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=20
near the edge of allowable timing error, it may not work.
I think about 2400 is maximum safe speed without a crystal. Maybe less.

1 start, 8 data, 1 stop:

Over 9 bits the frequency must be such that the sampling can still be=20
inside the sending 1 or 0 signal or by the 9th bit you are reading too=20
early or too late. Thus you can work out the % accuracy needed. The Baud=20
Rate clock application note shows how to calculate what error there is=20
with given dividing ratios assuming a good enough crystal and what=20
maximum allowable error is. Add the % possible error of the internal=20
clock ...
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.