At 01:18 PM 2/26/02 -0500, Byron A Jeff wrote:
>On Tue, Feb 26, 2002 at 10:36:00AM -0500, Olin Lathrop wrote:
> > > The grief saving answer is no. The true answer is "maybe if the
> conditions
> > > are right." Conditions being that the bit rate is slow enough and there
> > > isn't drastic changes in temperature.
> >
> > The absolute bit speed has nothing to do with it.  The relative speed error
> > is what matters.  You'd like to be within 3%, whether that's 300 baud +-9,
> > or 115.2Kbaud +- 3500.
>
>But if you slow it down then the absolute amount of time before the error
>is exceeded is lengthned.
>
>Change your examples to a time scale by inverting:
>
>300 bPS cell width: 3.3333 ms +- 0.3ms
>115k bPS cell width: 0.00886805 mS +- 0.000256 ms

Byron - please forgive me for disagreeing with you.  Olin is more correct
than you.

The baud rate is directly related to Fosc.  If Fosc changes by 1%, so does
the baud rate.  You are correct in that one can have a slight amount more
error at higher baud rates because of rounding error but that is a problem
only if Fosc is not an exact multiple of the desired baud rate.

Using the internal RC oscillator (4 MHz) *can* result significant rounding
error at high baud rates - *IF* you are using standard baud rates.  For PIC
to PIC communications, one can choose a non-standard baud rate with no
rounding error at the desired speed.

However, having said all that, I feel that using the internal RC oscillator
for the timebase for serial comms is iffy at best.  I don't use it in my
products - I have to operate over the temperature range of -40 through +85C
in almost everything I ship.  The internal RC oscillator just isn't stable
enough over that wide a temperature range.

One can design a serial protocol that adjusts to the correct baud rate
automatically.  This involves measuring the width of one or more bits in
terms of the number of clock cycles, then adjusting the sample delays or,
in the case of hardware UART, adjusting the baud rate registers.  But this
requires designing an appropriate protocol from the ground up.  Again - so
long as both ends of the link understand the requirements - no problem.

dwayne


Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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