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'[PIC:] Oscilator on Timer1 hangs up !!!!'
2004\01\08@111235 by Gabriel Caffese

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face
Hello everyone,

           I4m using Timer1 with a 32,768Khz cristal for waking up an
16F628
every 15 seconds. Everything works fine, TILL CRISTAL STOPS OSCILLATING !!!
               I4ve put two 27pf caps to ground on each pin of cristal, but it ussually
stops
oscillating after a few minutes. If I put the scope probe on T1OSO or T1OSI,
it starts oscillating
again.
               For solving this problem, have put a 4,7Mohm R in parallel with cristal.
Have changed it up to 10Mohm,
but what only did, is change time till it stops running.
               Also changed cap values, used other cristals, and other 16F6284s
!!!!!!!!!!!

               Does anyne know how should I fix this ??????


               Thanks in advance,

                                               Gabriel.-

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2004\01\08@113312 by Spehro Pefhany

picon face
At 01:18 PM 1/8/2004 -0300, you wrote:
>Hello everyone,
>
>             I4m using Timer1 with a 32,768Khz cristal for waking up an
>16F628
>every 15 seconds. Everything works fine, TILL CRISTAL STOPS OSCILLATING !!!
>                 I4ve put two 27pf caps to ground on each pin of cristal,
> but it ussually
>stops
>oscillating after a few minutes. If I put the scope probe on T1OSO or T1OSI,
>it starts oscillating
>again.
>                 For solving this problem, have put a 4,7Mohm R in
> parallel with cristal.
>Have changed it up to 10Mohm,
>but what only did, is change time till it stops running.
>                 Also changed cap values, used other cristals, and other
> 16F6284s
>!!!!!!!!!!!

Series resistor value?  You'll likely overdrive and destroy the crystal
without a fairly high value resistor. Too high and it won't start
reliably.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2004\01\08@150020 by Jinx

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> two 27pf caps to ground on each pin of cristal

> Also changed cap values, used other cristals, and other
> 16F6284s !!!!!!!!!!!

I always use 47pF. It's a little slow starting but it keeps going

>, but it usually stops oscillating after a few minutes. If I put the
> scope probe on T1OSO or T1OSI, it starts oscillating again

Sounds like a physical fault. I'd check for a short and remove
any flux that may be bridging tracks. How's the humidity where
you are ? But you say you've changed the caps and crystal. Often
that soldering/de-soldering would change or remove the fault

Are the caps really grounded ?

I would have suggested a weirdness in your s/w that turns TMR1
off or does something strange to RB6 / RB7. That doesn't explain
why the turn-off time changes wrt a parallel resistor

However, as you don't know the cause of the problem, you can't
rule anything out, neither obvious nor obscure

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