> > >Russell McMahon wrote:
> > > > A ceramic resonator has ample accuracy to guarantee the required
> > > > accuracy if  used at both ends.
>
> > Vasile:
> > >   I'll retranslate a little this phrase:
> > >
> > >   A ceramic resonator *might* have the accuracy to guarantee if used at
> > >both ends. Is much better if slow rate are used.

David:
> > As has been said many times before, the accuracy is NOT dependent
> > on the baud rate. Its a percentage thing.

Vasile:
>   Ok, maybe I misunderstand the translation of "accuracy" which according
>to my Webster's is:
>   -- the condition or quality of being true,
>      correct, or exact; precision; exactness.
>I had yesterday a problem with choosing an "accurate" ceramic resonators
>and I can assure you, in my PIC-PC connection, a slow rate may work with a
>"wrong value" resonator and a high rate not, using the same basic code
>which is working perfectly with a Xtal.
>Everything depends if the samples are or not taken at the middle of the
>receptioned bit value. And I'm not talking about using USART but
>bit-banged.

I assume by "wrong value" resonator you mean not exactly on-frequency?
But at the slower baud rate & the correspondingly longer bit period, the PIC
will still sample in the same position within that bit won't it?  Sure, the
bits
are longer at lower baud rates, but the clock error is multiplied time-wise &
makes it the same timing error percentage-wise anyway. To put it another
way, the error amount (time) is more with a lower baud rate but the bits are
also longer making it the same percentage of the bit time. Where it comes
into play however is when you don't have enough clock cycle resolution to
generate exact delays when doing bit-bang comms. When using the UART
this makes that much less of a problem. It's important to carefully calculate
the loop delays when bit-banging to get it as close as you can so that you
don't contribute to the overall error percentage. Does this make sense? :-)
I'm not trying to criticise, just clear up the confusion.
David...
___________________________________________
David Duffy Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362 Fax: +61 7 38210281
New Web: www.audiovisualdevices.com.au
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