From: David VanHorn <microbrix@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 07:50:20 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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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 start
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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