From: Neil <picdude3@narwani.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 09:02:26 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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18F45Q10.  But I would expect that the EUSART is the same on both. I'll=20
check the datasheet.

But my point though is that isn't there a crystal that would for which=20
there is a SPBRG value that results in 0% error?




On 10/20/2020 11:10 AM, John Lawton 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 star=
t
>> 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
>>> View/change your membership options at
>>> http://mailman.mit.edu/mailman/listinfo/piclist
>>>

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