By high volume you probably mean thousands. For me it's hundreds. Anyway we
are shipping units in hundreds using the internal osc of a 12C509 for short
strings of RS232 at 9600 baud. I will qualify this and say if I had it to
do all over again I would add a $0.40 crystal and two $0.05 caps to
guarantee a more accurate osc frequency. We do not do any tweaking with the
present OTP's.  Just be sure you don't overwrite the built in osc cal
values. (Maybe you can't overwrite them I don't remember now)



                    Bruce Cannon
                    <bcannon@JPS.N        To:     PICLIST@MITVMA.MIT.EDU
                    ET>                   cc:
                    Sent by: pic          Subject:     Re: RS232 possible using
internal
                    microcontrolle        oscillator?
                    r discussion
                    list
                    <PICLIST@MITVM
                    A.MIT.EDU>


                    08/13/99 01:11
                    PM
                    Please respond
                    to pic
                    microcontrolle
                    r discussion
                    list





Hello all:

I never do a good enough job of explaining myself to start with when I ask
questions on this list!  Sorry.  I should have mentioned that I've already
done serial comm with the parts, from 2400 to 9600, in my shop.  I noticed
some occasional comm problems and dug deeper into the data sheets, where I
saw that (in the 508A-519 parts at least), the range for the calibrated osc
varies from 3.55 to 4.31MHz.  Which I think is another way of saying you
could be 450kHz off.

And so while it's clear you could tweak by trial and error at both ends to
get reliable transmissions based on specific parts for playing around or
small volume, it seems it might be tougher to mass produce a cheap high
volume product which talks to a PC at, say, 4800bps.  (The PC isn't going
to
tweak, it's just going to choke).  What I really meant to ask about was
about anyone's experience with a high volume design which works.

Thanks for the input,

Bruce Cannon
Style Management Systems
1228 Ceres ST Crockett CA 94525
(510) 787-6870
http://www.jps.net/bcannon

Remember: electronics is changing your world...for good!


> I use the 12C509 @ 2400 baud & the 12C672 @ 9600 baud with no
> problems.
> The graph for '672 shows 50-100KHz variation near 25 Celcius
> not 500KHz.
> With a regulated 5V supply there should be no problems. Even
> 100KHz off
> would be about 2.5% error - still only 25% over a 8N1 byte
> length. If you
> sample
> right in the middle of each bit that still leaves a good
> margin. Our serial
> routines
> are bit-bashed and I used the simulator to tweak the basic bit timing.
> Regards...
> ___________________________________
> David Duffy               Audio Visual Devices
> Ph: +61 7 38210362   Fax +61 7 38210281
> AVD@uq.net.au  Unit 8, 9-11 Trade Street
> Cleveland, Qld, 4163, Australia.
> ___________________________________
>