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'Weird crystal problem'
2004\08\21@075255 by Weiss, Bob

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I am trying to get a DS1305 RTC and a 16f76 to work together and all seems
fine with the communication part but I hit a snag with the crystal end. The
clock refuses to run with a barrell type crystal connected to the X1 and X2
pins but if I put a sqaure wave on X2 it works great. I enabled to
ocscillator in the control register and haven't a clue to what it could be.
I though maybe the crystal was bad so I connected it to the pic instead and
it worked fine. Is there something I'm missing here? Any help appreciated...

Bob

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2004\08\21@085429 by Russell McMahon

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> I am trying to get a DS1305 RTC and a 16f76 to work together and all seems
> fine with the communication part but I hit a snag with the crystal end.
The
> clock refuses to run with a barrell type crystal connected to the X1 and
X2
> pins but if I put a sqaure wave on X2 it works great. I enabled to
> ocscillator in the control register and haven't a clue to what it could
be.
> I though maybe the crystal was bad so I connected it to the pic instead
and
> it worked fine. Is there something I'm missing here? Any help
appreciated...

1.    Welcome to the list - or welcome back perhaps? (Nothing on list from
you in last 18 months plus so i assume you are new.)

2.    There are some miracle working problem solvers here, but maximum data
makes the miracles easier.

Mentat reply: you are doing something out of spec :-)

Full reply:    More data needed.

I assume this MAY be an eg 32 kHz crystal but it's better if you tell us.

What frequency is the crystal? If you don't know, what frequency does the
PIC run at when crystal is connected to the PIC. If you don't know this you
can find out with an oscilloscope or other instrument. If no suitable
instrument, write a blinking LED program that should blink at eg 1 second
rate with an XXX Mhz crystal and work it out from there.

What capacitors are you using with the crystal?

What frequency range crystals does the DS1305 wish to have?

What does the DS1305 say about crystals, capacitors, etc. Are you doing what
it says?


With the above information (and anything else that you know associated that
may OR MAY NOT seem relevant) we can probably be much more helpful. (eg
supply voltage ...)



       Russell McMahon

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2004\08\21@094452 by Dave VanHorn

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>
>Mentat reply: you are doing something out of spec :-)

Mentat Dave agrees.

Also, 32kHz tuning fork crystals are more delicate than AT cut uP crystals.
Also check the crystal spec for cap values, and drive level.

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2004\08\21@114129 by Weiss, Bob

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Yes I am new to the list and thanks for the welcome!

I am using a 32khz tuning fork crystal and the cap. rating is 12pf. The chip
calls for this frquency and a 6pf cap level. I have 2 or those crystals and
neither work so I am ruling out a bad crystal. The only thing I can come up
with is that there is something differnt with tuning fork crystals and
regular rectangular type. The pic is running on a 4mhz freq. One thing I did
notice about the DS1305 chip is that it doesn't require you to use caps on
the outside, they are built in the chip. Do you think the pf difference
between what it requires and what the crystal is would cause it not to
start?



{Original Message removed}

2004\08\21@114955 by Dave VanHorn

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At 10:40 AM 8/21/2004, Weiss, Bob wrote:

>Yes I am new to the list and thanks for the welcome!
>
>I am using a 32khz tuning fork crystal and the cap. rating is 12pf. The chip
>calls for this frquency and a 6pf cap level.

The chip does not determine the capacitance.
If the xtal spec says 12pF, then roughly 18pF should be right.


> I have 2 or those crystals and
>neither work so I am ruling out a bad crystal. The only thing I can come up
>with is that there is something differnt with tuning fork crystals and
>regular rectangular type.

Well, they are constructed entirely differently, and the fork types are much more sensitive to overdrive.

>The pic is running on a 4mhz freq. One thing I did
>notice about the DS1305 chip is that it doesn't require you to use caps on
>the outside, they are built in the chip. Do you think the pf difference
>between what it requires and what the crystal is would cause it not to
>start?

The DS1305 is looking for a crystal with 6pF spec.

Have a read through this, at least the first page.
http://www.dvanhorn.org/Micros/All/Crystals.php

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