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'[PIC]: Downsides to these oscillators?'
2001\06\10@210453 by Nick Veys

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Hey, I've been looking for a nice, simple oscillator to use in my projects.
My last one, a 16F84 project used a 4MHz full can style oscillator, didn't
use any caps on it, and it was a fairly time-critical project and it worked
just fine.

Now I'm on a 16F870, using a 20MHz CTS Reeves MXO45 1/2-can oscillator.
Again w/o caps, testing my busy waits shows it to be very accurate (not a
perfect test, but still).  I'm wondering what the downside to this is.
Anyone care to shed some light on the adv/disadv of doing this?

I'm assuming the caps are for voltage stability?  So in my final build I'll
probably toss them in.  But how necessary are they really?

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2001\06\10@220144 by Bob Ammerman

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If you are using oscillator modules you neither need nor want caps.

If you are using crystals you need and want caps.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

2001\06\11@090559 by Olin Lathrop

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> Now I'm on a 16F870, using a 20MHz CTS Reeves MXO45 1/2-can oscillator.
> Again w/o caps, testing my busy waits shows it to be very accurate (not a
> perfect test, but still).  I'm wondering what the downside to this is.
> Anyone care to shed some light on the adv/disadv of doing this?

Mostly cost and power, and possibly board space.  The oscillator duplicates
some of the circuitry already in the PIC.  They also tend to draw
surprisingly large current.  Since PICs come with crystal driving circuitry,
I just use a bare crystal and caps about 95% of the time.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

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2001\06\11@143104 by iklas Wennerstrand

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I'm not familiar to the oscillator you wrote about but if you use a ordinary
crystal you can go on reading.
The advantages with a crystal is that its less costly and that you get a
sinus output. A caned oscillator generates square wave and that can generate
frequencies not desirable in the design. (A sharp edge contains over stored
amount of sinus frequencies)
You cant toss away the caps, they are needed. You can experiment with their
values to get the oscilator design to fit your microcontroller. Chose
capacitors from what the crystal manufacturer suggest and then test it in
your system with the PICmicro. Test the design in absolute maximum and
minimum conditions. Max and min Vdd. Test the Vdd min by toggle an I/O far
away from de oscillator and measure with a oscilloscope (crystal oscillator
is very sensitive and can be affected of your probe or even by dirt on the
PCB). It should generate a even square signal. If it is unreliable you have
to play with the C1 and C2. Do the same test at Vdd max but then you need to
measure on the oscillator. The signal should be a nice and smooth sinus
signal. If its cut in the top it can generate unwanted frequencies and
during long time use even damage the crystal. If the amplitude is to big,
try to lower it with a serial resistor or change  c1 and c2. After this is
done go back to test the Vdd min as it might have generated to low amplitude
in that range of the voltage. If you can combine this two extreme in your
design you are quite sure you have done a good oscillator design.
If you search Microchips application notes you find a good oscillator design
guide that if I remember right is called AN588.

Niklas Wennerstrand

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Ämne: [PIC]: Downsides to these oscillators?

Hey, I've been looking for a nice, simple oscillator to use in my projects.
My last one, a 16F84 project used a 4MHz full can style oscillator, didn't
use any caps on it, and it was a fairly time-critical project and it worked
just fine.

Now I'm on a 16F870, using a 20MHz CTS Reeves MXO45 1/2-can oscillator.
Again w/o caps, testing my busy waits shows it to be very accurate (not a
perfect test, but still).  I'm wondering what the downside to this is.
Anyone care to shed some light on the adv/disadv of doing this?

I'm assuming the caps are for voltage stability?  So in my final build I'll
probably toss them in.  But how necessary are they really?

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2001\06\11@152152 by Dale Botkin

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On Mon, 11 Jun 2001, Niklas Wennerstrand wrote:

> You cant toss away the caps, they are needed. You can experiment with their
> values to get the oscilator design to fit your microcontroller.

I'm just curious.  Am I the only one who has never used capacitors with
the crystal, and never had one fail to start?  Or is this one of those "it
will work for more repetitions than you can test for one-offs and
prototypes but don't trust it for production" things?

Dale
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2001\06\11@153541 by Alice Campbell

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Yes.  Sooner or later you will be wasting hours and hours
wodering why a device works at home, but dies in the car 10
miles from home.  With all my other bad habits, i just cant
bear leaving an easy thing like caps to stray capacitance.
I prefer tempting fate other ways.

alice

{Quote hidden}

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2001\06\11@160414 by iklas Wennerstrand

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Dale wrote: I'm just curious.  Am I the only one who has never used
capacitors with
the crystal, and never had one fail to start?  Or is this one of those "it
will work for more repetitions than you can test for one-offs and
prototypes but don't trust it for production" things?

Answer: If you use a crystal it is just plain luck that makes it to work or
maybe the capacitance in the wire that makes things rock. When I have used
Weroboards and wires hanging in the air I have seen this in my own design
but I would definitely not recommend it. Generally oscillator design is not
tested as much as I describe things and usually things work by just applying
some capacitor with approximately right value but then there are those cases
when you experience strange behaviour in your design. Even if my suggestions
are basic oscillator design testing there is a lot of qualified designers
out there that never test this. If you do not do similar test a design can
be failing when the customer using it, and you do not want that. If you have
a marginal design in production it can start failing if the temperature goes
up or down or if the capacitor ages and get slightly different values.
It is also good to do a oscillator design well to enable transitions to
other PICmicro or newer revisions as the internal inverter amplification in
the PICmicro can change. The amplification of the internal inverter is not
mention in the data book. Its not there because it's a parameter that is not
that important as its the whole oscillator design that matters (witch
oscillator used, manufacturer of crystal, capacitor and PCB layouts). Its up
to the designer to develop a secure oscillator design that takes differences
in consideration. To test at max and min Vdd makes the design to be just
perfect. If its perfect it does not matter if the internal inverter
amplifies a bit more or a bit less or if the capacitor ages and change
values slightly. You are secure anyway.

Niklas Wennerstrand


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För Dale Botkin
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Ämne: Re: [PIC]: Downsides to these oscillators?

On Mon, 11 Jun 2001, Niklas Wennerstrand wrote:

> You cant toss away the caps, they are needed. You can experiment with
their
> values to get the oscilator design to fit your microcontroller.

I'm just curious.  Am I the only one who has never used capacitors with
the crystal, and never had one fail to start?  Or is this one of those "it
will work for more repetitions than you can test for one-offs and
prototypes but don't trust it for production" things?

Dale
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2001\06\11@161928 by David VanHorn

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At 10:08 PM 6/11/01 +0200, Niklas Wennerstrand wrote:
>Dale wrote: I'm just curious.  Am I the only one who has never used
>capacitors with
>the crystal, and never had one fail to start?  Or is this one of those "it
>will work for more repetitions than you can test for one-offs and
>prototypes but don't trust it for production" things?
>
>Answer: If you use a crystal it is just plain luck that makes it to work or
>maybe the capacitance in the wire that makes things rock.

No, it's gain and phase shift.
In a closed loop system, with gain >1, you will oscillate at the frequency
where the phase shift = 360 degrees, including whatever shift is in the
amplifier.

The crystal and the caps provide the phase shift.
The crystal provides 180 degrees of phase shift at one particular
frequency, and the amplifier in the chip provides the gain, and the other
180 degrees of shift.

The caps trim it out to the proper frequency, because the circuit interacts
with the crystal. Rather than leave this to chance, we apply a larger value
of C to swamp out the parasitics. The crystal is designed to work with this
larger C value.

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2001\06\11@162118 by David VanHorn

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At 02:25 PM 6/11/01 -0500, Dale Botkin wrote:
>On Mon, 11 Jun 2001, Niklas Wennerstrand wrote:
>
> > You cant toss away the caps, they are needed. You can experiment with their
> > values to get the oscilator design to fit your microcontroller.
>
>I'm just curious.  Am I the only one who has never used capacitors with
>the crystal, and never had one fail to start?  Or is this one of those "it
>will work for more repetitions than you can test for one-offs and
>prototypes but don't trust it for production" things?

I once said this oughtta be a faq:

http://www.dvanhorn.org/Micros/All/Crystals.php


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differentiate a signature line from the text of an email, I am forbidden to
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2001\06\11@170409 by Dale Botkin

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On Mon, 11 Jun 2001, David VanHorn wrote:

> I once said this oughtta be a faq:
>
> http://www.dvanhorn.org/Micros/All/Crystals.php

All I was asking is whether I was the only one who has never -- not once
-- seen a board fail to start without caps.  8080, 8085, Z80, 8048, 8051,
8052, Lord knows how many 7404-based oscillators driving various circuits
and half a dozen PIC flavors, construction ranging from wire wrap to
perfboard to PCB to dead bug, and I've never seen an oscillator not start
since I started playing with these things in about 1979 or '80.  Granted,
I have never put anything into volume production, but that's why I was
asking.

I've read the app notes.  I understand the theory.  That's what makes me
curious.  I'll quit asking questions now before I get a reputation for
stirring up trouble...

Dale (n0xas)
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2001\06\11@180113 by iklas Wennerstrand

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Dale wrote: I'll quit asking questions now before I get a reputation for
stirring up trouble...

Dale,
Do not stop asking questions. I looked at the link from David VanHorn and
got some input that I have not been given if you haven't popped the question
you did.
Applied to humans as a spices: We learn as long as we live. Or was it we
live as long as we learn ;-)
Niklas




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Ämne: Re: [PIC]: Downsides to these oscillators?

On Mon, 11 Jun 2001, David VanHorn wrote:

> I once said this oughtta be a faq:
>
> http://www.dvanhorn.org/Micros/All/Crystals.php

All I was asking is whether I was the only one who has never -- not once
-- seen a board fail to start without caps.  8080, 8085, Z80, 8048, 8051,
8052, Lord knows how many 7404-based oscillators driving various circuits
and half a dozen PIC flavors, construction ranging from wire wrap to
perfboard to PCB to dead bug, and I've never seen an oscillator not start
since I started playing with these things in about 1979 or '80.  Granted,
I have never put anything into volume production, but that's why I was
asking.

I've read the app notes.  I understand the theory.  That's what makes me
curious.  I'll quit asking questions now before I get a reputation for
stirring up trouble...

Dale (n0xas)
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On my desk I have a workstation...

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2001\06\11@210749 by Olin Lathrop

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> I'm just curious.  Am I the only one who has never used capacitors with
> the crystal, and never had one fail to start?  Or is this one of those "it
> will work for more repetitions than you can test for one-offs and
> prototypes but don't trust it for production" things?

Yes.  The capacitors are not there to help startup.  They reduce the loop
gain for harmonics and cause the crystal to be driven with more of a
sinusoid.  Without caps, the crystal could end up oscillating at a harmonics
of its value, and its accuracy may be a bit off if it gets banged with hard
edges.  Some crystals can even be damaged by this.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, RemoveMEolinspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\06\12@042452 by Graeme Zimmer

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Dale,

> All I was asking is whether I was the only one who has never -- not once
> -- seen a board fail to start without caps.  8080, 8085, Z80, 8048, 8051,
> 8052, Lord knows how many 7404-based oscillators driving various circuits
> and half a dozen PIC flavors, construction ranging from wire wrap to
> perfboard to PCB to dead bug, and I've never seen an oscillator not start
> since I started playing with these things in about 1979 or '80.


It's not that they won't oscillate with the wrong capacitance.........
there is so much gain in the circuit that it will almost certainly oscillate
at one frequency or another.

The question is whether the Xtal is properly controlling the frequency or
not.

With the wrong capacitance the Xtal will certainly be on the wrong frequency
and it will very likely do unexpected things like "mode hop"
(e.g. randomly switch between series and parallel resonance),
or run on overtones (say three times the frequency). It may even destroy
itself.

It is most educational to listen to the frequency of your latest oscillator
using a good HF receiver
(with BFO switched on).

If you vary the volts, touch the leads, vary the temperature, etc, a good
oscillator just keeps putting out a nice clean steady tone.

A bad one however sounds horribly sproggy and hops and jumps around all over
the place..........

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

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2001\06\12@092825 by Dale Botkin

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Ahh, so that's it.  Thanks.  I gues it never bothered me because nothing I
have done has been terribly frequency-dependent.  Serial data can tolerate
a lot of speed variation (relative to crystals anyway).

Dale

On Tue, 12 Jun 2001, Graeme Zimmer wrote:

{Quote hidden}

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2001\06\12@102324 by mike

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On Tue, 12 Jun 2001 08:31:57 -0500, you wrote:

{Quote hidden}

...and if you have enough stray capacitance to make it run reliably
without caps,  your layout is probably rather poor!

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2001\06\13@142623 by Peter L. Peres

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Dale, the '7404' oscillator uses the crystal in series mode and besides
not being 'on' the parallel frequency it does not require caps.

The 8085,8051,8052 etc have a larger pin capacitance than many other chips
and they will often run without caps or with only one cap.

In practice a 4MHz oscillator can be 'pulled' about 400Hz without too much
trouble. If it is used w/o caps it may be pulled 200Hz by environmental
factors. If used with caps the environmental factors become a small
percentage of the caps and the pull is reduced to under 10Hz (over
temperature, humidity, etc). That is a 20:1 improvement.

Peter

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2001\06\13@142635 by Peter L. Peres

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Dale Botkin <daleEraseMEspam.....BOTKIN.ORG> wrote:
> I'm just curious.  Am I the only one who has never used capacitors
> with the crystal, and never had one fail to start?  Or is this one of
> those "it will work for more repetitions than you can test for
> one-offs and prototypes but don't trust it for production" things?

Many (most) breadboarded projects work w/o caps due to longer leads and
extra stray capacitance. Also some oscillator designs do not require
capacitors at all (see lambda diode oscillator), but this is not the case
with a PIC and other micros, whose oscillator is a badly mangled Pierce
oscillator. So the capacitors should be there.

The problem with missing caps is not missing the start but running on a
frequency that is just far enough 'off' to cause problems in the
application and instability due to changing conditions in the box. It
could also jump modes and do other nasty things.

Peter

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