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. I've searched for
> an application note, but haven't found one. Please go easy on me, as I
> have never used this forum before and don't even know if it is going
> to get through.
>
> Thanks,
>
> Steven
>

If I wanted to be sure of performance, I would use a crystal or=20
resonator. However, under "normal" conditions the internal oscillator=20
can work okay, I have used it a few times in non-critical applications=20
(my own projects) with no problems, though I wouldn't use it in a=20
commercial product - as Joe says you have to decide whether saving the=20
cost is worth the risk (I can't really see how complexity is an issue=20
here? - you can get cheap resonators with internal caps for a one part=20
solution) It depends on how reliable you need it to be for your application=
..
If you really don't want to use a crystal/resonator, then you could=20
maybe look at calibrating the internal oscillator using the RX timing (a=20
synch character) as a reference - check out the auto-baud detect=20
information in the datasheet. At 9600 you should have reasonable=20
tolerance with 16 bit BRG. You could implement a similar thing but tune=20
the actual oscillator using the OSCTUNE register also, the datasheet I=20
looked at says this is the preferred solution.
I couldn't find any app notes on the OSCTUNE method, but I found these=20
that might be helpful on autobaud and general UART use:
http://ww1.microchip.com/downloads/en/AppNotes/00944A.pdf
http://ww1.microchip.com/downloads/en/AppNotes/00712a.pdf
http://ww1.microchip.com/downloads/en/AppNotes/Autobaud%2000962a.pdf

--=20
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