Hi Mark,

Thanks for your response.

What is special about the number 3.6864 MHz?  Does it work equally well
for hardware and software implementations?  If I want to operate the
link at 38.4kbps, and use a 3.6864 MHz crystal, what kind of error rate
are we looking at?  Should I use software or hardware USART?  If
software, should I use an interrupt-driven implementation?  If so, can
you suggest any to me?

Thanks again,
Adam Smith

> Adam Smith wrote:
> >
> > So I can handle the memory and the output fairly easy using the
methods
> > described for them, but I'm not so sure what to do about the first
one.
> > My chip of choice right now is a 16F872.  I was considering going
for a
> > 16F873 which has a USART peripheral and a little more program space
but
> > is double the cost.  I was thrown off, however, when I was reading
the
> > error rates in its data sheet; it says about 6%.  I need it down to
0.1%
> > or 0.01%.
>
> First, a 16F873 isn't "double the cost" of the 16F872, at least not so
> far as I can tell. "1.5x the cost" would be more accurate. Still a
> significant difference, though.
>
> The error rates you're talking about are the difference between the
> theoretical bit rate and the one that you get from the BRG. It is true
> that the error can be significant if you're using one of the
"standard"
> clocks (4 MHz, 20 MHz, etc.). You can reduce the error to zero for the
> usual serial rates by using a non-standard clock frequency of 3.6864
> MHz, or 2x or 4x that frequency. If your serial communication is PIC
to
> PIC, don't worry about the errors at all, since they will be the same
at
> both ends of the connection.
>
> A bit rate generator error of 6% doesn't mean that you'll see an error
> rate of 6% when receiving characters! If the connection is short and
> noise-free, you would probably see no character errors at all.
However,
> the noise immunity of your connection would be poor. I've heard
varying
> advice on what amount of error is acceptable, but you certainly want
to
> keep it low, say 2% or less.

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