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'[PIC:] Argh, more crystal questions.'
2003\12\03@214125 by Keith L. Kovala

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Ok, I've read AN826 from Microchip, and while informative... I'm still not
sure whether I want a parallel or series resonant crystal, and if parallel
does it really matter for basic circuits what the load cap value is (other
than it be between 12 and 32 pF)??  I suspect series resonant will work, but
can I get some advice from the pros on this one?

Why can't this just be easy?  All I want to do is play with the chips... why
does this have to be so difficult?  (That's a rhetorical question and I
don't expect an answer to that. GRiN)

BTW: Thanks everyone for the input on my previous post about A/D converting.

Keith L. Kovala
spam_OUTklkTakeThisOuTspamrenderedelement.com

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2003\12\03@220242 by John J. McDonough

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Grab any old crystal, stuff it in there, forget the caps.  99%+ shot it will
work.  OK, maybe that 20 MHz crystral will oscillate at 20.0004 MHz, is that
such a big deal?

72/73 de WB8RCR    http://www.qsl.net/wb8rcr
didileydadidah     QRP-L #1446 Code Warriors #35

{Original Message removed}

2003\12\03@221938 by steve

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> Ok, I've read AN826 from Microchip, and while informative... I'm still
> not sure whether I want a parallel or series resonant crystal, and if
> parallel does it really matter for basic circuits what the load cap
> value is (other than it be between 12 and 32 pF)??  I suspect series
> resonant will work, but can I get some advice from the pros on this
> one?

A few snips from my webpage (http://www.tla.co.nz/xtal1.html)
There's more there if you want it.
Otherwise (as someone already suggested) just plug in a 20MHz crstal
and a couple of 20pF caps and it will nearly always work.

Serial or Parallel resonance.
When a crystal is described as being a particular frequency series or
parallel resonant, it is analogous to any other part being specified at a
temperature of 20 or 25 degrees. The part is the same but the
frequency has been determined under those defined parameters.

Load capacitance
Once again, the crystal has been measured under the stated conditions.
In this case, the load capacitance and it is only relevant at parallel
resonance. If you use a different load capacitance, the crystal is in a
different situation from that stated on the can and will behave differently.
This may be a small shift in the frequency or may prevent the oscillator
from starting, depending on how far away from the stated value you
take it.

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: .....steveKILLspamspam@spam@tla.co.nz                      fax +64 9 820-1929
=========================================

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2003\12\03@223223 by D Yates

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----- Original Message -----
From: "Keith L. Kovala" <klkspamKILLspamKSU.EDU>


> Ok, I've read AN826 from Microchip, and while informative... I'm still not
> sure whether I want a parallel or series resonant crystal, and if parallel
> does it really matter for basic circuits what the load cap value is (other
> than it be between 12 and 32 pF)??  I suspect series resonant will work,
but
> can I get some advice from the pros on this one?

Well I'd hardly consider myself a pro, but I can tell you what works for me.
Just about everything when it comes to making oscillators!  If you just want
to play around with the chips try a ceramic resonator with built in
capacitors.  Its accurate enough for 95% of the things you can do with a
pic, and extremely easy to connect.  It comes with 3 pins... connect the
outer two pins to OSC1 and OSC2 and the center one to ground, then set the
pic oscillator mode to HS and boot it up.  I found some cheap ceramic
resonators at allelectronics.com:
www.allelectronics.com/cgi-bin/category.cgi?category=197&item=CER-20&type=store
for $1 each and I believe they have internal capacitors b/c they work fine
without any.

If you happen to already have a crystal lying there and want to use it
instead, put it across OSC1 and OSC2 with a 220 ohm resistor, then tie each
crystal pin to ground with a 10-50pf capacitor (or whatever you have laying
around).  If it doesn't work you won't hurt the pic (just use some common
sense...), so just take it apart and try different values.

XTAL:

GND -----------------------------------
      C1   =         =   C2
               |           |
              <XTAL>
               |           |
              R1        |
               |           |
            OSC1   OSC2
               ||          ||
------------------------------
                  PIC

100 ohms < R1 < 1k
10 pf  <  C1,2  < 50 pf


> Why can't this just be easy?  All I want to do is play with the chips...
why
> does this have to be so difficult?  (That's a rhetorical question and I
> don't expect an answer to that. GRiN)

It's not that hard, microchip did all the real work for us!  The pic is
waiting to run.. it just needs a little nudge.

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2003\12\03@225304 by Dan Devine

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RESONATORS baby...make your life simple.  They're rugged, simple, and
small.

Digikey part number: X909-ND ; 20mHz Ceramic with caps...$0.54 ea

Ground the center pin and tie the outer two to the OSC pins....dead
simple, only thing to remember is the MPLAB project doesn't default to
HS, you'll have to set that yourself.



DD

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2003\12\03@225305 by John J. McDonough

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Actually, Microchip says you MAY need the series resistor for AT cut
crystals.  And from what I can tell, you will need to get farads in there to
keep the thing from oscillating.  I may not have been clear before when I
said to stuff a crystal in there.  I meant between OSC1 and OSC2.  It will
almost always work.  If you use a parallel cut crystal with a 20pF load spec
(the most common type) and 20pF caps, the frequency is going to be very,
very close to what is marked on the crystal.  If you put some other value in
there, it will only be very close.  Like D Yates said, it only takes a
little nudge.  It's pretty tough to get very far wrong.  The book may say
15-33 pF, but it works just fine with zero, and I'd be surprised if it
didn't work with a 0.1u in there. It's terribly non-critical unless you are
really hung up about getting the frequency spot on.

I never did get the ceramic resonator thing.  Microprocessor crystals are
more stable, and no more expensive.  The C-301 case jobs are also a lot
smaller.  The HC49 cases are a little larger but cheaper.  But in all cases,
we're talking under a buck (that's about 80% of a Euro these days), so it
seems worth it to go for a little extra stability unless you're going to buy
a thousand of the things.

You can also stick an RC in there if you are more comfortable with that.
Not as stable, nor as accurate, but you probably know how to calculate that.
Like DY said, the oscillator is in there, it just needs a little poke.

72/73 de WB8RCR    http://www.qsl.net/wb8rcr
didileydadidah     QRP-L #1446 Code Warriors #35


{Original Message removed}

2003\12\03@231208 by Denny Esterline

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Agreed!
But Digikey's order proccess is prety painful or just a couple resonators,
look towards Randy Jones (http://www.glitchbuster.com). From his web site:
CR-4 4 MHz Ceramic Resonator ECS ZTT 3-pin w/caps $0.48
CR-10 10 MHz Ceramic Resonator ECS ZTT 3-pin w/caps $0.48
CR-20 20 MHz Ceramic Resonator ECS ZTT 3-pin w/caps $0.48
No minimum order and shipping less than $2

I bought from him and reccomend him to others.
-Denny


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2003\12\03@231622 by D Yates

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> ----- Original Message -----
> From: "Keith L. Kovala" <.....klkKILLspamspam.....KSU.EDU>

<snip>
> I'm still not sure whether I want a parallel or series resonant crystal,
<snip>

It doesn't make any difference unless you're into *very* timing critical
applications, or into very long term applications where a couple instruction
cycles eventually add up to a significant error.  If you run the pic at
20.01mhz instead of 20.00 for an entire day, you'll be off by a total of
about 45 seconds.  After a month that adds up to over 20 minutes...  and
after a year to over 4 hours.  It's starting to sound like my watch...

D

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2003\12\03@232701 by Liam O'Hagan

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Off topic but interestingly, my brother in law just bought a rediculously
expensive omega watch, somewhere around the $2.5k mark, and the instruction
book (not just a little brochure mind you, but a hard cover bound volume)
indicates that the expected accuracy is only +/- 40 seconds a day or so...

> {Original Message removed}

2003\12\03@234608 by Ken Pergola

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Hi Keith,

If you have not done so, please read Microchip's application note AN588:
PICmicro Microcontroller Oscillator Design Guide. This should address most
of your questions.

There are a lot of things to think about when choosing a crystal if you want
to have a solid design over temperature, voltage, and time. It's definitely
not as simple as throwing any crystal and load capacitors at the PIC and
you're all set. Always observe and follow the crystal manufacturer's specs
and follow Microchip's advice in the data sheets as well.

When buying crystals or resonators, look carefully at the operating
temperature specs. Ensure that it is rated to operate in the temperature
your end-product will be subjected to.

Best regards,

Ken Pergola

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2003\12\04@002135 by William Chops Westfield

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On Wednesday, Dec 3, 2003, at 19:30 US/Pacific, D Yates wrote:

>  I found some cheap ceramic
> resonators at allelectronics.com:
> http://www.allelectronics.com/cgi-bin/
> category.cgi?category=197&item=CER-20&type=store
> for $1 each and I believe they have internal capacitors b/c they work
> fine
> without any.
>
yeah, but that's NOT cheap.  At least, i see surplus crystals (and even
canned
oscillators) available on the surplus market all the time at well under
$1 each,
and hardly ever see ceramic resonators at similar prices (even in odd
freqs.)

( http://sales.goldmine-elec.com/prodinfo.asp?prodid=3654  $0.02 each
for 13.5Mhz crystals.
eBay has similar "deals", but not on resonators.)

BillW

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2003\12\04@002143 by William Chops Westfield

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Short of having a very accurate frequency counter, how does one go about
figuring out the type and load capacitance requirements for the average
'no specs' crystal used by hobbyists?

(I like the idea of "just connect it"...  Too bad about all those other
messages I've read where people have had trouble getting their crystals
to oscillate...)

BillW

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2003\12\04@021928 by Vasile Surducan

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On Wed, 3 Dec 2003, John J. McDonough wrote:

> Grab any old crystal, stuff it in there, forget the caps.  99%+ shot it will
> work.

 True. The problem is what are you doing IF you are in the 1%,
 and the pcb is already finished...
 And believe me, in one day you will be there !

 PIC16F630 with external crystal and small Q factor is:
 oscillating-not oscillating-oscillating-not oscillating

 etc...

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2003\12\04@030611 by

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William Chops Westfield wrote:

> (I like the idea of "just connect it"...  Too bad about all
> those other messages I've read where people have had trouble getting
> their crystals to oscillate...)

If I'm not wrong, most of those cases has been caused by the
trouble to realy be sure of that the osc actualy *does't* run.
Like trying to observe some outout pin and when there's no "output",
people are poiting to the osc instead of their faulty app or
wrong CONFIG settings...

Jan-Erik.

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2003\12\04@034553 by Vasile Surducan

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On Thu, 4 Dec 2003, Jan-Erik Soderholm XA (TN/PAC) wrote:

> William Chops Westfield wrote:
>
> > (I like the idea of "just connect it"...  Too bad about all
> > those other messages I've read where people have had trouble getting
> > their crystals to oscillate...)
>
> If I'm not wrong, most of those cases has been caused by the
> trouble to realy be sure of that the osc actualy *does't* run.
> Like trying to observe some outout pin and when there's no "output",
> people are poiting to the osc instead of their faulty app or
> wrong CONFIG settings...
>
 Hmm. Maybe you are too sure that's the only one problem... :)
 A deep analyses of oscout signal shape and amplitude with a good
oscilloscope and a 1:10 or 1:100 probe with known internal capacity,
will tell you many things about this "0 problem" external Xtal and
how it varies the amplitude with the oscout capacitor, pcb topology
and crystal's (or resonator) quality factor. In about 3% of the
situations I've met, the oscillator is running, but is exactly at the
limit (or a little above) with the amplitude, no matter how you vary the
capacitors values in the range specified by the data sheet.


best,
Vasile

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2003\12\04@054444 by hael Rigby-Jones

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> -----Original Message-----
> From: John J. McDonough [EraseMEwb8rcrspam_OUTspamTakeThisOuTARRL.NET]
> Sent: 04 December 2003 03:53
> To: PICLISTspamspam_OUTMITVMA.MIT.EDU
> Subject: Re: [PIC:] Argh, more crystal questions.

> I never did get the ceramic resonator thing.  Microprocessor
> crystals are
> more stable, and no more expensive.  The C-301 case jobs are
> also a lot
> smaller.  The HC49 cases are a little larger but cheaper.
> But in all cases,
> we're talking under a buck (that's about 80% of a Euro these
> days), so it
> seems worth it to go for a little extra stability unless
> you're going to buy
> a thousand of the things.

They are much more rugged than crystals, so if your product is going
somewhere likely to experience shocks and vibration, a resonator is much
more likely to survive.  Also, as you say, in large volume production the
cost savings could be significant.


Mike




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2003\12\04@084924 by John J. McDonough

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The original poster indicated he was just experimenting, and I got the
impression (maybe wrong) that he was a hobbyist.

If I'm tinkering, I'll just stuff a crystal in there.  I'm sort of a fan of
these little cylindrical things, 20 pF, exactly what Microchip calls for are
about 80 cents (quantity 1), so it's not worth dinking around unless I'm
building a counter or something where a few tenths of a percent error is a
big deal.

If I expect even one person to attempt to duplicate what I'm doing, I'll
show the caps.  But it's not exactly rocket science to go buy a 20 pF
crystal and use a couple of caps.

If I was going to build a million of them, then yes, I would take a few
hundred, measure them, understand the ranges I'm dealing with, and design
accordingly.  Depending on my manufacturing costs, I may well need to be
sure I fully understand the envelope.  But for a one-off, the thing is
almost never going to fail to oscillate.  My hands are going to be too old
and shaky to hold a soldering iron before I run across that one in a million
that won't work.  Heck, even for small production runs, simply applying the
thing according to the datasheet is plenty adequate.

These things are pretty tolerant.  Unless you are applying them right on the
edge, you can be pretty cavalier about lots of things.  Sure, the larger
your numbers the more likely you are to experience that combination that
doesn't work, but for smaller runs it's not worth sweating the small stuff.

72/73 de WB8RCR    http://www.qsl.net/wb8rcr
didileydadidah     QRP-L #1446 Code Warriors #35


{Original Message removed}

2003\12\04@091553 by Keith L. Kovala

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Yeah, I get the rating of hobbyist and not tinker'er.  GRiN  Yes, this is by
no means for production stuff... most of the stuff will be one off.
Ocassionally I might write something for others to reproduce the design...
but I am by no means using PICs in production stuff.

Though I did find it interesting when sampling some PICs recently the drop
down option of "Adult Toys" was available as the end product the PICs were
going to be used for.  Hmm.... PICs in an adult toy.  That's produced some
interesting conversations, especially among the girl geeks.  GRiN

Keith L. Kovala
KILLspamklkKILLspamspamrenderedelement.com

> {Original Message removed}

2003\12\04@092801 by Olin Lathrop

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Keith L. Kovala wrote:
> Ok, I've read AN826 from Microchip, and while informative... I'm
> still not sure whether I want a parallel or series resonant crystal,

Parallel.

> and if parallel does it really matter for basic circuits what the
> load cap value is (other than it be between 12 and 32 pF)??

It will make a small difference in the accuracy.  If the caps are too far
out of line, the crystal may not oscillate at all or at a harmonic.

> Why can't this just be easy?

It is.  If you don't care about the last few PPM of accuracy, just get a
20MHz parallel rated crystal, put a 22pF cap on either side to ground, and
be done with it.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\12\04@093211 by Intosh, Ph.D.

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Hash: SHA1

source= http://www.piclist.com/piclist/2003/12/03/231208a.txt?

Denny Esterline wrote:
Agreed!
But Digikey's order proccess is prety painful or just a couple resonators,
look towards Randy Jones (http://www.glitchbuster.com). From his web site:
CR-4 4 MHz Ceramic Resonator ECS ZTT 3-pin w/caps $0.48
CR-10 10 MHz Ceramic Resonator ECS ZTT 3-pin w/caps $0.48
CR-20 20 MHz Ceramic Resonator ECS ZTT 3-pin w/caps $0.48
No minimum order and shipping less than $2

- ----

Yesterday I ordered an oscillator from Mouser.  Some of their catalog
numbers are:
Part No.  520-TCH368 3.6864 MHz (384 x 9600) $1.70
 520-TCF1966 19.6608 MHz (2048 x 9600)

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=327200

They don't have a minimum order.  They will save several BOM's for you.  I
figure that oscillators always start.  I may move to a VCO when I port ntp
to the PIC :-)



- ---
Aubrey  McIntosh
http://www.piclist.com/member/AM-vima-Y84
PIC/PICList FAQ: http://www.piclist.com


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2003\12\04@095249 by Bob Axtell

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I guess I should add my experiences with crystal oscillators.

First- NO question about it- ceramic resonators are very durable and more
reliable
than piezo crystals, particularly in a possible high-shock environment.
They are also slightly less expensive. But the negatives are that they are
susceptible to moisture and their freq stability is much worse. BUT, I
always use ceramic resonators whenever possible and I've never been
disappointed yet.

With shock, crystals will fracture then oscillate at a wrong frequency. If
you layout the PCB to allow shock-mounting a crystal, such as floating the
crystal case in a drop of silicone caulk, with the flying leads formed to
prevent shock energy from being transferred, reliability will improve
100-fold. It follows that SMD crystals, which have NO leads, will be the
least reliable: and they are.

The caps are used to force the phase shift of the oscillator, and have
little effect except at startup. It's best to use the MC-recommended
values. A lower cap value will usually cause the oscillator startup time to
change. Overall, a slight shift in cap values doesn't seem to make any
difference, especially if both are the same.

In high-precision situations, such as 14.31818Mhz video situations, I
prefer to use a guaranteed external oscillator package instead of the uP
oscillator.

--Bob

At 03:42 AM 12/4/2003, you wrote:
> > {Original Message removed}

2003\12\04@095833 by Mike Harrison

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On Thu, 4 Dec 2003 07:32:34 -0700, you wrote:

{Quote hidden}

A handy tip I got from a microchip seminar regarding xtal caps was the amplitude measured (with a
high-z, low-c probe) on both xtal pins well  be about the same with the correct cap values.

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2003\12\04@100457 by hael Rigby-Jones

picon face
> -----Original Message-----
> From: Bob Axtell [RemoveMEcr_axtellTakeThisOuTspamYAHOO.COM]
> Sent: 04 December 2003 14:33
> To: spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU
> Subject: Re: [PIC:] Argh, more crystal questions.
>
> In high-precision situations, such as 14.31818Mhz video situations, I
> prefer to use a guaranteed external oscillator package
> instead of the uP
> oscillator.

They are quite current hungry compared to the PIC's oscillator however.

Mike




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2003\12\04@101458 by Olin Lathrop

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John J. McDonough wrote:
> and I'd be surprised if it didn't work
> with a 0.1u in there.

I wouldn't be.  It would kill the signal to the point where the whole thing
wouldn't oscillate anymore.

Consider the impedence of 100nF at 20MHz, then compare that to what the PIC
can drive and the crystal can pass.


*****************************************************************
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(978) 742-9014, http://www.embedinc.com

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2003\12\04@103541 by DAVE L

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part 1 453 bytes content-type:text/plain; x-avg-checked=avg-ok-3CFE68DE; charset=us-ascii; format=flowed (unknown type 8bit not decoded)

One of these posts implied that the frequency would/could be altered with
the loading caps
I would not have thought it would have much impact in this configuration?

I do seem to remember the HAMS would use L's and C's to drag their
frequencies about?

D

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2003\12\04@104823 by Paul Hutchinson

picon face
That's got to be a misprint, 40 seconds per day is > 400ppm error. I don't
think they even make 32kHz watch crystals that inaccurate any more. Maybe it
has a ceramic resonator instead of a crystal :-).

My guess is that it should be either 40 seconds per month if they are using
medium quality crystals (< 20ppm) or 40 seconds per year if they are using
the best crystals (< 2ppm).

Back in the late 70's early 80's when I started in the meteorological
instrument industry, clock movements with better than 2ppm accuracy (1
minute/year) were inexpensive and readily available from a number of German
manufacturers. Now those companies have all been bought out by Asian
manufacturers and the standard for clock movements is 20ppm (~ 1
minute/month).

Paul

> {Original Message removed}

2003\12\04@104852 by llile

flavicon
face
4 MHz vs 10 mHZ vs other frequencies:

I tend to like frequencies for crystals that can divide down evenly to
baud rates.  For instance the 18.432 Mhz crystal will divide down with nil
errors to 9600 baud, whereas 4 Mhz or 10 MHZ always have some error in the
baud rate.  Why design in problems?  It is trouble enough to get a PIC to
talk back to your computer.

RC oscillators almost never work for RS232 serial comms in my experience.
The actual frequency is dependant on the tolerance of the parts.  even if
you get one working, the next will not.

If you find a 10MHz crystal in a device, you might ask "why was that
frequency picked?"  it is just like in mechanical engineering, if you find
a part that is exactly one inch long, you might ask "why did they pick an
inch?"  generally because it was handy, not because it was thought through
and intentionally designed.

I pick a lot of 4MHz RC oscillators for designs.  Why?  Because the
internal RC oscillators default to 4MHz, and I am used to designing at
that speed, and in many of my designs tight control of frequency is not
needed.  Habit, in other words. This is a bad reason to design something.
Every aspect of a design should really be intentional and not just plucked
out of the air.




-- Lawrence Lile


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2003\12\04@105515 by

picon face
llile@SALTONUSA.COM wrote :

> I pick a lot of 4MHz RC oscillators for designs.  Why?
> Because the internal RC oscillators default to 4MHz,...

Or 8 Mhz, as is the case with the newer "nanoWatt" PICs
(both 18 an 16 series). And you can "change gear" in
7 steps down to 125kHz on-the-fly from your application...

:-)

Jan-Erik.

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2003\12\04@105931 by Wouter van Ooijen

face picon face
> Every aspect of a design should really be intentional and not
> just plucked out of the air.

I disagree. Every part of a design should receive the amount of
attention that is economically justified. For some aspects this might
boil down to 'just choose something' because it just does not matter (or
does not matter *significantly*), for other parts it might start a
country-wide research program to find the optimal solution.

Wouter van Ooijen

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2003\12\04@124810 by Mike Hord

picon face
>I never did get the ceramic resonator thing.  Microprocessor crystals are
>more stable, and no more expensive.  The C-301 case jobs are also a lot
>smaller.  The HC49 cases are a little larger but cheaper.  But in all
>cases,
>we're talking under a buck (that's about 80% of a Euro these days), so it
>seems worth it to go for a little extra stability unless you're going to
>buy
>a thousand of the things.

Ceramic resonators are less shock sensitive.  Crystals don't like being
banged
on too much.

Mike H.

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2003\12\04@133858 by llile

flavicon
face
>> Every aspect of a design should really be intentional and not
> just plucked out of the air.

>I disagree. Every part of a design should receive the amount of
attention that is economically justified.

Good Point , Wouter!   For one-offs I commonly bang together a
Microengineering Labs prototype board, 18.432 MHz crystal, PIC877, ISP
port and TO220 5V regulator with an 18V 500mA wall wart without
considering them at all.  All of the components have way more "Ummph" than
I will ever need in most of my one-off's, but putting this together takes
little attention other than some soldering.  I will often make two just to
have one laying around handy.  I certainly don't consider the design at
all, just throw in big parts that will kill the application I am working
on.  For instance, one of these turned into a current-controlled stopwatch
that actuates a Radio Shack Stopwatch and needs tenth-second accuracy.
Coulda done it with a 12C508 or even a relay but why bother.

I guess my point is, though, that at the minimum you must decide whether a
specificaiton matters or not.   In other words, if you decide to not pay
attention to something, make sure that is a conscious decision.  It may
take 500 milliseconds to decide "Oh just use the same old crystal I am
used to and have a drawer full of" and that decision has repercussions all
through the project.




-- Lawrence Lile





Wouter van Ooijen <RemoveMEwouterspamTakeThisOuTVOTI.NL>
Sent by: pic microcontroller discussion list <PICLISTEraseMEspam.....MITVMA.MIT.EDU>
12/04/2003 09:58 AM
Please respond to pic microcontroller discussion list


       To:     EraseMEPICLISTspamMITVMA.MIT.EDU
       cc:
       Subject:        Re: [PIC:] Argh, more crystal questions.


> Every aspect of a design should really be intentional and not
> just plucked out of the air.

I disagree. Every part of a design should receive the amount of
attention that is economically justified. For some aspects this might
boil down to 'just choose something' because it just does not matter (or
does not matter *significantly*), for other parts it might start a
country-wide research program to find the optimal solution.

Wouter van Ooijen

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2003\12\04@150700 by steve

flavicon
face
> One of these posts implied that the frequency would/could be altered
> with the loading caps I would not have thought it would have much
> impact in this configuration?

You are right. The frequency is largely determined by the inductive
component of the crystal when operated at the parallel resonance (ie.
most applications).  Often a 32kHz real time clock crystal will be
operated in the series resonant mode and the frequency can be pulled
by varying the capacitance.
A parallel resonant circuit uses an inverting amp and a series resonant
amp uses a non inverting amp.

> I do seem to remember the HAMS would use L's and C's to drag their
> frequencies about?

That's slightly different. A crystal has several frequencies that it will
resonate at. The fundamental, the third overtone, fifth overtone, etc.
Assuming that the drive level is correct, it will tend to go for the lowest of
those frequencies. However, if you put a trap (or a filter) in the circuit for
the fundamental frequency, you can force the circuit to oscillate at an
overtone. That means that you can get to a higher frequency with a
much thicker (ie. cheaper) crystal.

This is the best way of determining if your oscillator circuit is working
correctly. Write a program that toggles a pin and watch that on a scope.
Mess with temperatures, voltages, etc and if your circuit is on the edge,
you'll see the frequency of your pulses jump back and forth as the
crystal jumps to the next overtone. The biggest oscillator design error is
layout and this method is the only way to really determine the
performance of the actual circuit.

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
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=========================================

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2003\12\04@150701 by steve

flavicon
face
> A handy tip I got from a microchip seminar regarding xtal caps was the
> amplitude measured (with a high-z, low-c probe) on both xtal pins well
>  be about the same with the correct cap values.

I hope you didn't pay too much for that seminar.
Did they mention why they bother having gain in the oscillator if the
output voltage is going to be the same as the input ?
There is a large range of equivalent resistance values for crystals and
resonators. That observation would be correct for a low ESR resonator
but wrong for a higher frequency crystal.
Both cases will oscillate because the amplifier has the gain available to
compensate.

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: RemoveMEstevespam_OUTspamKILLspamtla.co.nz                      fax +64 9 820-1929
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2003\12\04@151454 by Denny Esterline

picon face
> 4 MHz vs 10 mHZ vs other frequencies:
>
> I tend to like frequencies for crystals that can divide down evenly to
> baud rates.  For instance the 18.432 Mhz crystal will divide down with
nil
> errors to 9600 baud, whereas 4 Mhz or 10 MHZ always have some error in
the
> baud rate.  Why design in problems?  It is trouble enough to get a PIC to
> talk back to your computer.
>
That's all fine and good if a serial connection is the primary concern, but
many devices don't have a serial port. They may be more concerned with even
division to seconds, or something else.


-Denny

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2003\12\04@163901 by John J. McDonough

flavicon
face
----- Original Message -----
From: "Olin Lathrop" <RemoveMEolin_piclistTakeThisOuTspamspamEMBEDINC.COM>
Subject: Re: [PIC:] Argh, more crystal questions.


> John J. McDonough wrote:
> > and I'd be surprised if it didn't work
> > with a 0.1u in there.
>
> I wouldn't be.  It would kill the signal to the point where the whole
thing
> wouldn't oscillate anymore.
>
> Consider the impedence of 100nF at 20MHz, then compare that to what the
PIC
> can drive and the crystal can pass.

I haven't run the experiment, but I know it doesn't work real good with a
short to ground, and maybe a 0.1 is getting pretty close to a short!

--McD

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2003\12\04@170012 by Olin Lathrop

face picon face
John J. McDonough wrote:
>> Consider the impedence of 100nF at 20MHz, then compare that to what
>> the PIC can drive and the crystal can pass.
>
> I haven't run the experiment, but I know it doesn't work real good
> with a short to ground, and maybe a 0.1 is getting pretty close to a
> short!

There is no experiment to run.  Just do the calculation.  100nF at 20MHz
looks like about 80mOhms.  There is no chance at all it will oscillate with
that kind of shunt to ground.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\12\04@172718 by Spehro Pefhany

picon face
At 04:59 PM 12/4/2003 -0500, you wrote:
>John J. McDonough wrote:
> >> Consider the impedence of 100nF at 20MHz, then compare that to what
> >> the PIC can drive and the crystal can pass.
> >
> > I haven't run the experiment, but I know it doesn't work real good
> > with a short to ground, and maybe a 0.1 is getting pretty close to a
> > short!
>
>There is no experiment to run.  Just do the calculation.  100nF at 20MHz
>looks like about 80mOhms.  There is no chance at all it will oscillate with
>that kind of shunt to ground.

It might oscillate at 50 or 200kHz, but the crystal won't be doing squat.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspamspamspamBeGoneinterlog.com             Info for manufacturers: http://www.trexon.com
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2003\12\05@065059 by Russell McMahon

face
flavicon
face
Some comments from a friend follow my comments.
I agree that crystal oscillators can be FAR more tricky than may have been
suggested by some of the comments on this thread. While often enough the
"paralllel resonant crystal with 2 x 20 pF caps" is all that's needed, this
certainly isn't always the case. Some processor & crystal combinations can
be extremely finnicky and may need fine tuning of capacitors and then some.
Worst case I have ever met involved a batch of Zilog Z8+s that required an
inductor across the oscillator pins to ensure reliable startup in all cases.
A most unusual requirement, fortunately.



       RM

______________________________

Crystal (and resonator) startup is a very complex phenomena and something of
a black art.

Philips (and other uC manufacturers) have written quite detailed treatises
on crystal behaviour and it makes sobering reading for anyone manufacturing
in high volumes.  Epson also have done a detailed manual on crystals and
resonators and their use in reliable oscillators.  Murata also have a good
manual on resonators and their application.

Incidenatlly a major difference between crystals and resonators is their
startup time  - resonators are typically 10-100 times faster and in
low-power uC applications their faster startup can be a real advantage both
with regard to power consumption and reliability.  We have seen an
application where a crystal was substituted for a resonator in such a system
and while it worked in some cases it didn't in others.  Turned out that the
system periodiacally woke up the uC which in most cases then communicated
with an I2C bus device exterally before going back to sleep.  Problem was
that the crystal was very slow to start and ran the uC very erratically
while doing so such that the I2C bus transactions were screwed up.  In fact
in most cases the uC went back to sleep (believing it had done its job)
before the crystal had really got going.  With a resonator (as originally
designed) the oscillator was up to full amplitude and stable 100's of us
before the I2C transaction, while with the crystal oscillation had typically
only reached about 20% of full amplitude after several ms when it went back
to sleep.

You can find the Philips app notes on their website or (more easily) from
ours at:

http://www.acqura.com/technotes.asp?ID=483

http://www.acqura.com/technotes.asp?ID=486

The second is a rework of the first and contains quite a bit of additional
theory and coverage of additional issues.

Regards,

Ken Mardle

Applied Digital Research Ltd.  Tel    : +64 9 415-2514
P.O. Box 6480                  Fax    : +64 9 415-3514
Wellesley St                   Mobile : +64 25 879-648
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> {Original Message removed}

2003\12\05@143441 by Peter L. Peres

picon face
> short of having a very accurate frequency counter, how does one go about
> figuring out the type and load capacitance requirements for the average
> 'no specs' crystal used by hobbyists?

In theory a Q-meter should yield this answer. Many grid dip meters have a
Q setting. You need to account for the instrument capacitance and set the
crystal for intended mode (parallel for all pics). The frequency will be
exact (subject to manufacturer's precision) when Q is maximum. You put a
small variable in parallel with the crystal and tune it. The required caps
for Pierce (the kind of oscillator used on pics) are then
((Cx+Cinstrument+Cstray)*2 - Cpin) each. The adjustment can be very hard
at 20MHz.  I only used it at 4.9 and 1.0MHz.

It's been a long time.

Peter

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