problem with a 32768Hz crystal

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xpzwhisper
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problem with a 32768Hz crystal

Post by xpzwhisper » Wed Mar 31, 2010 2:50 pm

Hi, everyone

i'm posting first time here :)
I have a problem with a 32768Hz crystal. I have set it up like shown on the picture I've provided and I divided the frequnecy with three 4024 7-bit (÷128) ripple counters. It works but the length of the "second" is too short (i'd say maybe 900ms), I noticed it by watching obviously but confirmed it with a stopwatch.. I don't get what is wrong.. I have some suspicions, though. I think the 1M resistor has got something to do with the time, because when I change its value the length of the time changes as well.. That makes me ask a question: how come the frequency can be dependant on the resistor, I thought the crystal has got its natural oscillating frequency..And if you're asking where did I get this configuration it's from A level Electronics book.

but yeah, I am a little confused.

hope you guys know what i'm doing wrong :)
thanks in advance
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leon_heller
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Re: problem with a 32768Hz crystal

Post by leon_heller » Wed Mar 31, 2010 6:43 pm

The crystal isn't controlling the oscillator! Use an oscillator circuit designed for that type of crystal.
Leon Heller
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riden
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Re: problem with a 32768Hz crystal

Post by riden » Wed Mar 31, 2010 7:01 pm

Crystals have specific loading requirements and it looks like your circuit is making the grade for the crystal. Is this a "standard" watch crystal? If so, perhaps the following oscillator might work better: http://www.elecfree.com/electronic/3276 ... h-crystal/
Ralph

xpzwhisper
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Re: problem with a 32768Hz crystal

Post by xpzwhisper » Wed Mar 31, 2010 11:42 pm

Thanks you for your replies.
I don't know much about crystal. I will try this "32.768 KHz oscillator using a watch crystal". I'd like to understand how does the crystal physically work and also I would like to know what sort of crystal should I use in order to use the circuit I have.

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leon_heller
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Re: problem with a 32768Hz crystal

Post by leon_heller » Thu Apr 01, 2010 3:04 am

Most crystal manufacturers have lots of information on crystal oscillator design.
Leon Heller
G1HSM

xpzwhisper
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Re: problem with a 32768Hz crystal

Post by xpzwhisper » Thu Apr 08, 2010 1:02 pm

okay.. but does anyone know what crystal I can use in such configuration I have provided.. there must be a type of crystal.
and also in the page http://www.eleccircuit.com/32768-khz-os ... h-crystal/ there is a variable capacitor required.. can you tell me about that a little bit.. like what valuse should it be ranging between and where to get it :)

n1ist
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Re: problem with a 32768Hz crystal

Post by n1ist » Thu Apr 08, 2010 4:46 pm

If you look at the link in your last post, the crystal is *across* the inverter, not in series with the signal. As for the cap, I'd guess a 30 or so pF trimcap. You could also just try caps around 30pF and see if things are close enough.
/mike

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