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Author Topic: Inaccurate DS1307  (Read 3078 times)
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Lancashire, England
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OK, how do I find that out - without a datasheet for the crystal? I've tried to test it with my multi-meter, but it seems to show open circuit with the crystal unconnected to anything. Is there a way I can try and read the ESR?
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You would need the datasheet for the crystal.

This could be a half-baked idea -- if you set the DS1307 square wave
output to 32768Hz this should output the crystal frequency 1:1. You could
measure the frequency with an oscilloscope (or another Arduino). Any deviation
from 32768 would affect the time keeping accuracy.

(* jcl *)

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Dallas also recommends a separate ground plane for the xtal and related IC pins.  Check the datasheet for details.

-j
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@jluciani
Unfortunately I neither have access to an oscilloscope nor another Arduino, so testing the output would be hard. To be honest I'm not experienced enough at the moment to do that measurement with an Arduino anyway.

@kg4wsv
I'm very green with regards to electronics at the moment, so if grounding the body of the crystal isn't sufficient to achieve the 'ground plane' then can you point me at a resource so I can learn how to do this correctly.
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I've just found a datasheet for the crystal, it has an ESR of 35kOhm, so it should be within tolerance.

http://www.farnell.com/datasheets/75563.pdf
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Looking at the Frequency Range table it looks like 45K. It does not say whether the values are typical or maximum specifications.

Also mentioned in the datasheet is a special crystal "specifically designed for the Dallas
RTC". Did you get the standard crystal or the special one?

(* jcl *)
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I forgot to add the crystals that I like to use with the Dallas/Maxim chips are the
Citizen CFS series. These are specified for a maximum ESR of 35K
(as long as you use a 3x8 or 2x6).

My board is at http://tinyurl.com/y8exe7k

(* jcl *)

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« Last Edit: December 02, 2009, 07:49:18 pm by jluciani » Logged


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Yeah, on re-reading the datasheet I see that it has an ESR of 35kOhm @32.768kHz..

I have no idea. I have the one linked in the original post, but there is no indication if it's specifically for the RTC or not. However, there are only a very limited range of crystals from that supplier, and I suspect that it's the general one. It's handy for me to use CPC (part of the Farnell group) as they have a warehouse outlet next door to where I work.

All the indications at the moment are that the second crystal is working just fine, it appears that the first one may just be faulty.
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Crystals are, er, crystal (quartz crystal), and subject to damage from shock.  If it got dropped on a tile or concrete floor, it could be damaged.

A specific crystal ground plane would be something for a custom PCB design.

-j
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I have SparFuns pre assembled and mine is of by 6 sec/day, will get a new crystal to see if this one is damaged.
Else i will get a DS3231, both cheaper and has a temp compensation.
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