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'[PIC] About the Precision Internal Oscillator'
2008\06\11@221851 by Matthew Miller

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Hi everyone, this is just a curiosity question that came to me. Many of the
newer PICs have this Precision Internal Oscillator and it does seem to be
pretty good. I know that with a 12F629, I had set the OSCCAL register in
order for a software UART to work correctly. But, I'm now using a 16F689 and
with this Precision Internal Oscillator the built in EUSART works just fine
without any calibration.

So my question is this: how has the technology changed? Is a mini ceramic
resonator present in these new chips? :) Or is it just a better designed RC
oscillator?

Thanks for any insight! Matthew

2008\06\11@224228 by M. Adam Davis

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Better designed internal oscillator, less sensitive to temperature and
voltage change (perhaps temperature compensated?).

Don't count on it for RS-232 and other timing applications if you plan
to run it to its extremes in temperature or voltage - it can vary as
much as +/-3.2% or so ( see figures 18-39 to 18-42 for graphs) which
is greater than the 2.5% I usually restrict myself to when talking
about RS-232 acceptable clock error.

Still, if you drive it with a reasonable voltage, and stay within a
restricted temperature range, then it should work reasonably well (ie,
+/- ~1%).

-Adam

On Wed, Jun 11, 2008 at 10:18 PM, Matthew Miller <spam_OUTnamiller2TakeThisOuTspamnaxs.net> wrote:
{Quote hidden}

> -

2008\06\11@230356 by Xiaofan Chen

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On 6/12/08, Matthew Miller <.....namiller2KILLspamspam@spam@naxs.net> wrote:
> So my question is this: how has the technology changed? Is a mini ceramic
> resonator present in these new chips? :) Or is it just a better designed RC
> oscillator?

In the case of 16F689, it is just a better designed RC oscillator. Maybe this
just because of the process. Maybe Microchip added some circuitry.

Microchip's internal RC is still not as good. For example, you can not
use the internal RC oscillator for USB 18F PICs (like 18F2550/18F4550).
Silicon Labs C8051F 3xx USB MCUs can actually use internal
RC oscillator for USB operations. I think they add quite some
circuitry to achieve that.


Xiaofan

2008\06\11@231031 by Xiaofan Chen

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On 6/12/08, Matthew Miller <namiller2spamKILLspamnaxs.net> wrote:
> But, I'm now using a 16F689 and
> with this Precision Internal Oscillator the built in EUSART
> works just fine without any calibration.
>

Actually you can still use OSTUNE register to change the
internal RC oscillator frequency (HFINTOSC only, not the
LFINTOSC) if you really want for 16F689.

Microchip has several implementations of tuning the
internal RC oscillator frequency. Some of them use
OSCCAL written in one of the Flash Bytes. Some
of them use the OSTUNE register, some of them
use one or two configuration words (0x2008 or
0x2008/2009).

Xiaofan

2008\06\11@232432 by Matthew Miller

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On Wed, Jun 11, 2008 at 10:42:24PM -0400, M. Adam Davis wrote:
> Better designed internal oscillator, less sensitive to temperature and
> voltage change (perhaps temperature compensated?).
>
> Don't count on it for RS-232 and other timing applications if you plan
> to run it to its extremes in temperature or voltage - it can vary as
> much as +/-3.2% or so ( see figures 18-39 to 18-42 for graphs) which
> is greater than the 2.5% I usually restrict myself to when talking
> about RS-232 acceptable clock error.
>
> Still, if you drive it with a reasonable voltage, and stay within a
> restricted temperature range, then it should work reasonably well (ie,
> +/- ~1%).

Right now at 5V and room temp. 9600 buad and 1 minute measurements work well
with the 16F689. I'll soon see how that 1 minute timer works in full sun
shine and using 3AA batteries. Data transmission over serial can occur
in-doors, but hopefully the 1 minute interval won't change too much.

Has any manufacture placed ceramic oscillators in their chip packages
before?

Matthew

2008\06\11@233212 by Apptech

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> Better designed internal oscillator, less sensitive to
> temperature and
> voltage change (perhaps temperature compensated?).
>
> Don't count on it for RS-232 and other timing applications
> if you plan
> to run it to its extremes in temperature or voltage - it
> can vary as
> much as +/-3.2% or so ( see figures 18-39 to 18-42 for
> graphs) which
> is greater than the 2.5% I usually restrict myself to when
> talking
> about RS-232 acceptable clock error.
>
> Still, if you drive it with a reasonable voltage, and stay
> within a
> restricted temperature range, then it should work
> reasonably well (ie,
> +/- ~1%).

Almost just saying "what he said". Almost.

RS232 with N bits will just mis-sample at ~~= 100/(2N+1) %
total error or about 5% total error.
That's error between both ends so if you got opposing 3%+
per end you would be dead, as Adam says.

Also, your received needs to be a bit smart or lucky to work
right out to bit edges OK.
Really clever code can try to resynch on bit data
transitions, but much isn't that clever, and that may fail
for some data. If the drifts are slow, as you'd expect, then
bit timed clock speed recovery can allow much better
results. Even cleverer code needed.

Also as he says, if you can get both ends to drift the same
way (eg common temperature change) or limit drift then all
may be well.




       Russell


2008\06\12@092724 by olin piclist

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Matthew Miller wrote:
> I'm now using a 16F689 and with this Precision Internal Oscillator
> the built in EUSART works just fine without any calibration.

That is only OK if the PIC is powered from at least 2.5V and is kept with
0-85C temperature.  Otherwise you can have up to 5% baud rate error.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\06\12@133332 by Vasile Surducan

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On 6/11/08, Matthew Miller <.....namiller2KILLspamspam.....naxs.net> wrote:
> On Wed, Jun 11, 2008 at 10:42:24PM -0400, M. Adam Davis wrote:
> > Better designed internal oscillator, less sensitive to temperature and
> > voltage change (perhaps temperature compensated?).
> >
> > Don't count on it for RS-232 and other timing applications if you plan
> > to run it to its extremes in temperature or voltage - it can vary as
> > much as +/-3.2% or so ( see figures 18-39 to 18-42 for graphs) which
> > is greater than the 2.5% I usually restrict myself to when talking
> > about RS-232 acceptable clock error.
> >
> > Still, if you drive it with a reasonable voltage, and stay within a
> > restricted temperature range, then it should work reasonably well (ie,
> > +/- ~1%).
>
> Right now at 5V and room temp. 9600 buad and 1 minute measurements work well
> with the 16F689. I'll soon see how that 1 minute timer works in full sun
> shine and using 3AA batteries. Data transmission over serial can occur
> in-doors, but hopefully the 1 minute interval won't change too much.

My experience with 16F676 and internal oscillator at 115200 is not very good.
How many boards have you tested and on how many computers ?
Sice then I'm not using only zero error crystals for communications
and believe me it worth.

Vasile

2008\06\12@182834 by Mark Rages

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On Thu, Jun 12, 2008 at 12:33 PM, Vasile Surducan <EraseMEpiclist9spam_OUTspamTakeThisOuTgmail.com> wrote:
{Quote hidden}

The error doesn't change with baudrate.  I've used the internal 8MHz
with 16f688 at 115200 baud, 120 copies and had no problems.  The
message format had a checksum, however.  And they were fed
linear-regulated +5V and kept at indoor-office temperatures.

Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
@spam@markragesKILLspamspammidwesttelecine.com

2008\06\13@141207 by Vasile Surducan

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On 6/12/08, Mark Rages <KILLspammarkragesKILLspamspamgmail.com> wrote:
{Quote hidden}

The error is changing with different sprg value (for PIC's with
USART). And with different PC's on which are connected. PIC16F676 has
no usart. Maybe that makes difference.

Vasiliii

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