From: Neil <picdude3@narwani.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 11:54:05 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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Oh, I hadn't seen that, thanks.


On 10/20/2020 1:46 PM, John Lawton wrote:
> Well not that specifically, but the datasheet mentioned:
>
> 3.6864MHz, 11.0592MHz, 18.432MHz for 0% error @ 115.2kBaud
>
> J
>
>
>
> On 20/10/2020 18:11, Neil wrote:
>> 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% err=
or).
>>>> 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 cl=
ose
>>>>>>> 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 "exa=
ct"
>>>>>>> crystal.
>>>>>>> Shouldn't I be able to find one of those "odd" values to get exactl=
y
>>>>>>> 115,200 baud at around 64Mhz?
>>>>>>>
>>>>>>> Cheers,
>>>>>>> -Neil.
>>>>>>>

--=20
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