Michael Watterson wrote 2011-05-19 09:17:
> On 19/05/2011 03:24, Steven Ramon wrote:
>> I am planning to use a PC18F2455 for RS232. To help save cost and
>> reduce complexity, I want to use the internal oscillator. However,
>> I'm concerned about baud rate error. We'll be operating at 9600 8N1
>> and 5V signal levels. Can someone point me to documentation that
>> discusses using the internal oscillator with RS232.
>
> The accuracy of internal oscillator is such that if the other end is
> near the edge of allowable timing error, it may not work.
> I think about 2400 is maximum safe speed without a crystal. Maybe less.

The baud rate has nothing as such with this to do.
Only the relleative baud rate error (which does not change
with the baudrate).

Excluding errors from "rounding" in the baud rate generator, of
course, but that is another question/issue.

Jan-Erik.


>
> 1 start, 8 data, 1 stop:
>
> Over 9 bits the frequency must be such that the sampling can still be
> inside the sending 1 or 0 signal or by the 9th bit you are reading too
> early or too late. Thus you can work out the % accuracy needed. The Baud
> Rate clock application note shows how to calculate what error there is
> with given dividing ratios assuming a good enough crystal and what
> maximum allowable error is. Add the % possible error of the internal
> clock ...
--=20
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