On Jan 3, 2013, at 11:36 PM, Perry Curling-Hope wrote:

> The accuracy of the clock is determined entirely by the crystal

Well, plus the load capacitors/etc, and how often do people take care to ge=
t that right instead of slapping a couple 18pf on the board.

(where does that 6ppm number come from anyway?  If I plug 16MHz crystal int=
o digikey's search engine, I get a selection of crystals ranging from 9ppm =
stability and 10ppm accuracy to 150ppm stability and 50ppm accuracy.)

I'm not saying you can't implement an accurate clock with a crystal, I'm ju=
st saying that it frequently doesn't seem to happen.  I'm pretty sure I've =
seen a fair number of crystal-controller micros that have quite noticeable =
drift when used as clocks (which is, after all, a pretty common hobbyists a=
pplication=85)

BillW



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