From: David VanHorn <microbrix@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 11:25:25 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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Errors have consequences. Poor designers will assume most or all the error
budget is theirs to spend.   In one application I am required to hold +/-
80 hz on transmit and an earlier design was typically 30 hz off. My design
using correct crystal caps is 3 hz off. Receivers in this application
benefit greatly from having the narrowest possible bandwidth.  Which
transmitter would you bet your life on?  You don't get to know the
bandwidth or frequency error of the other recievers..

On Tue, Oct 20, 2020, 12:06 PM Nicola Perotto <nicola@nicolaperotto.it>
wrote:

> I don't understand...
> 1) 0% error doesn't exists, never!
> 2) crystals have error...
> You have to maintain the error in a KNOWN, acceptable range!
>        N
>
> PS: with a "round" crystal it's simpler to calculate timings :-)
>
>
> On 20/10/2020 18:34, Neil wrote:
> > Okay, I found one... 14.7456Mhz crystal --> 59.9824Mhz Fosc, which is
> > close enough to 64Mhz.
> > Using SPBRG =3D 127 with that gives me spot-on 115,200 baud (so 0% erro=
r).
> > And there are 10+ suppliers for 3.2 x 2.5mm SMD crystals with that
> > frequency so that's common enough to be low-cost and not have to worry
> > about being discontinued in the near future.
> > Wish I didn't have to trial and error this though.
> >
> > Cheers,
> > -Neil.
> >
> >
> >
> >
> >
> > On 10/20/2020 12:27 PM, Neil wrote:
> >> I had also created my own, but it's been trial-and-error (xtal freq
> >> and SPBRG value) to find a spot-on crystal.
> >> Now, if your spreadsheet runs through DIgikey's crystal frequency
> >> selections and picks the spot-on option for me, then you have my
> >> attention :)
> >>
> >> Cheers,
> >> -Neil.
> >>
> >>
> >>
> >> On 10/20/2020 12:08 PM, Nicola Perotto wrote:
> >>> Hi Neil & all,
> >>> I use the attached spreadsheet to calculate baud and errors.
> >>> Enter values in yellow cells.
> >>>        N
> >>>
> >>>
> >>> On 20/10/2020 16:42, Neil wrote:
> >>>> A bit academic, but this is bugging me... I was trying to get as clo=
se
> >>>> to 115,200 baud on a PIC18F running at 64Mhz (or thereabouts).
> >>>> At 64Mhz, best I can get is 0.8% error using SPBRG=3D34.
> >>>> Calculating backwards, to get exactly 115,200, I can't find an "exac=
t"
> >>>> crystal.
> >>>> Shouldn't I be able to find one of those "odd" values to get exactly
> >>>> 115,200 baud at around 64Mhz?
> >>>>
> >>>> Cheers,
> >>>> -Neil.
> >>>>
> >>>
> >>>
>
> --
>
> Nicola Perotto - NP Engineering
> Sistemi Informatici ed Elettronici
> skype: nicolap
> mob IT: +39 335 7162 582
> mob HU: +36 30 417 3780
> tel: +39 06 916 502 732
> fax: +39 0439 633 1191
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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