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From: email@example.com (Tim Shoppa)
Subject: Re: 100Hz from 25.6kHz crystal.
Date: 4 Dec 2002 15:27:50 -0800
References: <5Q1H9.32847$zX3.firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 4 Dec 2002 23:27:50 GMT
"John Jardine" wrote in message news:...
> John Fields wrote in message
> > On Tue, 3 Dec 2002 14:46:52 -0000, "John Jardine"
> > wrote:
> > >
> > >That is one weird crystal frequency!. Crystals that work at such low
> > >frequencies are not much good at keeping time
> > ---
> > Like the 32768Hz. ones in most of the watches in the world?
> Yes ... The watch crystals keep 'customer acceptable' good time.
> Xtal wise, the watch manufacturers are limited in what they can choose to
> use and this is set by the power required by the xtal oscillator circuit and
> the following binary divider chains needed to get down to 1HZ. This results
> in the 32768Hz compromise.
> Compare this 32768Hz timekeeping, against that of the clock in your auto,
> where the designers didn't have to worry about power consumption.
> The vehicle clock will invariably be fitted with a higher frequency 4Mhz
> xtal. This xtal is naturally much more stable over time and temperature and
> results in about 10X better timekeeping!.
??? 32.768 kHz 20 ppm crystals cost about twenty cents each. 4 MHz 50 ppm
crystals cost several bucks - and most 4MHz crystals you find will be
100ppm or 200ppm.
TXCO's can be constructed with either. I really like the Dallas 32.768kHz
modules with TXCO's in 'em; they're typically good to 2 ppm over outdoor
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