From: David Van Horn <david.vanhorn@backcountryaccess.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sender: "piclist-bounces@mit.edu" <piclist-bounces@mit.edu>
Date: Tue, 20 Oct 2020 08:59:04 -0700
Subject: Re: [PIC] Crystal frequency selection
Thread-Topic: [PIC] Crystal frequency selection
Thread-Index:
 AQHWpu+Cye8XO8+oKEK1+gz9gEeJfamgksoAgAAFswCAAAInAIAABg4AgAAFOBs=
Message-ID:
 <CY4PR01MB240713AB1B8834ADAB43973BE01F0@CY4PR01MB2407.prod.exchangelabs.com>
References: <5F8EF750.1010301@narwani.org>
	<CAB115wHEgX58LcSnuGgfdj_jBHJqU=4FAJ9uGFj+uea2UOAaxQ@mail.gmail.com>
	<48194d83-b7bd-e7ce-bea1-7da72b0d6dbb@jle.co.uk>
	<CAB115wGqR8dknUup4K0RYArsoT6p+AitP18vf7rF75t_+518SA@mail.gmail.com>,
	<7c8a8645-4eea-455e-23e7-974dc429c5af@jle.co.uk>
In-Reply-To: <7c8a8645-4eea-455e-23e7-974dc429c5af@jle.co.uk>
Reply-To: Microcontroller discussion list - Public. <piclist@mit.edu>
Accept-Language: en-US
Content-Language: en-US
X-MS-Exchange-Organization-SenderIdResult: Pass
X-MS-Exchange-Organization-PRD: mit.edu
X-MS-TNEF-Correlator:
acceptlanguage: en-US
received-spf: Pass (protection.outlook.com: domain of backcountryaccess.com
	designates 40.107.102.90 as permitted sender)
	receiver=protection.outlook.com; client-ip=40.107.102.90;
	helo=NAM04-DM6-obe.outbound.protection.outlook.com;
dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 d=k2mdv.onmicrosoft.com; 	s=selector2-k2mdv-onmicrosoft-com;
	h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
	bh=BuAx35sNWXXsb4X70guseKlSKMBhDkPhtO/0xjwKhTc=;
	b=YxF244ln8QclF6BCt77ssUeno6tnaNWFJkqTpAl1byz4/XJrxVL+RVKOLALc8UOIHqDXBXneMq0Cn87Jpf9eemn9VsAGBSDRhnhvRSxiEG3mcFfFzL0Ef5wf+EhCucp195BQoELVuzigxrAtkuc5yxGByfwlNFSZALdmx2ZujGU=
authentication-results: spf=pass (sender IP is 40.107.102.90)
	smtp.mailfrom=backcountryaccess.com; mit.edu; dkim=pass (signature was
	verified) header.d=k2mdv.onmicrosoft.com; mit.edu;	dmarc=pass action=none
	header.from=backcountryaccess.com;
x-originating-ip: [67.172.157.116]
errors-to: piclist-bounces@mit.edu
list-id: "Microcontroller discussion list - Public." <piclist.mit.edu>
list-post: <mailto:piclist@mit.edu>
x-beenthere: piclist@mit.edu
x-mailman-version: 2.1.6
x-topics: [PIC]
x-content-filtered-by: Mailman/MimeDel 2.1.6
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0

Well that's certainly close enough.

________________________________
From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of John L=
awton <piclist@jle.co.uk>
Sent: Tuesday, October 20, 2020 9:40 AM
To: piclist@mit.edu <piclist@mit.edu>
Subject: Re: [PIC] Crystal frequency selection

>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.
>>>>
>>>> --
>>>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
>>>> View/change your membership options at
>>>> http://mailman.mit.edu/mailman/listinfo/piclist
>>>>
>> --
>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>

--
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.