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Thread: Anyway to create stable 115.2khz pulse
www.piclist.com/techref/index.htm?key=anyway+create+stable
face picon face BY : Olin Lathrop email (remove spam text)



>     It will be nearly impossible to maintain that kind of accuracy (2-5%)
>     with a 555 timer and the associated R/C components.
>
> Why do you say that?  Aren't the primary variations introduced by
> temperature variations of the RC component values?  How much temperature
> change is necessary for a 5% value shift in a typical component, anyway?
> Doesn't it tend to related to absolute temperature (ie room temp = 300K,
> 5% of which is 15 degrees, which is a LOT for any sort of temperature
> controlled environment (say, indoors where people work), assuming your
> circuit doesn't use enough power to changes its own temperature much.

First of all, I was talking about 2.9%.  You added the 2-5% comment in
parenthesis.  I wouldn't want to run RS-232 with 5% clock mismatch.

The problem is not as much stability as initial accuracy.  It will be very
difficult to find a resistor and capacitor that together have no more than a
3% error.  Most of this error will come from initial part tolerances, but
you should probably allow 1% or so for drift due to time and temperature.
This leaves 2% for the resistor plus the capacitor tolerance.

Yes, you could use a pot and tweak it at production time, but that will be
MUCH more expensive and less reliable than just using a crystal or ceramic
resonator in the first place.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, TakeThisOuTolinKILLspamspam.....embedinc.com, http://www.embedinc.com

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