ...  You really should
> do a full design ... But few
> do.

This is hard to disagree with, but it is my experience that if you pick 
a crystal that billed as being suitable for microprocessors and load 
capacitors that are in the ballpark, it will oscillate at darned close 
to the rated frequency.  Take, for example, my STK500 expansion board.  
The crystal traces wander all over the STK500 itself to pick up each of 
the target sockets, then up the expansion connector to the crystal, 
which is in a socket, and the load capacitors, which are 20 pF without 
any fancy math.  And yet, every combination of microcontroller and 
crystal I've plugged in has worked.  My guess is that the OP in this 
thread is looking at a problem far more fundamental (pun not intended) 
than being a few picofarads off in his selection of load capacitor.

Graham.