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NNTP-Posting-Date: Tue, 03 Dec 2002 12:01:05 -0600
From: email@example.com (John Fields)
Subject: Re: 100Hz from 25.6kHz crystal.
Date: Tue, 03 Dec 2002 18:06:34 GMT
Organization: Austin Instruments, Inc.
References: <5Q1H9.32847$zX3.firstname.lastname@example.org> <email@example.com>
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On Tue, 3 Dec 2002 16:05:03 -0000, "John Jardine"
>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!.
I've got the data sheets for an Epson C-2 type 32768Hz and an Epson
CA-310 4MHz crystal in front of me and here are some interesting
TYPE FREQ TOL TEMPCO AGING SHOCK
C-2 20ppm -0.04ppm/°C² 5ppm/yr 5ppm
CA-310 30ppm +/- 0.714ppm/°C 5ppm/yr 10ppm
In the tempco column, the C-2 data is exactly as taken from their
data sheet, while the CA-310 is represented as +/-50ppm over the
range of -10 to 60°C. on their data sheet. I don't really
understand the notation for the C-2 tempco and that's the only
reference they make to it on the data sheet, but if it really _is_
0.04ppm, then it seems the C-2 is clearly a better crystal at
32768Hz than the CA-310 is at 4MHz! Here's Epson's URL for
Professional circuit designer
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