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'[PIC]: PIC oscillator'
2002\02\16@005731 by Jinx

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Does anyone know the internal structure of a PIC's oscillator
section ? I'm trying to sort out a circuit that can have a crystal
and a pair of caps plugged in that doesn't require tuning, and
works over the range of 4-20MHz. I've tried both of the circuits
suggested by MC for parallel/series crystals but so far neither
is flexible enough or needs fiddling with over the full range of
crystals that a PIC (or indeed any other micro) will accept

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2002\02\16@011423 by sirish

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On Saturday 16 February 2002 00:51, you wrote:
> Does anyone know the internal structure of a PIC's oscillator
> section ? I'm trying to sort out a circuit that can have a crystal
> and a pair of caps plugged in that doesn't require tuning, and
> works over the range of 4-20MHz. I've tried both of the circuits
> suggested by MC for parallel/series crystals but so far neither
> is flexible enough or needs fiddling with over the full range of
> crystals that a PIC (or indeed any other micro) will accept

i think your problem may be in the Oscillator mode in the CONFIG register.
With _XT_OSC, i have been able to use crystals upto 12MHz. The 16 MHz crystal
needed the _HS_OSC to work.

I am not sure, but most likely this is the problem.

Any Comments? others ?

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2002\02\16@013109 by Graeme Zimmer

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----- Original Message -----
From: "Jinx" <spam_OUTjoecolquittTakeThisOuTspamCLEAR.NET.NZ>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Saturday, February 16, 2002 4:51 PM
Subject: [PIC]: PIC oscillator


>  I'm trying to sort out a circuit that can have a crystal
> and a pair of caps plugged in that doesn't require tuning

Jinx, I've never had the standard "Pye" type circuit NOT work,
as long as normal cut Xtals were used.

I would think that it is more a matter of avoiding weird Xtals.

What problems do you notice?

..................... Zim

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2002\02\16@020106 by steve

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> Does anyone know the internal structure of a PIC's oscillator
> section ? I'm trying to sort out a circuit that can have a crystal and
> a pair of caps plugged in that doesn't require tuning, and works over
> the range of 4-20MHz. I've tried both of the circuits suggested by MC
> for parallel/series crystals but so far neither is flexible enough or
> needs fiddling with over the full range of crystals that a PIC (or
> indeed any other micro) will accept

If you can afford the frequency tolerance, it might be a good time to
look at your new found friend, the ceramic resonator. Layout for a 3
terminal version and plug in whichever frequency you need. They
are more forgiving to stray everythings than a crystal circuit.

Steve.

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TLA Microsystems Ltd         Microcontroller Specialists
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2002\02\16@054033 by Jinx

face picon face
> Jinx, I've never had the standard "Pye" type circuit NOT work,
> as long as normal cut Xtals were used.

> I would think that it is more a matter of avoiding weird Xtals.

> What problems do you notice?

> ..................... Zim

The immediate need is for a 3.6864MHz clock source that
can drive upwards of 6 PICs for an RS232 system. I'm not
aware of a resonator < 4MHz that could be used for 0%
comms unfortunately. There's a 7.37MHz though (2 x 3.685,
= 3.6864 x 0.9996)

(One option, just to keep things simple, if defeatist, would be
to run one PIC at 14.7456MHz and use Clkout = osc/4 for
the others)

The crystal is a Fox0368S. Maybe the S is for series ? I've
got Fox 10MHz crystals here that I use with the PICs and
they aren't marked with a P (unlike the back of our couch,
damn cat) so perhaps the S is meaningless in this context

In front of me now I have a Pierce circuit. A 3.6864 crystal in
parallel with an LS04 gate, 2 x 22p to ground, 470R across
the crystal (originally a 330k pot), like this one

http://vlsi.wpi.edu/webcourse/ch05/ch05.html

oscillating at 0.4V with a very nice 48MHz sine wave.

[update while off-line]

However, after re-reading the text with the text the link below
I've changed that to an HC and the o/p is now 11.092MHz (3rd
harmonic) at 4.5V, so that's an improvement. By changing the
feedback resistor to 4k7 in series with a 100k pot I can tune
the circuit to o/p the required 3.6864MHz. Grounding the crystal
case helps a lot. But it doesn't take much for the waveform
and frequency to go nutso, and it doesn't start reliably

Now, this one here (bottom of page in black), uses a series
resistor as a filter

http://www.gaby.de/z80/uexosc.htm

but I don't know what calculation he's using to work out the
value of the resistor. Some 1/ sqr rt pi rfc I guess

The others on that page look like the examples in the PIC
manual. The parallel one is what I tried first. But as you read
in my original request, you don't have to twiddle with pots on
a PIC. The best so far is the series circuit. With 470R feedback
resistors, no caps and an HC00, it will start reliably with either
a 3.6864MHz or a 9.8304MHz crystal, at good voltage and
wave quality

What I don't understand is that these are parallel cut crystals,
the type I use all the time with micros (which require parallel
cut), yet the series circuit seems to work so much better

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2002\02\16@070111 by Dave Dilatush

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Jinx wrote...

>What I don't understand is that these are parallel cut crystals,
>the type I use all the time with micros (which require parallel
>cut), yet the series circuit seems to work so much better

I'm no expert on crystal oscillators; so I could be wrong here.  But
AFAIK there isn't anything structurally different between "parallel"
resonant and "series" resonant crystals; all crystals have parallel and
series resonant frequencies, as I understand it, and a particular unit
is defined as parallel or series simply because that's the resonant mode
that'll give you the frequency that's marked on the can.  
Either mode will "work" but only the mode specified will give the right
frequency.  The series mode resonance and parallel mode resonant
frequencies may be very close together.  I think.

In any case, there's a good application note at Fairchild, "HCMOS
Crystal Oscillators", at
  http://www.fairchildsemi.com/an/AN/AN-340.pdf

which might be worth a read.

Need more coffee...

Cheers,

Dave

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2002\02\16@071600 by Jinx

face picon face
>  http://www.fairchildsemi.com/an/AN/AN-340.pdf

Despite their attempts, I'm afraid it's still all a bit of a black
art. The drivers in micros must be more complex than these
parallel circuits, the type I've found so flakey. Do micros have
AGC or some detector in them so they will operate over a
wide range of frequencies ? For example, what is switched
in/out when the micro is set to LP, XT or HS ?

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2002\02\16@073119 by Dave Dilatush

picon face
Jinx wrote...

>>  http://www.fairchildsemi.com/an/AN/AN-340.pdf
>
>Despite their attempts, I'm afraid it's still all a bit of a black
>art. The drivers in micros must be more complex than these
>parallel circuits, the type I've found so flakey. Do micros have
>AGC or some detector in them so they will operate over a
>wide range of frequencies ? For example, what is switched
>in/out when the micro is set to LP, XT or HS ?

Alas, it is indeed a black art.  I don't know what's being switched in
the PICs between LP, XT, and HS, but Intersil has a CMOS crystal
oscillator chip, the HA7210, that has a pair of frequency range select
inputs which change the internal biasing on the internal amplifier to
optimize its transconductance and power dissipation depending on the
operating frequency.  Perhaps PICs are similar?  (BTW, the HA7210 was
the first thing I thought of in response to your original question; but
it's only guaranteed to work up to 10 MHz.)

More coffee...

Dave

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2002\02\16@073932 by Jinx

face picon face
> (BTW, the HA7210 was the first thing I thought of in
> response to your original question; but it's only
> guaranteed to work up to 10 MHz.)

I use it occassionally as a 32kHz buffer

> More coffee...

Oscillators have done me in for today

SLEEP  SLEEP
GOTO    SLEEP

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2002\02\16@075223 by Dave Dilatush

picon face
Jinx wrote...

>Oscillators have done me in for today
>
>SLEEP  SLEEP
>GOTO    SLEEP

Nitey nite...
Meanwhile, I'll try to think of something.

Dave

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2002\02\16@080254 by Jinx

face picon face
> Nitey nite...

> Meanwhile, I'll try to think of something.

Cheers. If you suggest a circuit, don't forget to give it
a lewd name

(Benny Hill is not dead - he's Elvis' chaffeur)

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2002\02\18@052304 by Vasile Surducan

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On Sat, 16 Feb 2002, Dave Dilatush wrote:

> Jinx wrote...
>
> >What I don't understand is that these are parallel cut crystals,
> >the type I use all the time with micros (which require parallel
> >cut), yet the series circuit seems to work so much better
>
> I'm no expert on crystal oscillators; so I could be wrong here.  But
> AFAIK there isn't anything structurally different between "parallel"
> resonant and "series" resonant crystals; all crystals have parallel and
> series resonant frequencies, as I understand it, and a particular unit
> is defined as parallel or series simply because that's the resonant mode
> that'll give you the frequency that's marked on the can.
>
 Well, my clocks and Roman's Bresenham accurate timing methode said that
never the real oscillating frequency are marked on the can...
There are some important diferences about serial and parallel resonant
frequencies. Any cuartz crystal have three major cutting axis. The most
important one is the thermal ax. An unadvised user of a cuartz
oscillator will say: "what big deal ? we use an owen an keep the
oscillator at the same temperature " If you'll have in your hands a
crystal which was cutted on electrical or mechanical axis the
thermostating effect can be worst than keeping the whole oscillator on
ambiant temperature. And everything depends by distance of the cutted
material measured to thermal axis.
Why the quartz producers don't cut only on thermal axex ?
Easy, because of money, 1/3 from the quartz crystall must be dropped to
the garbage. The resonant modes of any cuartz take care about the
thickness of the cuartz and the cutting axis.
BTW, the best accuracy I've got with bresenham methode was +/- 2
sec/month. Could be better ?

best regards, Vasile

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2002\02\18@054956 by Alan B. Pearce

face picon face
>>What I don't understand is that these are parallel cut crystals,
>>the type I use all the time with micros (which require parallel
>>cut), yet the series circuit seems to work so much better

>I'm no expert on crystal oscillators; so I could be wrong here.  But
>AFAIK there isn't anything structurally different between "parallel"
>resonant and "series" resonant crystals; all crystals have parallel and
>series resonant frequencies, as I understand it, and a particular unit
>is defined as parallel or series simply because that's the resonant mode
>that'll give you the frequency that's marked on the can.

Correct. There may be slight differences in the processing to change the ESR
of a series mode specification, versus the parallel resonant frequency at a
certain parallel capacitance. In parallel resonant mode the processing will
most likely include minimising the close (and harmonic) spurs that the
crystal may try and oscillate at. If you have a look at the picture in the
Fairchild App note and expanded that to a wider frequency, you would see
more of the same reactance's (although of different amplitudes) quite close
to the nominal resonant frequency. It is always worth having a look at a
crystal using a spectrum analyser with tracking oscillator - it makes you
wonder how the thing ever finds the correct frequency. Marconi used to have
an App Note that showed how to do this with the 2370 Spectrum Analyser that
they built back in the 1970's.

>Either mode will "work" but only the mode specified will give the right
>frequency.  The series mode resonance and parallel mode resonant
>frequencies may be very close together.  I think.

Correct again. It may also be worth remembering that the old oscillator IC
for the 8080 series micros had a tank circuit to ensure the crystal started
in the correct mode.

Typically to have a crystal work in series mode will require a series tank
circuit (both C and L) to force it into series mode - especially running in
overtone mode.

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2002\02\18@065058 by steve

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> Correct again. It may also be worth remembering that the old
> oscillator IC for the 8080 series micros had a tank circuit to ensure
> the crystal started in the correct mode.
>
> Typically to have a crystal work in series mode will require a series
> tank circuit (both C and L) to force it into series mode - especially
> running in overtone mode.

Not quite right. The tank circuit is to supress the fundamental (or
rather, prevent the total circuit from providing the required phase
shift at the fundamental resonance). That's unrelated to the series-
parallel issue.

When a crystal is operating at its series resonant point it has no
reactive component and appears purely resistive. To make an
oscillator with one, requires that all of the required 360 degree
phase shift is provided by the amplifier.
As the speed goes up, that becomes more difficult as any delay is
a phase shift.
Also, because the impedance curve is close to horizontal at the
series resonant point, it only takes a small amount of capacitance
to move the frequency around. This is good if you want it to be
(tuning RTC crystals) and bad if you don't want it to be (flux
residues, fingers, etc).

When a crystal is operating at its parallel resonant point, it appears
as a huge inductor, which together with the capacitors, provides
180 degrees of phase shift. Add another 180 degrees from your
amplifier and away it goes.

The tank in the 8080 circuit would allow you to use an overtone
crystal which would have been cheaper. The fundamental frequency
of a crystal is inversely proportional to its thickness so high
frequency parts are harder to make.

Steve.


======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: .....stevebKILLspamspam.....tla.co.nz      fax +64 9 820-1929
======================================================

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2002\02\18@074142 by Graeme Zimmer

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Hi all,

Dunno if this has come up before.....


Intel  AP-155  "Oscillators for Microcontrollers"

http://developer.intel.com/design/mcs51/applnots/230659.htm

Well worth reading.


........................... Zim

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2002\02\18@075223 by Graeme Zimmer

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More Xtal Osc App notes:

http://www.fairchildsemi.com/an/AN/AN-118.pdf

and

http://www.linear.com/pdf/an12fa.pdf



............................. Zim

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2002\02\18@083017 by Vasile Surducan

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On Mon, 18 Feb 2002, Graeme Zimmer wrote:

> More Xtal Osc App notes:
>
> http://www.fairchildsemi.com/an/AN/AN-118.pdf


this one is nice, but says nothing about how to choose
a stable xtal with various cmos family...

Vasile

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2002\02\18@095212 by Bob Ammerman

picon face
> BTW, the best accuracy I've got with bresenham methode was +/- 2
> sec/month. Could be better ?
>
> best regards, Vasile
>

This represents an error of about 0.8 ppm. This is pretty amazing, given
that crystals are generally rated an order of magnitude or 2 worse than
1ppm. (or are you using an oven, etc).

Bob Ammerman
RAm Systems

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2002\02\19@031452 by Vasile Surducan

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Well, my last monday morning mail ( having my tongue tiwsted a little for
Shakespeare's language ) I want to say the truth there is no need for an
owen to achieve extremely stable clocks ( near 25C ! )
The clock ethalon I have used is the german radio broadcasting which
covers all romanian teritory and a standard ( 24 hour with seconds)
commercial clock.
And again, thermostating a cuartz oscillator can gave worst results mostly
because:
Any cuartz crystal can be defined as a series of an Re ( equivalent
resistance ) and Xe ( equivalent reactance ) no matter if they have serial
or parallel configuration ( AT or X axis cutting ). Imagine the graphic
showing the dependence of Xe versus frequency. Any crystal quartz have
two points in which the crystal is pure resistive. These points are
defined as resonant frequency ( fr ) and atiresonant frequency ( fa ).
Any parallel resonant quartz will oscillate somewhere between these two
frequencies and depends of capacitive load on the cuartz.
As an example the standard ( AT axis cutted ) 32768 quartz have fr at
about 32760 and the fa at about 32790. Let say we've got a stable
oscillation at exactly 32768Hz at 25C. Thermostating at higher temperature
( usual at 50...60C ) will shift the oscillating frequency because of
strong variation of Xe with temperature. If the Xe/frequency
characteristic have a high variation at thermostated temperature the effect
will be a high frequency deviation for a very small temperature deviation.
This is happening all the time when insufficient datasheet are comming
with the crystal and your oven is not perfect...
Staying on piclist from about 2 years I heard a lot of funny explanation
about why is needed the series resistor on PI microcontroller oscillating
network. The funniest I ever heard ( and my mouth is now open up to my
ears ) is that some small quartz can be damaged if this resistor is not
used.
AGREE, ONLY IF YOU HAVE USE A 2KG HAMMER ! and smash the litte quartz with
him...
The PI network ( capacitor to gnd, cuartz oscillator, capacitor to gnd )
can be used only with an 180 degree phase shift amplifier. The most common
amplifier on microcontroller is an inverter gate. The phase shift
generated by PI network is extremely sensitive at frequency deviation,
this is the input condition for a stable global oscillation ( with cuartz
crystal ) To have a stable oscillation the equivalent resistance Re must
be zero. Case which is rarely found in practice.
The serial resitance is used to modify a little the phase shift and the
load on the crystal, but only that load created by input/output of the
oscillator amplifier, nothing more.
Some time this help, some time create problems, and this depends by cuartz
structure and phase shift stability of the whole network.
I hope you have understood me now.
For romanian readers of this list, there is an old good book treating this
subject: "CMOS integrated circuits" by Iulian Ardelean and others at
Editura Tehnica, Bucuresti, 1986

Best from Transilvania,

Vasile


On Mon, 18 Feb 2002, Bob Ammerman wrote:

{Quote hidden}

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2002\02\19@034107 by Laszlo Kohegyi

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Hi Vasile,

>about why is needed the series resistor on PI microcontroller oscillating
>network. The funniest I ever heard ( and my mouth is now open up to my
>ears ) is that some small quartz can be damaged if this resistor is not
>used.
>AGREE, ONLY IF YOU HAVE USE A 2KG HAMMER ! and smash the litte quartz with
>him...

Well, I have never used a series resistor yet but once I DID crack a quartz.
I was experimenting with a 32k one with different cmos gates (1-2-3-4 of them)
and different capacitors. I even tried extremeli big values such as 1nF
and the quartz was blown in a few seconds!

> BTW, the best accuracy I've got with bresenham methode was +/- 2

Could you show me any link about this?

Best regards
Les

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2002\02\19@042112 by Vasile Surducan

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On Tue, 19 Feb 2002, Laszlo Kohegyi wrote:

>
> > BTW, the best accuracy I've got with bresenham methode was +/- 2
>
> Could you show me any link about this?
>
 of course, is our loved Roman Black's site:

  http://www.ezy.net.au/~fastvid/one_sec.htm

take also a look at Dwaine Reid ideea by searching the list with topic:
  " Zero error 1 second timer - again"

best regards from neighborhood
Vasile

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