From: Neil <picdude3@narwani.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 20 Oct 2020 10:46:05 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
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As I mentioned at the opening, it was academic.




On 10/20/2020 1:29 PM, WA1RHP_Android wrote:
> The real question is: why do you need an exact baud rate?Sent from my Ver=
izon, Samsung Galaxy smartphone
> -------- Original message --------From: Neil <picdude3@narwani.org> Date:=
 10/20/20  12:16 PM  (GMT-05:00) To: piclist@mit.edu Subject: Re: [PIC] Cry=
stal frequency selection I'm using datasheet 40001996D for the PIC18F45Q10.=
  The table of baud rate formulas on page 515 continued into page 516, so I=
 missed the option with SYNC=3D1 and BRGH=3D1.  But same as the K22 part.  =
And yes I get the 0.08% error now.But still, shouldn't some easily availabl=
e crystal value work out to 0% error?Cheers,-Neil.On 10/20/2020 11:40 AM, J=
ohn Lawton wrote:>   From the datasheet DS41412F page 383:>> SYNC =3D 0 BRG=
H =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 s=
peed) =3D 64MHz.>> J>> On 20/10/2020 16:18, David VanHorn wrote:>> What div=
isor?  I get 555.55...>>>> On Tue, Oct 20, 2020, 9:11 AM John Lawton <picli=
st@jle.co.uk> wrote:>>>>> Which device?>>>>>> For the 18F25K22, -0.08% erro=
r at 64MHz clock according to th!
 e datasheet.>>>>>> John>>>>>> On 20/10/2020 15:50, David VanHorn wrote:>>>=
> 115200 might be tough since the divisor is getting small.>>>> 7.372800 wo=
uld 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 a=
n>>>> estimate of the parasitics (5 to 6 pf is a good first guess)>>>> Runn=
ing 22pF caps puts you a bit high, and the oscillator might not start>>>> i=
n all conditions.>>>>>>>>>>>>>>>> On Tue, Oct 20, 2020 at 8:44 AM Neil <pic=
dude3@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 S=
PBRG=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 "od=
d" values to get e!
 xactly>>>>> 115,200 baud at around 64Mhz?>>>>>>>>>> Cheers,>>>>> -Neil.>>>=
>>>>>>> -->>>>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list ar=
chive>>>>> View/change your membership options at>>>>> http://mailman.mit.e=
du/mailman/listinfo/piclist>>>>>>>> -->>> http://www.piclist.com/techref/pi=
clist PIC/SX FAQ & list archive>>> View/change your membership options at>>=
> http://mailman.mit.edu/mailman/listinfo/piclist>>>-- http://www.piclist.c=
om/techref/piclist PIC/SX FAQ & list archiveView/change your membership opt=
ions athttp://mailman.mit.edu/mailman/listinfo/piclist



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