AVR-Chat

On 2/14/07, weston54 <weston54@yahoo.com> wrote:
>
> 33pf caps??  The crystal is 10mhz, and stamped right on the package
> from Digikey is that it's rated for 18pf caps.  I'm not sure I
> understand why you recommend 33pf caps.


Sigh..

The Cl value is NOT the size of caps to use.
See app notes at FOX crystal, they are probably the most easy to understand
that I've seen.

The crystal sees the loading caps as being in series, so the proper way to
size the caps is to assume maybe 5pF for stray capacitance, and since the
caps are equal value you can simplify the parallel capacitor equation to
twice the desired value, so (18*2) = 36, - 5 = 31.  Closest standard value
is 33pF.


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