AVR-Chat

On Wed, Apr 16, 2008 at 9:27 PM, Graham Davies <Yahoo37849@ecrostech.com> wrote:
> --- 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.


Sorry, long day.. Still the difference isn't large, and is way closer
than "22pF xtal, 22pF caps"
:)


>  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.

Agreed, if the rock is wrong enough to cause serial to notice it, then
something is VERY VERY wrong.   I asked also about VCC bypassing,
didn't hear a reply.  Some folks think those caps are optional.