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'[PIC]: 16F628 internal oscillator'
2002\06\29@122515 by Richard Mellina

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I am new to PICs and I emailing concerning the reliability of the 16F628's
internal oscillator. Is it reliable enough to time the USART or am I crazy
to even think about it?

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2002\06\29@140407 by A.J. Tufgar

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Richard,
       I once read on the list that the internal osc won't vary more
then 3%.  I've communicated with modems using the intrc and never seen a
problem.  However I use my own code and not the usart.

It would depend on what you are trying to communicate with and how picky
it was. i.e. a 57.6K modem would most likely have problems but a 2400
would work with no problems.

What are you communicating with and at what rate?

Aaron

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2002\06\29@143600 by Olin Lathrop

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>         I once read on the list that the internal osc won't vary more
> then 3%.  I've communicated with modems using the intrc and never seen a
> problem.  However I use my own code and not the usart.
>
> It would depend on what you are trying to communicate with and how picky
> it was. i.e. a 57.6K modem would most likely have problems but a 2400
> would work with no problems.

This makes no sense because it is the percentage error that counts.  3%
clock error should work if the other end has very little error, but is
uncofortably close to the margin.  This is true at 300baud or 57.6Kbaud.


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2002\06\29@145919 by uter van ooijen & floortje hanneman

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> I am new to PICs and I emailing concerning the reliability of the 16F628's
> internal oscillator. Is it reliable enough to time the USART or am I crazy
> to even think about it?

You might get away with it (read: when you test it it will probably work,
especially if you tune your code a bit), but I would not want to be
responsible for the servicing of a product based on this idea, unless the
chips all run from a 5.00 Volt and are kept at a constant temperature.

Note that the datasheet of the (future) 12F chips suggests a much more
stable internal oscillator.

Wouter van Ooijen
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2002\06\29@164929 by lexandre_Guimar=E3es?=

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Hi,


> >         I once read on the list that the internal osc won't vary more
> > then 3%.  I've communicated with modems using the intrc and never seen a
> > problem.  However I use my own code and not the usart.
> >
> > It would depend on what you are trying to communicate with and how picky
> > it was. i.e. a 57.6K modem would most likely have problems but a 2400
> > would work with no problems.
>
> This makes no sense because it is the percentage error that counts.  3%
> clock error should work if the other end has very little error, but is
> uncofortably close to the margin.  This is true at 300baud or 57.6Kbaud.

   Depending on how you implement the serial routines you can get a little
better results than the hardware UART ! If you use the rising edge of the
'1' bits you can resyncronize the timing within a byte and that gives you
better margin. At the worst case of just '0' bits you loose the advantage
but it is a minority of the bytes anyway. If you are using a protocol that
has error correction you are OK with the rare errors that may come up. Using
a more sofhisticated algorithm you can even time the start bit and use that
timing to determine the position of the other bits. That way you can get
away with errors bigger than 5% ! The timing varies with temperature and
that variation should not be large inside a single byte.

Best regards,
Alexandre Guimaraes

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2002\06\29@195802 by Dwayne Reid

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At 11:21 AM 6/29/02 -0500, Richard Mellina wrote:
>I am new to PICs and I emailing concerning the reliability of the 16F628's
>internal oscillator. Is it reliable enough to time the USART or am I crazy
>to even think about it?

Not quite close enough to be reliable.  You might get away with it if your
supply voltage and temperature are constant but all bets are off if those
vary widely.

Bottom line: *I* won't rely upon the internal oscillator for async serial
comms.  Spend the 30 cents and add a ceramic resonator.

dwayne

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2002\06\30@102608 by rusque (Listas)

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Hello Richard,

> I am new to PICs and I emailing concerning the reliability of the 16F628's
> internal oscillator. Is it reliable enough to time the USART or am I crazy
> to even think about it?

   I've made a little cheap device that communicates with the internal
USART of a PIC16F628 using the INTRC, but wouldn't make it again.

   The speed of the INTRC on the F628's I have is more than 5% slower than
the nominal 4MHz IIRC. I had to adjust SPBRG=26 (instead of SPBRG=25) for it
to work at 9600bps and still it's not very confiable.

   I've made some tests to see how the INTRC speed would drift with
temperature and found very little (almost negligible) variation.

   Best to use a XTAL or resonator. If you need more pins, use a PIC16F870
(US$0.60 more expensive than the F628 here in Brazil) and be happy.

   Best regards,

   Brusque

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