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'Re[2]: (OT, sort of)Crystals oscillating at 3x mar'
1997\10\15@040043 by Scott Walsh

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    I think Steve has probably identified your problem.

    You may have overlooked this parameter when specifiying the crystal
    that you are using, this is easy to do as it is not an intuitive thing
    to check for!

    AFAIK they call a third overtone XTAL an 'XT cut' and a fundamental
    mode XTAl an 'AT cut' ... anybody know what this is?

    BTW: Do you really need to have the accuracy and expense of a cystal,
    you will find a resonator cheaper although less accurate? But you can
    get people like Kyocera to build a resonator matched to your type of
    micro if you need greater accuracy!

    regards,
    SW.


______________________________ Reply Separator _________________________________
Subject: Re: (OT, sort of)Crystals oscillating at 3x marked freq
Author:  pic microcontroller discussion list <spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU> at
INTERNET
Date:    14/10/97 17:20


Crystal is not a third overtone type is it. If so try fundimental type this
may resolve the problem. Or could the value of bypass capacitors be marginal
check device data sheets for correct window of values Or use disamilar values
for bypass capacitors Or use ceramic resanator with inbuilt bypass capacitors
Or add series resistor 500R or so with the crystal.

Cant think of any more off hand but I am sure others will

Cheers Steve.....

1997\10\15@074314 by Steve Baldwin

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>      I think Steve has probably identified your problem.

I think Steve is on the right track, but not quite on the nail.
If the subject line is correct, the crystal is oscillating in the third
overtone of the marked frequency. If it was a third overtone xtal and
things were going wrong, it would be running at a third of the marked
frequency (ie fundamental).

Crystals are usually operated in fundamental mode up to around 20MHz where
they get too thin to be economical. I can't remember what frequency was
being used in this instance, but it seems unlikely that it would be that
high for a DTMF chip.
When you use a third overtone crystal, you have to put a filter in place to
suppress the fundamental, leaving the overtone the only one with the right
phase to make things happen.
IIRC, the original fault manifested itself when a finger was stuck on the
board.
If the normal trimming capacitors you put around are only 10-30pF (these
are pretty small values of capacitance), shoving 100lbs of skin encased
biological slop onto the board is likely to upset things. ie. A high pass
filter with finger-nails.

>      AFAIK they call a third overtone XTAL an 'XT cut' and a fundamental
>      mode XTAl an 'AT cut' ... anybody know what this is?

Again, almost but not quite. The cut (AT, XT, etc) describes how the
crystal physically moves in it's oscillation mode. Whether it gets
fatter/thinner or twists along one axis, etc. An AT cut is the most common
and has good temperature characteristics. The other common format for low
frequencies (32kHz) is the tuning fork.

>> Crystal is not a third overtone type is it. If so try fundimental type
this
>> may resolve the problem. Or could the value of bypass capacitors be
marginal
>> check device data sheets for correct window of values Or use disamilar
values
>> for bypass capacitors Or use ceramic resanator with inbuilt bypass
capacitors
>> Or add series resistor 500R or so with the crystal.

A couple of other things to check:
I think the layout was described as being good. ie. short tracks, etc.
Does this include a blob of copper ? Is this blob of copper grounded ?
If not, it makes a nice capacitor that you probably don't have on your
schematic.

Are there any fast switching things nearby ? For example, AC series logic
is a killer.

Is the chip designed to use the style of crystal in use ?
I used a Motorola RTC once,  that would power up configured for a 4MHz AT
crystal which would overdrive the 32kHz tuning fork I was using. That sent
the crystal into spasms and I couldn't get any sense out of it until the
clock was configured by software to use the lower power part.

Steve. (a different one)

======================================================
 Very funny Scotty.  Now beam down my clothes.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680                email: .....stevebKILLspamspam@spam@kcbbs.gen.nz
New Lynn, Auckland           ph  +64 9 820-2221
New Zealand                  fax +64 9 820-1929
======================================================

1997\10\15@092252 by John Shreffler

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part 0 1583 bytes
-----Original Message-----
From:   Scott Walsh [SMTP:Scott.WALSHspamKILLspamPLANTRONICS.COM]
Sent:   Wednesday, October 15, 1997 11:45 AM
To:     .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
Subject:        Re[2]: (OT, sort of)Crystals oscillating at 3x marked freq

    I think Steve has probably identified your problem.

    You may have overlooked this parameter when specifiying the crystal
    that you are using, this is easy to do as it is not an intuitive thing
    to check for!

    AFAIK they call a third overtone XTAL an 'XT cut' and a fundamental
    mode XTAl an 'AT cut' ... anybody know what this is?

    BTW: Do you really need to have the accuracy and expense of a cystal,
    you will find a resonator cheaper although less accurate? But you can
    get people like Kyocera to build a resonator matched to your type of
    micro if you need greater accuracy!

    regards,
    SW.


______________________________ Reply Separator _________________________________
Subject: Re: (OT, sort of)Crystals oscillating at 3x marked freq
Author:  pic microcontroller discussion list <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU> at
INTERNET
Date:    14/10/97 17:20


Crystal is not a third overtone type is it. If so try fundimental type this
may resolve the problem. Or could the value of bypass capacitors be marginal
check device data sheets for correct window of values Or use disamilar values
for bypass capacitors Or use ceramic resanator with inbuilt bypass capacitors
Or add series resistor 500R or so with the crystal.

Cant think of any more off hand but I am sure others will

Cheers Steve.....

1997\10\15@092257 by John Shreffler

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face
part 0 1583 bytes
-----Original Message-----
From:   Scott Walsh [SMTP:Scott.WALSHspamspam_OUTPLANTRONICS.COM]
Sent:   Wednesday, October 15, 1997 11:45 AM
To:     @spam@PICLISTKILLspamspamMITVMA.MIT.EDU
Subject:        Re[2]: (OT, sort of)Crystals oscillating at 3x marked freq

    I think Steve has probably identified your problem.

    You may have overlooked this parameter when specifiying the crystal
    that you are using, this is easy to do as it is not an intuitive thing
    to check for!

    AFAIK they call a third overtone XTAL an 'XT cut' and a fundamental
    mode XTAl an 'AT cut' ... anybody know what this is?

    BTW: Do you really need to have the accuracy and expense of a cystal,
    you will find a resonator cheaper although less accurate? But you can
    get people like Kyocera to build a resonator matched to your type of
    micro if you need greater accuracy!

    regards,
    SW.


______________________________ Reply Separator _________________________________
Subject: Re: (OT, sort of)Crystals oscillating at 3x marked freq
Author:  pic microcontroller discussion list <KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU> at
INTERNET
Date:    14/10/97 17:20


Crystal is not a third overtone type is it. If so try fundimental type this
may resolve the problem. Or could the value of bypass capacitors be marginal
check device data sheets for correct window of values Or use disamilar values
for bypass capacitors Or use ceramic resanator with inbuilt bypass capacitors
Or add series resistor 500R or so with the crystal.

Cant think of any more off hand but I am sure others will

Cheers Steve.....

1997\10\15@092452 by John Shreffler

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face
part 0 1583 bytes
-----Original Message-----
From:   Scott Walsh [SMTP:RemoveMEScott.WALSHTakeThisOuTspamPLANTRONICS.COM]
Sent:   Wednesday, October 15, 1997 11:45 AM
To:     spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU
Subject:        Re[2]: (OT, sort of)Crystals oscillating at 3x marked freq

    I think Steve has probably identified your problem.

    You may have overlooked this parameter when specifiying the crystal
    that you are using, this is easy to do as it is not an intuitive thing
    to check for!

    AFAIK they call a third overtone XTAL an 'XT cut' and a fundamental
    mode XTAl an 'AT cut' ... anybody know what this is?

    BTW: Do you really need to have the accuracy and expense of a cystal,
    you will find a resonator cheaper although less accurate? But you can
    get people like Kyocera to build a resonator matched to your type of
    micro if you need greater accuracy!

    regards,
    SW.


______________________________ Reply Separator _________________________________
Subject: Re: (OT, sort of)Crystals oscillating at 3x marked freq
Author:  pic microcontroller discussion list <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU> at
INTERNET
Date:    14/10/97 17:20


Crystal is not a third overtone type is it. If so try fundimental type this
may resolve the problem. Or could the value of bypass capacitors be marginal
check device data sheets for correct window of values Or use disamilar values
for bypass capacitors Or use ceramic resanator with inbuilt bypass capacitors
Or add series resistor 500R or so with the crystal.

Cant think of any more off hand but I am sure others will

Cheers Steve.....

1997\10\15@140832 by David Schmidt

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Maybe, I've just been ordering
3.579545Mhz Parallel Resonant crystals from jameco or DigiKey.

Have never seen the listing for a 3rd overtone type.  Aren't they
mostly used for much higher frequency crystals?
Dave


At 08:44 AM 10/15/97 -0700, you wrote:
>     I think Steve has probably identified your problem.
>
>     You may have overlooked this parameter when specifiying the crystal
>     that you are using, this is easy to do as it is not an intuitive thing
>     to check for!
>
>     AFAIK they call a third overtone XTAL an 'XT cut' and a fundamental
>     mode XTAl an 'AT cut' ... anybody know what this is?
>

1997\10\15@141502 by David Schmidt

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>A couple of other things to check:
>I think the layout was described as being good. ie. short tracks, etc.
>Does this include a blob of copper ? Is this blob of copper grounded ?

No blob of copper, only traces from chip to xtal and resitor.
Traces from the chip to the crystal to the resistor
are only about 3/8" total in length.

>Are there any fast switching things nearby ? For example, AC series logic
>is a killer.

Nope, real simple board, 9V battery powered, output goes to a PIC 52' series.


>Is the chip designed to use the style of crystal in use ?

Yes, The data sheet says you only need to use a parallel cut crystal and a 1M
parallel resistor.


I sent an e-mail to Motorola for help. Still waiting for a reply.
Dave
>

1997\10\16@214833 by Dave Mullenix

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>Yes, The data sheet says you only need to use a parallel cut crystal and a 1M
>parallel resistor.
>
>
>I sent an e-mail to Motorola for help. Still waiting for a reply.

Lots of chips are designed this way.  The CD22212 DTMF chip is an example.
One crystal, one resistor, no caps.  I've seen spec sheets that say you must
sometimes use a series resistor to prevent overdriving some crystals.  This
may be what's happening here.

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