From: John Lawton <piclist@jle.co.uk>
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Date: Tue, 20 Oct 2020 08:40:06 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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 From the datasheet DS41412F page 383:

SYNC =3D 0 BRGH =3D 1, or SYNC =3D 1, BRG16 =3D 1

Actual speed =3D 115.11k, error -0.08% SPBRGHx: SPBRGx (decimal) =3D 138

I should have said Fosc, (not clock speed) =3D 64MHz.

J

On 20/10/2020 16:18, David VanHorn wrote:
> 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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