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'[PIC]: RC oscillator'
2000\05\20@010721 by MCMANUS, Don

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Hi,

       I have an application which requires a an RC oscillator.  Since the
design will be subjected to high vibration in the field, it is felt that a
crystal or resonator would be too fragile.

       Right now we are operating at 4MHz, but this is may end up at 1 MHz
or below.

       The problem is that when we test the oscillator over temperature it
moves several percent between 25C and 50 C, at which point it can no longer
maintain serial communications because the clock has drifted too far. This
is unexpected because the we are using top quality high stability resistors
and capacitors, which should only move about 20-30 PPM/C each.

       I thought I had seen graphs of the stability of the RC mode from
device to device and over temperature, but I can't locate these in the
datasheet or app notes.

       I would like to know if anyone out there has experience with the RC
oscillators performance over extended temperature(we'd like to go up to
125C).  Hopefully I could get a few suggestions to stabilize our design.
Thanks in advance for any help

Don McManus
Bently Nevada Corporation

2000\05\20@013240 by Spehro Pefhany

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At 08:42 AM 5/18/00 -0700, you wrote:

>        I thought I had seen graphs of the stability of the RC mode from
>device to device and over temperature, but I can't locate these in the
>datasheet or app notes.

I don't think that the RC mode is suitable for serial communications,
not in a production product that has to operate over a wide temperature
range.

Why don't you put an SMT or a 20 cent leaded resonator on there? You will get
guaranteed acceptable performance (without a lot of margin, but O.K.)
Resonators are much more robust than crystals.

Only a percent or two change and you will have decreased serial noise
immunity,
at 5% or so it typically will stop working.

I have used LC oscillators on PIC's to improve the guaranteed accuracy, but
the
accuracy certainly not good enough for this application. The stability,
OTOH,
might be just ok for asynchronous serial comms.


Best regards,
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2000\05\20@233820 by Russell McMahon

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>        I have an application which requires an RC oscillator.  Since the
>design will be subjected to high vibration in the field, it is felt that a
>crystal or resonator would be too fragile.


Don,

RC oscillators can be made stable enough for controlling serial async data
transfer but in your circumstances I would first consider whether a
resonator or crystal is really as fragile as you think it is. How strong is
a capacitor compared to a resonator?

What sort of environment are you in?
Properly mounted a resonator or crystal will survive massive accelerations.
I don't know details on what they use but the military fire GPS units in
artillery shells (!)
Unless you are using this equipment to tamp in your sticks of gelignite you
are unlikely to get higher accelerations than that.
Equipment with survivable accelerations of in the order of 1000 g is quite
standard.
Close attention to mounting detail and massive common sense seems to cover
most of it.
I don't know about the internal mounting structures in crystals but
resonators "seem" to be rather more monolithic.

You could try an easy test.
Mount a resonator on a steel block - hold in place with epoxy and press
firmly into place against the steel as the epoxy sets.
Drop steel block from various heights onto paper sheet on steel plate on
concrete floor (resonator uppermost :-)) from various heights.
If one assumes about zero compression of the steel-steel interface any
deceleration is carries out over the thickness of the paper (or less as it
will have some compression thickness).

Acceleration in g's = Fall distance/Stopping-distance
eg for a 0.010" = 0.25mm = 0.00025m thick stopper
A drop from 250mm results in a deceleration of 0.250/0.00025
= 1000g

IF this kills your resonator try embedding it in a very small amount of
stiffish padding and try again.
The g forces will be MUCH reduced (as seen above) by allowing it even a
small amount of "compliance".

regards


     Russell McMahon
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'[PIC]: RC Oscillator'
2002\01\10@095130 by Jafta
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Hi All

Is there a rule-of-thumb to be used when using RC on F876's?  The book says 3K - 300K etc, but how do I know the frequency?  Can I just use T=RC?

TYIA

Chris A

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2002\01\10@104719 by Dave Dilatush
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Chris A wrote...

>Is there a rule-of-thumb to be used when using RC on F876's?  The
>book says 3K - 300K etc, but how do I know the frequency?  Can
>I just use T=RC?

The PIC Mid-Range Reference Manual (and many of the device data sheets
too, if I recall correctly) gives some data on RC oscillator
performance.  Table 31-1 on page 628 of the MRRM (document DS31031A)
gives oscillator frequency as a function of various resistor/capacitor
values.

Dave

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2002\01\10@110744 by Alan B. Pearce

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>>Is there a rule-of-thumb to be used when using RC on F876's?  The
>>book says 3K - 300K etc, but how do I know the frequency?  Can
>>I just use T=RC?

>The PIC Mid-Range Reference Manual (and many of the device data sheets
>too, if I recall correctly) gives some data on RC oscillator
>performance.  Table 31-1 on page 628 of the MRRM (document DS31031A)
>gives oscillator frequency as a function of various resistor/capacitor
>values.

Jinx used to have curves on his website IIRC that were frequencies he
measured with a 16F84 series device. This should not be too different to any
of the other 16... series devices at similar voltage to what he used. I
cannot remember the URL unfortunately.

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2002\01\10@111826 by Jafta

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Thanks Dave, I have not read to page 628 yet....glad you did.  You solved my
4MHZ problem.

Alan, I'm going to check Jinx's page, thanks.  Maybe I can get a R:C
relation.

Regards

Chris A

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2002\01\10@144231 by Jinx

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home.clear.net.nz/pages/joecolquitt/0f84rc.html

Try a couple of values with the 876, see how it goes

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2002\01\10@150329 by Jinx

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I just realised that if you were asking about RC speeds
then you probably don't have a way of measuring the
frequency directly. The simplest way I can think of to do
it indirectly would be to set up a timer IRQ and measure
how long it takes for a certain number of IRQs. eg if the
RC you choose gets you 1MHz, then that will be approx
1000 IRQ/sec  (= 1,000,000/4/256). Using a pre-scaler
and counters to extend the measurement period and a
stopwatch should give you a pretty good idea of what the
actual RC frequency is. It'll be as close as you could expect
with T & V dependent RC anyway

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2002\01\10@152855 by Jafta

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Jinx

Although timing is not critical, to use a delay loop - delay_100ms( 4 ) -
would make more sense if I knew the osc freq.  I thought that there would be
a rule-of-thumb to calculate these things.

Regards

Chris A

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2002\01\11@013433 by Jinx

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> I thought that there would be a rule-of-thumb to calculate
> these things

Not that I'm aware of, but maybe you could deduce a formula
from my chart. It's reasonably straight-line. Not that I bothered,
sorry. Didn't need to. There is some variance between different
PICs (both individually and families), as you'll see from the loose
specs in the data sheet (ISTR 7% ?) and tolerances of caps
and resistors plus temperature fluctuations.

If you need a 100ms delay, work out how many instruction cycles
to do that, that is both in the internal 00-FF loop and the external
counter incrementation

So a basic loop takes 1 + 256 +1 + (256 * 2) = 770 cycles (did I
get that right ? Been a very very busy day)

         clrf  count1            (1)
loop   incfsz count1,f      (1) or (2)
         goto  loop              (2)

At 4MHz this would take 770us. Bump another counter 128 times
(just test its bit7) to give you the 100ms, give or take the "incf" and
"goto loop" and test for the second counter byte, which will be a
small fraction of the overall time

But you still need, I think, to set up a timer IRQ routine as I
suggested to actually measure the frequency if you have no
scope or freq counter

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2002\01\11@024031 by Jafta

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Hi Jinx

For the sake of curiosity, I will measure, and post my findings.

Regards

Chris A

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