From: David VanHorn <microbrix@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 08:18:26 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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What divisor?  I get 555.55...

On Tue, Oct 20, 2020, 9:11 AM John Lawton <piclist@jle.co.uk> wrote:

> Which device?
>
> For the 18F25K22, -0.08% error at 64MHz clock according to the datasheet.
>
> John
>
> On 20/10/2020 15:50, David VanHorn wrote:
> > 115200 might be tough since the divisor is getting small.
> > 7.372800 would work, 14.745600, and so on.
> >
> >
> > One thing a lot of people miss, which causes baud rates to be off, is
> > selecting the right crystal caps.
> > A crystal with CL of 22pF wants (2*CL-Cp) =3D 44pF - Cp where Cp is an
> > estimate of the parasitics (5 to 6 pf is a good first guess)
> > Running 22pF caps puts you a bit high, and the oscillator might not sta=
rt
> > in all conditions.
> >
> >
> >
> > On Tue, Oct 20, 2020 at 8:44 AM Neil <picdude3@narwani.org> wrote:
> >
> >> A bit academic, but this is bugging me... I was trying to get as close
> >> 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 "exact"
> >> crystal.
> >> Shouldn't I be able to find one of those "odd" values to get exactly
> >> 115,200 baud at around 64Mhz?
> >>
> >> Cheers,
> >> -Neil.
> >>
> >> --
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> >>
>
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