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'[EE] DS1307 presicion'
2004\04\29@104913 by Mauricio Jancic

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part 1 757 bytes content-type:text/plain; (decoded 7bit)


Hi all,
       I've been working for a long time with the DS1307 and have
encountered always the presicion issue quite anoying. For example, the
attached JPG containd both top and bottom layer and schematics of a board
that I make and that layout make my circuit that after 2 months it was 10
minutes behaind another Dallas clock (the one that comes inside the iButton)

       Any ideas?

Best regards

Mauricio

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part 2 45349 bytes content-type:image/jpeg; (decode)

2004\04\29@111624 by Samuel BOUQUET

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If i remember correctly, you have to use a specific crystal, with a 6pF
capacitance. (that's what is use with DS1302). Check the latest
datasheet on maxim's website, they have modified dallas datasheets, and
some data are more explicit...

SamB

-----Message d'origine-----
De : pic microcontroller discussion list [.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU]
De la part de Mauricio Jancic
Envoyé : jeudi 29 avril 2004 16:48
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Objet : [PICLIST] [EE] DS1307 presicion



Hi all,
       I've been working for a long time with the DS1307 and have
encountered always the presicion issue quite anoying. For example, the
attached JPG containd both top and bottom layer and schematics of a
board that I make and that layout make my circuit that after 2 months it
was 10 minutes behaind another Dallas clock (the one that comes inside
the iButton)

       Any ideas?

Best regards

Mauricio

--
Outgoing mail is certified Virus Free.
Checked by AVG Anti-Virus (http://www.grisoft.com).
Version: 7.0.230 / Virus Database: 262.9.10 - Release Date: 28/04/2004



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2004\04\29@160135 by Jose Da Silva

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On Thursday 29 April 2004 07:47, Mauricio Jancic wrote:
>Hi all,
>        I've been working for a long time with the DS1307 and have
>encountered always the presicion issue quite anoying. For example, the
>attached JPG containd both top and bottom layer and schematics of a board
>that I make and that layout make my circuit that after 2 months it was 10
>minutes behaind another Dallas clock (the one that comes inside the
> iButton)
>
>        Any ideas?

Crystals can be expected to be 5 digits steady, but you can't expect them to be 5 digits accurate from crystal to crystal to crystal. If you want to do that, then you will need a trimmer capacitor of some form (after all, capacitors are only 1% exact at best from cap to cap to cap).
The iButton one would be tuned in-factory versus your discrete version (which may be down to a polished science (like wrist watches)).
We used to have a trimmer capacitor for some products, while with other products we used hand-select different grades of capacitor to compensate for temperature as well (low power, low drift, mountaintop repeaters).

From past experience, if you want to be very accurate, you need to run your product atleast 24 hours before attempting tuning (you're slightly aging the crystal plus putting it at operating temperature).

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2004\04\29@165436 by Paul Hutchinson

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Be sure to use a 12.5pF crystal per the DS1307 data sheet. See Dallas App
note 58 for excellent information and troubleshooting instructions.

What is the accuracy of the 32.768kHz crystal you have? A 20ppm crystal is
accurate to about 1 minute/month at room temperature. A 200ppm crystal is
accurate to about 10 minutes/month at room temperature. The Dallas DS32KHZ
TCXO is accurate to about 1ppm at room temperature and about 2ppm (~1
minute/year) over a temperature range of 0 to 50 degC.

Your PCB layout is less than ideal, remove that piece of ground plane that
is between the crystal and IC (it is messing up the capacitance). See Dallas
App note 58 for a lot of good info including the preferred PCB pattern for
Dallas RTC's and crystals.

Hope this helps,
Paul

> {Original Message removed}

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