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'[PIC]: Oscillator not working at indicated freque'
2001\02\06@175941 by Jacob M Hartman

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Bizarre question:

I'm driving my PIC16F877 with a 19.6608 MHz oscillator (Digikey didn't have
the 20.000 MHz ones in stock :-).  Everything worked happily, until I
started experimenting with the hardware USART, at 9600 baud.  I set the BRG
to high speed, with its divisor at 127:  19.6608 MHz / (16 * (127 + 1)) =
9600, exactly.  Fine.  But when I ran my program, I got a slew of framing
errors, in both directions.  After much experimentation and pulling out of
hair, I discovered that the setup worked perfectly if I set the divisor to
121.  But, if we reverse the equation:  9600 baud * 16 * (121 + 1) = 18.7
MHz.

So, the question:  Why on earth does my crystal seem to be oscillating at a
frequency 5% less than it should?  Now that I've got everything working the
question is solely academic, but nevertheless I'm quite curious what could
be going on.  It seems unlikely (albeit not impossible) that Digikey
shipped me the wrong oscillator.

Thanks and best regards,
Jake

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2001\02\06@180847 by David VanHorn

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At 05:59 PM 2/6/01 -0500, you wrote:
>Bizarre question:

See http://www.dvanhorn.org/Micros/All/Index.php for some info on that.



>So, the question:  Why on earth does my crystal seem to be oscillating at
>a frequency 5% less than it should?

I'm doubtful that it's really that far off.
Do you have a paralell resonant crystal?
Right caps on it?

Start there.

Can you borrow a shortwave receiver with BFO?


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2001\02\06@210605 by Olin Lathrop

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> So, the question:  Why on earth does my crystal seem to be oscillating at
a
> frequency 5% less than it should?  Now that I've got everything working
the
> question is solely academic, but nevertheless I'm quite curious what could
> be going on.  It seems unlikely (albeit not impossible) that Digikey
> shipped me the wrong oscillator.

Did you use a parallel cut crystal?  Series crystals will usually oscillate,
but can be off by a few percent.


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Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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