No.  I believe the internal RC osc is rated at +- 8%.  That may not seem
like too much error, but when doing RS-232, the error is cumulative.  Over
a 10 bit word, you could drift up to 80% of a bit, pushing your sample well
into the next bit.

It is not impossible to use an RC though.  The maximum error occurs over
time, temperature, voltage and process.  It will not CHANGE rapidly.  If
you could calibrate the processor speed periodically, you could get away
with such large errors.  Doable, but requires a specific plan for dealing
with the uncertainties.

Resonators are accurate enough.  Most are better than 3% over temperature
and voltage, INCLUDING ageing.  That results in a maximum error of 1/3 bit.



At 09:37 AM 3/27/00 , you wrote:
>Pic List,
>Is the internal osc. of the 12C508/9 stable and accurate enough for RS-232?
>It seems to work, but can I depend on it?   Microchip says resonators are
>too variable for serial work, and so I have always used crystals.  Is the
>internal osc on these chips stable enough for reliable indoor operation?
>Thanks,
>John
>
>
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Jerry Merrill

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