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'Help -- series resistor for low freq osc'
1999\12\28@193508 by Ken Webster

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Could someone with experience please give me a ballpark figure for a series
resistor for using a watch crystal (32.768kHz) with a PIC?  I'm not sure how
much drive level these things can take but two of them have bit the dust
already so appearantly it isn't ok to use them without a series resistor.

If it makes any difference I am using them with the Timer1 oscillator on a
PIC16C73.

TIA,

Ken

1999\12\28@194743 by Erik Reikes

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At 05:35 PM 12/28/99 -0700, you wrote:
>Could someone with experience please give me a ballpark figure for a series
>resistor for using a watch crystal (32.768kHz) with a PIC?  I'm not sure how
>much drive level these things can take but two of them have bit the dust
>already so appearantly it isn't ok to use them without a series resistor.
>

Maybe make sure you are in LP mode?


I've used them (albeit not for too long) and didn't have any problems....

-Erik Reikes

>If it makes any difference I am using them with the Timer1 oscillator on a
>PIC16C73.
>
>TIA,
>
>Ken

1999\12\29@030809 by William K. Borsum

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At 05:35 PM 12/28/99 -0700, you wrote:
>Could someone with experience please give me a ballpark figure for a series
>resistor for using a watch crystal (32.768kHz) with a PIC?  I'm not sure how
>much drive level these things can take but two of them have bit the dust
>already so appearantly it isn't ok to use them without a series resistor.
>
>If it makes any difference I am using them with the Timer1 oscillator on a
>PIC16C73.

I use a 100K CARBON COMPOSITION resistor in series with the watch crystals.
All of the new die shrinks from microchip have trouble starting the
crystals in LP mode.  Our PCB's are so clean electronically there is
apparently not enough noise floating around in the system to kick the
crystals into oscillation.  Carbon Comp resistors are noisy! We get 100%
consistent--and fast--startup with them--and 100 % no start without.

Also, be sure the load caps are correct--most watch crystals I get from
Digikey or Mouser are around 12.5 pF--remember this includes trace
capacitance, etc.  NOT critical, but it does seem to help to be close.

kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<spam_OUTborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>San Diego, California, USA

1999\12\30@125827 by Andre Abelian

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Ken,

ballpark figure is not going to work because crystals are always different.
this is what you need to do.

1. connect 1k pot serially.
2. you need a scope to measure the sinusoid signal.
3. you must know that in the probe you have a capacitor about 15-45 pf
   when you probe it  will add that value to it and it may shift the wave
form
4. if you can not get a probe without the cap in it then use 2 same probes
by
  connecting on both crystal pins this way it will add equal values on both
crystal pins
 witch is much safer or if you already know  the value of your probe add an
other cap
 on the other side.
5. slowly turn the pot until you get sharp signal
6. once you got good signal check the value of the pot and match with
   closest resister you can get a few ohms + -  may not even affect .
7. I did this once using 4 mhz crystal up to 200 ohms I didn't even see any
changes.
   I ended up with 820 ohm.


Andre




> Could someone with experience please give me a ballpark figure for a
series
> resistor for using a watch crystal (32.768kHz) with a PIC?  I'm not sure
how
{Quote hidden}

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