PIC24F; dump external OSC?

classic Classic list List threaded Threaded
30 messages Options
12
Reply | Threaded
Open this post in threaded view
|

PIC24F; dump external OSC?

Lee Mulvogue
 

        I'd always read in the past to not use the internal OSC of a PIC as
it was too innacurate, but am now wondering if this is still the case
for a PIC24F?

        I've just started making a board based on a PIC24FJ64GA004 (my first
time with a 24F), and added the usual 20MHz crystal and supporting
caps; but are the modern PICs accurate enough to start considering
dumping this external setup?  The most time sensitive thing I'm doing
is interrupting @ 16kHz for PCM sound playback (current sample to PWM
output, set flag for Main loop to process the next sample), and simple
RS232 comms.  The datasheet seems to indicate a max variation of +/-
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,
resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal
without PLL.

        Thoughts?

        Lee

-------------------------
Msg sent via Webmail - http://hosting.myob.com/
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Jan-Erik Söderholm


Lee Mulvogue wrote 2012-09-25 07:48:
>
>
> I'd always read in the past to not use the internal OSC of a PIC as it
> was too innacurate,

Innacurate for *what* ?
I always use INTOSC as long as it is accurate enough.

> but am now wondering if this is still the case for a PIC24F?
>

Impossible to answer.

> I've just started making a board based on a PIC24FJ64GA004 (my first
> time with a 24F), and added the usual 20MHz crystal and supporting caps;
> but are the modern PICs accurate enough to start considering dumping
> this external setup?  The most time sensitive thing I'm doing is
> interrupting @ 16kHz for PCM sound playback (current sample to PWM
> output, set flag for Main loop to process the next sample), and simple
> RS232 comms.  The datasheet seems to indicate a max variation of +/-
> 5%,

That is over the range "-40°C -- +125°C", right ?

At 25°C it's +/- 2%.

Since you give no indications of the temp range where this
processor will be used, you have to decide yourself.

Jan-Erik.


> but I don't have a benchmark to compare that against.
>
> I'd possibly be running the 8Mhz internal with the 4x PLL multiplier,
> resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal without
> PLL.
>
> Thoughts?
>
> Lee
>
> ------------------------- Msg sent via Webmail -
> http://hosting.myob.com/
>
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
ivp
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

ivp
In reply to this post by Lee Mulvogue
> The datasheet seems to indicate a max variation of +/-
> 5%, but I don't have a benchmark to compare that against.

Write a small clock program and compare it with one on
the wall or a 32kHz crystal on T1OSC. You could toggle
a seconds LED and a minutes LED

1% per hour is 36 seconds and the PIC might be obviously
fast or slow. You'll probably get an idea of its outness after
only a minute or two

Joe
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Jan-Erik Söderholm


IVP wrote 2012-09-25 09:32:
>> The datasheet seems to indicate a max variation of +/-
>> 5%, but I don't have a benchmark to compare that against.
>
> Write a small clock program and compare it with one on
> the wall or a 32kHz crystal on T1OSC. You could toggle
> a seconds LED and a minutes LED
>
> 1% per hour is 36 seconds...

1% is 36 seconds per hour.

:-)

Or simply belive what the datasheet says.

Jan-Erik.


> and the PIC might be obviously
> fast or slow. You'll probably get an idea of its outness after
> only a minute or two
>
> Joe
>
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
ivp
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

ivp
 
> Or simply believe what the datasheet says

You could, although I've found the datasheet is usually quite
conservative, and the frequency is likely to be pretty accurate

If it were me I'd measure FRC just out of curiousity. However,
it will be temperature and voltage sensitive (if the module is
comparable to other PICs), so no matter how accurate it is
under certain conditions, it does have that against it

For what the OP is doing I think I might prefer the comfort
of a crystal, unless he finds FRC is stable enough over the
working conditions

Joe
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Glen Wiley
I would love to be able to ditch a few extra discrete components on my
boards, however I would love less to get a call about a defect in the
boards due to crappy oscillators.  For my purposes, a few visits to
installed sites by service technicians would more than outweigh the cost of
simply including external oscillators on the boards so I chose to keep the
parts.

One risk you take when you decide to disregard the data sheet is that while
you might have one or two parts that operate within your tolerances, but
you may not be able to rely on the majority of the parts exhibiting the
same behavior.


On Tue, Sep 25, 2012 at 5:57 AM, IVP <[hidden email]> wrote:

>
> > Or simply believe what the datasheet says
>
> You could, although I've found the datasheet is usually quite
> conservative, and the frequency is likely to be pretty accurate
>
> If it were me I'd measure FRC just out of curiousity. However,
> it will be temperature and voltage sensitive (if the module is
> comparable to other PICs), so no matter how accurate it is
> under certain conditions, it does have that against it
>
> For what the OP is doing I think I might prefer the comfort
> of a crystal, unless he finds FRC is stable enough over the
> working conditions
>
> Joe
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>



--
Glen Wiley

"A designer knows he has achieved perfection not when there is nothing left
to add, but when there is nothing left to take away." - Antoine de
Saint-Exupery
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Joe Wronski
In reply to this post by Lee Mulvogue
Check the requirements of the RS232.  You might want to consider the
case 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





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
> it was too innacurate, but am now wondering if this is still the case
> for a PIC24F?
>
> I've just started making a board based on a PIC24FJ64GA004 (my first
> time with a 24F), and added the usual 20MHz crystal and supporting
> caps; but are the modern PICs accurate enough to start considering
> dumping this external setup?  The most time sensitive thing I'm doing
> is interrupting @ 16kHz for PCM sound playback (current sample to PWM
> output, set flag for Main loop to process the next sample), and simple
> RS232 comms.  The datasheet seems to indicate a max variation of +/-
> 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,
> resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal
> without PLL.
>
> Thoughts?
>
> Lee
>
> -------------------------
> Msg sent via Webmail - http://hosting.myob.com/
>

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Isaac Marino Bavaresco
In reply to this post by Lee Mulvogue
Serial (UART) communications require oscillator precision better than 2%,

Isaac



Em 25/09/2012 02:48, Lee Mulvogue escreveu:

>  
>
> I'd always read in the past to not use the internal OSC of a PIC as
> it was too innacurate, but am now wondering if this is still the case
> for a PIC24F?
>
> I've just started making a board based on a PIC24FJ64GA004 (my first
> time with a 24F), and added the usual 20MHz crystal and supporting
> caps; but are the modern PICs accurate enough to start considering
> dumping this external setup?  The most time sensitive thing I'm doing
> is interrupting @ 16kHz for PCM sound playback (current sample to PWM
> output, set flag for Main loop to process the next sample), and simple
> RS232 comms.  The datasheet seems to indicate a max variation of +/-
> 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,
> resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal
> without PLL.
>
> Thoughts?
>
> Lee
>
> -------------------------
> Msg sent via Webmail - http://hosting.myob.com/

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Wouter van Ooijen
In reply to this post by Joe Wronski
> Check the requirements of the RS232.  You might want to consider the
> case 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.

Common misconception! Apart from minor influences like the switching
time of the hardware the accuracy requirement for asynch does NOT depend
on the baudrate. Hence you can not escape the accuray requirement by
running at a lower baudrate.

Wouter van Ooijen
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

RE: PIC24F; dump external OSC?

Michael Rigby-Jones-2
In reply to this post by Joe Wronski
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
> it was too innacurate, but am now wondering if this is still the case
> for a PIC24F?
>
> I've just started making a board based on a PIC24FJ64GA004 (my first
> time with a 24F), and added the usual 20MHz crystal and supporting
> caps; but are the modern PICs accurate enough to start considering
> dumping this external setup?  The most time sensitive thing I'm doing
> is interrupting @ 16kHz for PCM sound playback (current sample to PWM
> output, set flag for Main loop to process the next sample), and simple
> RS232 comms.  The datasheet seems to indicate a max variation of +/-
> 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,
> resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal
> without PLL.
>
> Thoughts?
>
> Lee

-----Original Message-----
From: [hidden email] [mailto:[hidden email]] 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 case
>> 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 actual baud rate.  An inaccurate baud generator clock will have the same effect 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. Reliability problems are likely to occur before you reach 5%, and I would want no more than about 3% total error including sending and receiving ends, obviously the lower the better.

Mike



=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Glen Wiley
This has been my observation in practice.  As you approach the edges of
tolerance you end up with odd failures that can be hard to troubleshoot.  I
have made it a practice to do my best to design within the constraints
provided by the datasheets of the components.

My conclusion was that he pain that I earn by not using accurate
oscillators is simply not worth the parts savings.

On Tue, Sep 25, 2012 at 8:20 AM, Michael Rigby-Jones <
[hidden email]> wrote:

> 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
> > it was too innacurate, but am now wondering if this is still the case
> > for a PIC24F?
> >
> >       I've just started making a board based on a PIC24FJ64GA004 (my
> first
> > time with a 24F), and added the usual 20MHz crystal and supporting
> > caps; but are the modern PICs accurate enough to start considering
> > dumping this external setup?  The most time sensitive thing I'm doing
> > is interrupting @ 16kHz for PCM sound playback (current sample to PWM
> > output, set flag for Main loop to process the next sample), and simple
> > RS232 comms.  The datasheet seems to indicate a max variation of +/-
> > 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,
> > resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal
> > without PLL.
> >
> >       Thoughts?
> >
> >       Lee
>
> -----Original Message-----
> From: [hidden email] [mailto:[hidden email]] 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
> case
> >> 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
> actual baud rate.  An inaccurate baud generator clock will have the same
> effect 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. Reliability problems are likely to occur before you reach 5%, and I
> would want no more than about 3% total error including sending and
> receiving ends, obviously the lower the better.
>
> Mike
>
>
>
> =======================================================================
> This e-mail is intended for the person it is addressed to only. The
> information contained in it may be confidential and/or protected by
> law. If you are not the intended recipient of this message, you must
> not make any use of this information, or copy or show it to any
> person. Please contact us immediately to tell us that you have
> received this e-mail, and return the original to us. Any use,
> forwarding, printing or copying of this message is strictly prohibited.
> No part of this message can be considered a request for goods or
> services.
> =======================================================================
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>



--
Glen Wiley

"A designer knows he has achieved perfection not when there is nothing left
to add, but when there is nothing left to take away." - Antoine de
Saint-Exupery
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Mike Harrison
In reply to this post by Wouter van Ooijen
On Tue, 25 Sep 2012 14:13:51 +0200, you wrote:

>> Check the requirements of the RS232.  You might want to consider the
>> case 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.
>
>Common misconception! Apart from minor influences like the switching
>time of the hardware the accuracy requirement for asynch does NOT depend
>on the baudrate. Hence you can not escape the accuray requirement by
>running at a lower baudrate.

Oscillator jitter can be a significant issue with RC oscillators, and this will affect higher
baudrates more than lower ones.

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
ivp
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

ivp
In reply to this post by Joe Wronski
> The baud rate accuracy requirement increases with baud rate

Error percentage is constant, whatever the speed. The only way
to do serial with any confidence when the clock is liable to drift
would be to use bit-banging, timing based on a bit length of the
external transmitter. Not as convenient as the PIC's serial module
but it can be done fairly easily

Joe
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

picram
In reply to this post by Joe Wronski
> Check the requirements of the RS232.  You might want to consider the
> case 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.

I knew this question would bring out all the old misconceptions regarding
RS232 baud rate accuracy.

First, the total worst case error budget in RS232 is about 6%. You have to
remember that both the transmitter and receiver may be off frequency.

Second, required baud rate accuracy is *not* a function of baud rate.
Everything just scales so that the same % error is possible at any baudrate.
(However, a caveat: if the baud rate is so fast that the rise and fall times
of the signal make a signficant difference then you might have to adjust the
error budget).

IMHO I would *not* attempt to do serial communications without a crystal (or
resonator).

-- Bob Ammerman
RAm Systems



--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Isaac Marino Bavaresco
In reply to this post by Michael Rigby-Jones-2
Em 25/09/2012 09:20, Michael Rigby-Jones escreveu:

> 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
>> it was too innacurate, but am now wondering if this is still the case
>> for a PIC24F?
>>
>> I've just started making a board based on a PIC24FJ64GA004 (my first
>> time with a 24F), and added the usual 20MHz crystal and supporting
>> caps; but are the modern PICs accurate enough to start considering
>> dumping this external setup?  The most time sensitive thing I'm doing
>> is interrupting @ 16kHz for PCM sound playback (current sample to PWM
>> output, set flag for Main loop to process the next sample), and simple
>> RS232 comms.  The datasheet seems to indicate a max variation of +/-
>> 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,
>> resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal
>> without PLL.
>>
>> Thoughts?
>>
>> Lee
> -----Original Message-----
> From: [hidden email] [mailto:[hidden email]] 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 case
>>> 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 actual baud rate.  An inaccurate baud generator clock will have the same effect 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. Reliability problems are likely to occur before you reach 5%, and I would want no more than about 3% total error including sending and receiving ends, obviously the lower the better.
>
> Mike


The UART samples near the middle of the bit time plus up to 1/4 or 1/16
of a bit time, depending on the oversampling configuration.

So, the first bit can be sampled at up to 75% of its duration for 4x
oversampling and 56.25% for 16x oversampling. There are 8 more bits to
be sampled (including the stop bit). Assuming an oversampling of 4x, you
have only 25% of a bit time of margin. That gives a maximum error of
3.125% but you must remember that the other end probably have some
imprecision too, so divide the error by two.

UARTS usually use a majority of three samples near the middle of the bit
time, what improves things a little.


ASCII Art (fixed pitch font):


Start-bit detected at its very beginning:
------------+                   +-------------------+
            |                   |                   |
            +-------------------+                   +---
            ^                        ^    ^    ^
  |    |    |    |    |    |    |    |    |    |    |    <- sampling points
            |                        |    |    |            (4x
oversampling)
        start bit                     first bit
      detected here                  sampled here


Start-bit late detect:
------------+                   +-------------------+
            |                   |                   |
            +-------------------+                   +---
                ^                        ^    ^    ^
 |    |    |    |    |    |    |    |    |    |    |    <- sampling points
                |                        |    |    |        (4x
oversampling)
            start bit                     first bit
          detected here                  sampled here


As you can see, as soon as the samples start to happen late, the third
sample will be out of the bit cell. If the middle sample also falls
outside of the bit cell you will get a sampling error.


Isaac

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Joe Wronski
In reply to this post by Wouter van Ooijen
On 9/25/2012 8:13 AM, Wouter van Ooijen wrote:

>> Check the requirements of the RS232.  You might want to consider the
>> case 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.
>
> Common misconception! Apart from minor influences like the switching
> time of the hardware the accuracy requirement for asynch does NOT depend
> on the baudrate. Hence you can not escape the accuray requirement by
> running at a lower baudrate.
>
> Wouter van Ooijen
>

True, I guess I was thinking more about interrupt latency with a slow,
but accurate clock and a software uart.

Joe W

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

Justin Richards
In reply to this post by Jan-Erik Söderholm
>
> Or simply belive what the datasheet says.
>
because datasheets are never wrong.
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

M.L.-2
On Wed, Sep 26, 2012 at 9:59 AM, Justin Richards
<[hidden email]> wrote:
>>
>> Or simply belive what the datasheet says.
>>
> because datasheets are never wrong.

Oscillator specs in a data sheet should never be wrong. Do you have
data to indicate that they sometimes are?

--
Martin K.
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

RE: PIC24F; dump external OSC?

Michael Rigby-Jones-2
-----Original Message-----
From: [hidden email] [mailto:[hidden email]] On Behalf Of M.L.
Sent: 26 September 2012 17:36
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] PIC24F; dump external OSC?

On Wed, Sep 26, 2012 at 9:59 AM, Justin Richards <[hidden email]> wrote:
>>>
>>> Or simply belive what the datasheet says.
>>>
>> because datasheets are never wrong.

>Oscillator specs in a data sheet should never be wrong. Do you have data to indicate that they sometimes are?

Microchips (and other vendors) datasheets have suffered from numerous errors over the years.  Why would oscillator specs be more immune to errors than any of the other parameters?

Mike

=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Reply | Threaded
Open this post in threaded view
|

Re: PIC24F; dump external OSC?

sd211
In reply to this post by ivp
IVP <joecolquitt <at> clear.net.nz> writes:

>
> > The baud rate accuracy requirement increases with baud rate
>
> Error percentage is constant, whatever the speed. The only way
> to do serial with any confidence when the clock is liable to drift
> would be to use bit-banging, timing based on a bit length of the
> external transmitter. Not as convenient as the PIC's serial module
> but it can be done fairly easily

This can be done with auto baud rate detection in some of the PICs.  I believe
PIC24 have it.  In this case, sender will send 1 byte to sync the clocks, while
receiver is in the auto baud detection mode.  I have a test setup that does this
with PIC24FJ32GA002 and an internal RC oscillator.  Works fine even with main
clock frequency variation as high as 4x ( +/- PLL).  The auto baud sync can be
done before each packet of information.  In this case, even crappy RC clock is
stable over short, ~10-100 ms, time.

Sergey Dryga

http://beaglerobotics.com




--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
12