On 9/25/2012 1:48 AM, Lee Mulvogue wrote:
>
>
> 	I'd always read in the past to not use the internal OSC of a PIC as=20
> it was too innacurate, but am now wondering if this is still the case=20
> for a PIC24F?
>
> 	I've just started making a board based on a PIC24FJ64GA004 (my first=20
> time with a 24F), and added the usual 20MHz crystal and supporting=20
> caps; but are the modern PICs accurate enough to start considering=20
> dumping this external setup?  The most time sensitive thing I'm doing=20
> is interrupting @ 16kHz for PCM sound playback (current sample to PWM=20
> output, set flag for Main loop to process the next sample), and simple
> RS232 comms.  The datasheet seems to indicate a max variation of +/-=20
> 5%, but I don't have a benchmark to compare that against.
>
> 	I'd possibly be running the 8Mhz internal with the 4x PLL multiplier,=20
> resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal=20
> without PLL.
>
> 	Thoughts?
>
> 	Lee

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 Joe Wronski
Sent: 25 September 2012 12:56
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] PIC24F; dump external OSC?

>> Check the requirements of the RS232.  You might want to consider the cas=
e
>> where one unit is running at +5% and the other at -5% (if both ends are =
the
>> same unit).  The baud rate accuracy requirement increases with baud rate=
,
>> IIRC.   Also, your PWM frequency will be affected, so, the pitch of your
>> sound may vary.  The answer is in the requirements, not the opinions of
>> people who don't have the numbers.
>>
>>
>>Joe W

Baud rate accuracy requirements (as a percentage) is independent of the act=
ual baud rate.  An inaccurate baud generator clock will have the same effec=
t at 300 baud as  115200 baud.

5% is already far too much and equates to half a bit slip over a 10bit word=
 (8,N,1) which will be threshold at which the comms simply can not work. Re=
liability problems are likely to occur before you reach 5%, and I would wan=
t no more than about 3% total error including sending and receiving ends, o=
bviously the lower the better.

Mike



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