From: Neil <picdude3@narwani.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 10:11:16 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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The 14.7456 crystal?




On 10/20/2020 1:00 PM, John Lawton wrote:
> But you didn't, it's all there is the datasheets :)
>
> John
>
> On 20/10/2020 17:34, Neil wrote:
>> Okay, I found one... 14.7456Mhz crystal --> 59.9824Mhz Fosc, which is
>> close enough to 64Mhz.
>> Using SPBRG =3D 127 with that gives me spot-on 115,200 baud (so 0% error=
).
>> And there are 10+ suppliers for 3.2 x 2.5mm SMD crystals with that
>> frequency so that's common enough to be low-cost and not have to worry
>> about being discontinued in the near future.
>> Wish I didn't have to trial and error this though.
>>
>> Cheers,
>> -Neil.
>>
>>
>>
>>
>>
>> On 10/20/2020 12:27 PM, Neil wrote:
>>> I had also created my own, but it's been trial-and-error (xtal freq
>>> and SPBRG value) to find a spot-on crystal.
>>> Now, if your spreadsheet runs through DIgikey's crystal frequency
>>> selections and picks the spot-on option for me, then you have my
>>> attention :)
>>>
>>> Cheers,
>>> -Neil.
>>>
>>>
>>>
>>> On 10/20/2020 12:08 PM, Nicola Perotto wrote:
>>>> Hi Neil & all,
>>>> I use the attached spreadsheet to calculate baud and errors.
>>>> Enter values in yellow cells.
>>>>         N
>>>>
>>>>
>>>> On 20/10/2020 16:42, Neil wrote:
>>>>> A bit academic, but this is bugging me... I was trying to get as clos=
e
>>>>> 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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