piclist 2002\10\11\224555a >
Thread: It's official: The Real-World Serial FAQ
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face BY : Russell McMahon email (remove spam text)



> > Sorry, I thought I'd made my point clear enough but obviously I  failed.
> > I'll try again.
>
> I'm jes' a little slow some days, I obviously misinterpreted.  I just
> object when it's stated as an absolute (like several times today)

I'll go for the hat trick.
Never, never, never do it EXCEPT under the conditions listed at the end of
my previous long post. ie you are aware of and willing to accept the
consequences.

> that
> using a resistor-fed PIC pin for RS-232 receive is completely nuts and
> destined to fail; neither are true.

I accept the challenge:

   It's nuts.
   It's destined to fail.
   This is true.

My first ever microprocessors were F8, SC/MP, 6800.
Long long long after that I met PIC. My first ever PIC experience with RS232
involved using a resistor in the PIC RS232 Receive lead. The circuit used (1
resistor & a PIC) was slavishly copied from the application note provided by
the fine folks at ME Labs. I had endless trouble and took a long long time
to find out what was happening. Despite my long prior experience with this
as an improper practice I fell into the trap. Changing the input circuit so
that the PIC operated within spec cured the problem instantly. Prior to that
the PIC's operation was inconsistent and non repeatable. AFAIK it behaved in
a similarly non similar manner with several PICs. IC used was, of course
:-),  a 16F84 (or maybe just possible a 16C84)

I can therefore say with certainty that the problem is a real one, can occur
with typical value of resistor as recommended by experienced companies who
want their customers to be able to use their software, and that the
maloperation, when it does occur, is no surprise.

> I'd never do it in a production
> device (though I'm sure others have), but I've seen it work repeatedly.
> In fact, it's never failed once for me, nor caused any problems at all.

The best you can say I suspect is ".... as far as I am aware." If your
systems are always trouble free you may well be correct.

> I often use it for those projects that will need to talk to a PC or laptop
> once in a while.  I haven't tried it in conjunction with ADC use, so I
> can't speak for that.
>
> To me it falls under the classification of something that should probably
> not be encouraged, but neither does it need to be completely denied.

See above.
Giving it one's benison in the presence of learners or people with limited
time available (ie anyone with no more than 168 hours a week) is ill
advised.


           Russell.

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