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'[PIC] 12c508a Timing'
2004\09\14@185049 by Joe Jansen

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I have been searching the datasheet for the 12c5xx  (# DS40139E) and
am probably overlooking it, but cannot seem to find what I am
searching for.

If I am using the internal RC on a 12c508a chip, and assuming that the
OSCCAL register is properly set, what is the frequency tolerance for
the clock?  ie:  4 MHz, +/- what %?

Table 13-3 (page 87) gives some min and max values that work out to
about 9.6% either way, which I assume (HOPE!!!) is the range that
OSCCAL can adjust it either way.  What I cannot find is what the max
variance is based on a properly set OSCCAL.

If this is obviously available, just point out where, and I can search
again from there.

Incidentally, and maybe this is the answer, since the 12c508a uses 6
bits for OSCCAL, and the min and max is 9.6% either way (roughly) for
a total variance of 19.2%,  taking .192 / 63 (bits resolution) gives
me an accuracy of 0.3% (again, roughly).  Does that seem reasonable?

Thanks!

--Joe
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2004\09\14@220633 by Bob Ammerman

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{Quote hidden}

I am afraid I have bad news for you: the specifications for MIN and MAX in
the datasheet are assuming that OSCCAL is set correctly.

Even the newest PICs are not doing real well for accuracy on internal RC.
Also, the RC accuracy seems to be a spec that mChip often ends up having to
loosen up (via an errata) after they actually get some silicon out there. In
other words: take the numbers on a newly announced/released chip with a
grain of salt.

Bob Ammerman
RAm Systems

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2004\09\14@221820 by Jose Da Silva

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It doesn't answer your question, but just thought you may want to know before
you accidently overwrite it...

You need to read the OSCCAL value within the chip and save the value
somewhere (for example if it's the ceramic reprogrammable chip, just jot it
down on the chip itself using a felt marker), or avoid programming over it
during programming sequence (OTP), since it is set at the factory for the
"best" value tested at the factory.

On Tuesday 14 September 2004 03:50 pm, Joe Jansen wrote:
> Incidentally, and maybe this is the answer, since the 12c508a uses 6
> bits for OSCCAL, and the min and max is 9.6% either way (roughly) for
> a total variance of 19.2%,  taking .192 / 63 (bits resolution) gives
> me an accuracy of 0.3% (again, roughly).  Does that seem reasonable?

0.3% sounds really nice if you can hold steady the other variables such as
voltage, current and temperature.   :-)

If we assume that the internal clock is an RC circuit, you may want to lump
accuracy at about 2 digits, so you may say 4.0MHz, but you do not want to
say 4.00MHz.
You may want to stick with 4.0+-.05MHz.
A ceramic resonator will get you 3 or 4 digits of accuracy.
A crystal gets you about 5 to 6 digits of accuracy... 4.0000 or 4.00000 mhz
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2004\09\14@223446 by Jinx

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> I am afraid I have bad news for you: the specifications for MIN and
> MAX in the datasheet are assuming that OSCCAL is set correctly.
>
> Even the newest PICs are not doing real well for accuracy on
> internal RC

I must say I was disappointed when first booting up a 12F675 with
internal RC. The "nominal" 4MHz turned out to be anything up to
4.3MHz. I plan to use a one-wire data transfer from several 675s
to another micro, and because of the uncertainty an autobaud routine
at the receiver is necessary



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2004\09\15@015613 by Wouter van Ooijen

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> I must say I was disappointed when first booting up a 12F675 with
> internal RC. The "nominal" 4MHz turned out to be anything up to
> 4.3MHz. I plan to use a one-wire data transfer from several 675s
> to another micro, and because of the uncertainty an autobaud routine
> at the receiver is necessary

You mean Dallas one-wire, or your own protocol? In both cases I think a
10% variation can easily be accomodated.

Wouter van Ooijen

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Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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2004\09\15@043701 by Andrew Warren

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Jinx <spam_OUTpiclistTakeThisOuTspammit.edu> wrote:

> I must say I was disappointed when first booting up a 12F675 with
> internal RC. The "nominal" 4MHz turned out to be anything up to 4.3MHz.
> I plan to use a one-wire data transfer from several 675s to another
> micro, and because of the uncertainty an autobaud routine at the
> receiver is necessary

Depends on the protocol you use.  Manchester encoding (with a decoder
written intelligently), for instance, can easily tolerate that much
variation in the clocks of the transmitter and receiver without any
autobaud calculations... And so could a pulse-width scheme where,
say, a 0 is a 50-microsecond (nominally) wide pulse, a 1 is a 100-
microsecond-wide pulse, and there's a short gap between each bit.

Or, easier, a scheme where a 0 is an x-microsecond gap followed by a
2x-microsecond pulse, and a 1 is a 2x-microsecond gap followed by an
x-microsecond pulse (with a "start pulse" in front of your message,
of course).

-Andy

=== Andrew Warren - .....aiwKILLspamspam@spam@cypress.com
===
=== Principal Design Engineer
=== Cypress Semiconductor Corporation


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2004\09\15@070133 by Bob Ammerman

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> > I must say I was disappointed when first booting up a 12F675 with
> > internal RC. The "nominal" 4MHz turned out to be anything up to
> > 4.3MHz. I plan to use a one-wire data transfer from several 675s
> > to another micro, and because of the uncertainty an autobaud routine
> > at the receiver is necessary
>
> You mean Dallas one-wire, or your own protocol? In both cases I think a
> 10% variation can easily be accomodated.
>
> Wouter van Ooijen

Dallas one-wire actually accommodates slaves with (almost) -50% to +100%
timing relative to the nominal bit time.

Bob Ammerman
RAm Systems

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2004\09\15@081109 by olin_piclist

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Joe Jansen wrote:
> Incidentally, and maybe this is the answer, since the 12c508a uses 6
> bits for OSCCAL, and the min and max is 9.6% either way (roughly) for
> a total variance of 19.2%,  taking .192 / 63 (bits resolution) gives
> me an accuracy of 0.3% (again, roughly).  Does that seem reasonable?

No, that's the adjustment precision, NOT the accuracy.  I sortof remember
these chips having around 5% overall oscillator accuracy with the factory
OSCCAL value.


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2004\09\15@085502 by Jinx

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> micro, and because of the uncertainty an autobaud routine at the
> > receiver is necessary
>
> Depends on the protocol you use.  Manchester encoding

Quite so. I've been playing around with Manchester in another
project and quite honestly I'd not made the obvious connection
to use it in this application too. It's been one of those weeks. As
you say, there are several ways it could be done, none of them
too difficult to achieve reliable transfer with. Autobauding just
requires timing a preamble bit, and it appears to be satisfactory
for the moment. Now that a data transfer has proved the h/w
works (there's not a simple physical connection between the
micros) it could be time to look at some optimisation in the s/w

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2004\09\16@083249 by Joe Jansen

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On Wed, 15 Sep 2004 08:11:06 -0400, Olin Lathrop
<olin_piclistspamKILLspamembedinc.com> wrote:
>
> No, that's the adjustment precision, NOT the accuracy.  I sortof remember
> these chips having around 5% overall oscillator accuracy with the factory
> OSCCAL value.

Is the 5% based on experience, or is it documented somewhere?

Hrm..  So I can control precisely where the frequency wanders around?
<sarcasm>  That's handy...  </sarcasm>

Since the 10F chips are internal oscillator only, are they any better
at holding a frequency?  Anybody with some of these looked into this
at all?  Table 12-4 claims 1%, but Temp and Vdd are TBD.

Thanks for all the feedback.  Not exactly what I had hoped to hear, tho...  <G>
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2004\09\16@113023 by William Chops Westfield

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On Sep 16, 2004, at 5:32 AM, Joe Jansen wrote:

> So I can control precisely where the frequency wanders around?
> <sarcasm>  That's handy...  </sarcasm>
>
Why, yes it is.  See the ATtiny11 for a different scheme. :-(
And frequency will be dependent on things like temp and supply
voltage, which can be controlled to some extent, so the freq
may not be as variable as the spec sheet (must) imply.


> Since the 10F chips are internal oscillator only, are they any
> better at holding a frequency?
>

I think so.  The 10F claims a 'precision' internal clock, like the
12f675, which is distinct from the simple 4MHz oscillator of the 12c508.
I think microchip got a message that people wanted the internal clock
to be good enough to do rs232...

BillW

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2004\09\16@114443 by Dave VanHorn

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At 10:30 AM 9/16/2004, William "Chops" Westfield wrote:

>On Sep 16, 2004, at 5:32 AM, Joe Jansen wrote:
>
>>So I can control precisely where the frequency wanders around?
>><sarcasm>  That's handy...  </sarcasm>
>Why, yes it is.  See the ATtiny11 for a different scheme. :-(

To be fair, the tiny11's scheme is atypical of the AVR line.
AFAIK, all the others have an Osccal register, and you can calibrate the internal osc.

>And frequency will be dependent on things like temp and supply
>voltage, which can be controlled to some extent, so the freq
>may not be as variable as the spec sheet (must) imply.

One would hope that the spec sheets always show the guaranteed limits.

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