From: David VanHorn <microbrix@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 07:55:13 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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58.982400 is as close as you'll get to 64 MHz and still divide evenly.

You have 10 bits, and 5% error would be marginal, so I would shoot for >1%
error so as to give the other system more than half of the error budget.

An obscure little bug these days was in the Atmel Mega128 which had it's
low power mode as the default.  Without exactly the right crystal (very low
CL value) the oscillator amplitude would be low, and parts of the chip
would sometimes not get clocked. (!)   This led to baud rates being a
little slow and some non-deterministic execution, but it would occur so
infrequently that you would more think it was a bug.  The issue was
uncovered for us when another batch of chips simply wouldn't start the
crystal osc at all.  Setting the chip to high power crystal cured all
ills.  I know a number of M128 users hit this problem.


On Tue, Oct 20, 2020 at 8:50 AM David VanHorn <microbrix@gmail.com> 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 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.
>>
>> --
>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
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>>
>
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