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'PIC16C84 LP Osc Problem'
1995\11\28@172852 by Varady Peter

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I'd like to operate a normal 4MHz, PIC16C84 with an 32.768kHz crystal
(the same one that used in clocks), but I had problems.
I have used of course LP config mode, and 100pF capacitors.
When I measured on OSC1 and OSC2 pins the signal shape and freq looked OK.
(OSC1 fast sinusoid, and OSC2 distorted square, just as at XT mode).
But all other operation was not adequate (no any sign of operation). What
can be the trouble??? Please help!
Thanks. Peter
email: spam_OUTvacaTakeThisOuTspambalu.sch.bme.hu

1995\11\28@183027 by Dr Ben Heller

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On Tue, 28 Nov 1995, Varady Peter wrote:

> I'd like to operate a normal 4MHz, PIC16C84 with an 32.768kHz crystal
> (the same one that used in clocks), but I had problems.
> I have used of course LP config mode, and 100pF capacitors.
> When I measured on OSC1 and OSC2 pins the signal shape and freq looked OK.
> (OSC1 fast sinusoid, and OSC2 distorted square, just as at XT mode).
> But all other operation was not adequate (no any sign of operation). What
> can be the trouble??? Please help!
> Thanks. Peter
> email: .....vacaKILLspamspam@spam@balu.sch.bme.hu
>

Did the chip work when the scope probe was held against the pins?  If so
then you will probably need more capacitance (we had the same problem
with this frequency of crystal).

Ben Heller (B.HellerspamKILLspamSheffield.AC.UK)

1995\11\29@035642 by David Knell

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>> I'd like to operate a normal 4MHz, PIC16C84 with an 32.768kHz crystal
>> (the same one that used in clocks), but I had problems.
>> I have used of course LP config mode, and 100pF capacitors.

>Did the chip work when the scope probe was held against the pins?  If so
>then you will probably need more capacitance (we had the same problem
>with this frequency of crystal).

We found we needed less (22pF each side, as I recall.)  The other thing
you might need is a drive limiting resistor in series with the crystal;
I think (and I might be well wrong here) that a distorted output from the
crystal (i.e. a distorted input to the PIC) indicates overdrive.

The clock oscillator gave more trouble than any other bit of the last
widget we designed..

Dave

1995\11\29@040715 by Tesaro Sandu

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hello people..

we actually found what the problem was.. It wasnt the caps at all :)..

we noticed when we heat them .. they work, then they stop again after 2-3
hours...

anyways.. cutting the long story short...

the problem was the flux we use contains some sort of salts...
when we wash the boards.. these salts wash off.. but they still remain
in the washing liquid... Then when we rince them .. still a little of
the solution remained in the liquid..  when the boards dried.. this very
very very very small amount of salts.. deposited round the crystal/caps
region.. (beacause thats on the bottom of the board)  anyway .. from the
salts it made the PCB conduct !!! Not much.. say 500k to 1 MEg within 2mm
BUT THATS WHAT MADE THE WHOLE PROBLEM!!  Arghh took me a good arm and a
leg to work that out.. would of never suspected that ..

Just thought i'd let yous know incase any have the same problem

reguards Tesaro
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