Timer 1 Oscillator Failure at high temperature

[Leslie, Shawn J]

I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as an RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz 12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal. After 2 years of successful builds we are suddenly having this circuit fail at high temp. The oscillator fails at 60-70C and a few even do not work at room temperature. I am having 40% fallout at temp. If you lower the temperature to <40C it starts to work while power is applied. The problem is made more difficult since the units are potted. We cannot get at the components to troubleshoot. We are trying to test unpotted units over temperature. In the meantime I have tried to find a threshold of failure on a working unpotted unit by increasing and decreasing the 27pF caps - amazingly the circuit works at room temperature with 47pF caps on the high end to 6pF on the low end. With 6pF I can make it fail if I raise the temperature to 70C, but as it cools it begins to wor!
 k. I have wrote Epson for crystal ideas and sent them my layout - any ideas from you guys? What have you all used? Maybe someone has a successful crystal oscillator design I could use.

Cheers,
Shawn



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[Bob Blick-4]

On Fri, 27 Jun 2008 11:12:56 -0600, "Leslie, Shawn J"
<sjlesli@...> said:

> I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as an
> RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz
> 12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal. After
> 2 years of successful builds we are suddenly having this circuit fail at
> high temp. The oscillator fails at 60-70C and a few even do not work at
> room temperature. I am having 40% fallout at temp. If you lower the
> temperature to <40C it starts to work while power is applied. The problem
> is made more difficult since the units are potted. We cannot get at the
> components to troubleshoot. We are trying to test unpotted units over
> temperature. In the meantime I have tried to find a threshold of failure
> on a working unpotted unit by increasing and decreasing the 27pF caps -
> amazingly the circuit works at room temperature with 47pF caps on the
> high end to 6pF on the low end. With 6pF I can make it fail if I raise
> the temperature to 70C, but as it cools it begins to wor!
>  k. I have wrote Epson for crystal ideas and sent them my layout - any
>  ideas from you guys? What have you all used? Maybe someone has a
>  successful crystal oscillator design I could use.

Hi Shawn,

I've had some experience with 32KHz problems on another chip, and there
are a few things that come to mind that may help you diagnose the
problem:

If you can raise the power supply voltage 1/2 volt and retest, if the
oscillator works better then it suggests the lower impedance and higher
gain of the oscillator driver is having some efect.

If you have no problem with units before potting it could be moisture
being potted along with the circuit.

Using a very low load scope probe you can see if you need to offset the
capacitance to one side of the oscillator or the other.

Scoping it quickly you can see if there's DC bias, that could suggest
board leakage.

I have a manufacturer's document somewhere that may have some useful
tidbits... let me look here and see if I can find it on the interwebs...
nope, I'll have to look for a reference to it on my computer at home.
I'll post another message tonight with a link.

Cheerful regards,

Bob

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[Marcel Duchamp]

I have no special ideas on this but Microchip has four or five apnotes
on various oscillator topics.

If you go here:
http://search.microchip.com/searchapp/searchhome.aspx?q=oscillator
there will be several articles in the top 10 results that may help
thinking about the problem.

Leslie, Shawn J wrote:
> I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as an RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz 12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal. After 2 years of successful builds we are suddenly having this circuit fail at high temp. The oscillator fails at 60-70C and a few even do not work at room temperature. I am having 40% fallout at temp. If you lower the temperature to <40C it starts to work while power is applied. The problem is made more difficult since the units are potted. We cannot get at the components to troubleshoot. We are trying to test unpotted units over temperature. In the meantime I have tried to find a threshold of failure on a working unpotted unit by increasing and decreasing the 27pF caps - amazingly the circuit works at room temperature with 47pF caps on the high end to 6pF on the low end. With 6pF I can make it fail if I raise the temperature to 70C, but as it cools it begins to w
or!
>  k. I have wrote Epson for crystal ideas and sent them my layout - any ideas from you guys? What have you all used? Maybe someone has a successful crystal oscillator design I could use.
>
> Cheers,
> Shawn
>
>
>

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[Marcel Duchamp]

To amplify some of Bob's points, check the frequency before and after
potting if possible to see if there is something going on.  If something
changed in the potting technique, the potting compound could be adding
mechanical stress to the components and maybe be pushing them towards
out of spec operation.

If potting compounds were changed from back when it seemed everything
used to work, look to see if the potting compound is exothermic to such
an extent that it damages components.  I have seen that one before.

One technique sometimes used when potting is producing mechanical stress
is to give the components a light coating of rubber like silicone RTV
(caulk, etc.) then encapsulating the unit.

I would contact Microchip just in the event a) they know something about
this and b) are willing to tell you.

Good luck!

Bob Blick wrote:

>
> Hi Shawn,
>
> I've had some experience with 32KHz problems on another chip, and there
> are a few things that come to mind that may help you diagnose the
> problem:
>
> If you can raise the power supply voltage 1/2 volt and retest, if the
> oscillator works better then it suggests the lower impedance and higher
> gain of the oscillator driver is having some efect.
>
> If you have no problem with units before potting it could be moisture
> being potted along with the circuit.
>
> Using a very low load scope probe you can see if you need to offset the
> capacitance to one side of the oscillator or the other.
>
> Scoping it quickly you can see if there's DC bias, that could suggest
> board leakage.
>
> I have a manufacturer's document somewhere that may have some useful
> tidbits... let me look here and see if I can find it on the interwebs...
> nope, I'll have to look for a reference to it on my computer at home.
> I'll post another message tonight with a link.
>
> Cheerful regards,
>
> Bob
>

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[Scott Touchton]

I had almost this exact problem several years ago.  Turns out the
increased permeativity of the potting compound caused too much parasitic
capacitance.  The low power oscillator design was marginal at best, and
this extra capacitance pushed it over the edge.

Removed the potting and all was well.  You might want to try that.


Also, I have had problems with Epson crystals damaged during reflow (in
a Quad oven running the correct profile).  Seems some crystals were
marginal and would not sustain the profile spec'd by Epson.  Changed to
another manufacturer and that problem went away.

Scott F. Touchton
 


-----Original Message-----
From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf
Of Leslie, Shawn J
Sent: Friday, June 27, 2008 1:13 PM
To: 'piclist@...'
Subject: [PIC] Timer 1 Oscillator Failure at high temperature

I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as
an RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz
12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal.
After 2 years of successful builds we are suddenly having this circuit
fail at high temp. The oscillator fails at 60-70C and a few even do not
work at room temperature. I am having 40% fallout at temp. If you lower
the temperature to <40C it starts to work while power is applied. The
problem is made more difficult since the units are potted. We cannot get
at the components to troubleshoot. We are trying to test unpotted units
over temperature. In the meantime I have tried to find a threshold of
failure on a working unpotted unit by increasing and decreasing the 27pF
caps - amazingly the circuit works at room temperature with 47pF caps on
the high end to 6pF on the low end. With 6pF I can make it fail if I
raise the temperature to 70C, but as it cools it begins to wor!
 k. I have wrote Epson for crystal ideas and sent them my layout - any
ideas from you guys? What have you all used? Maybe someone has a
successful crystal oscillator design I could use.

Cheers,
Shawn



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[Olin Lathrop]

Leslie, Shawn J wrote:
> I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator
>  as an RTC. I am driving the oscillator with a Epson FC-135 series
> 32.768KHz 12.5pF crystal good to 85C. I am hanging 27pF caps off the
> crystal. After 2 years of successful builds we are su

Your message got cut off probably because you are sending paragraphs as
single long lines (don't do that).

However, hanging at 27pF cap off one of these low frequency crystals is a
bad idea, especially since it's rated for a load less than half that.  The
drive levels are very low on LP mode.  A 1Mohm scope probe will usually kill
the oscillations.


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[phone].  Gold level PIC consultants since 2000.
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[Spehro Pefhany]

Quoting "Leslie, Shawn J" <sjlesli@...>:

> I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator  
> as an RTC. I am driving the oscillator with a Epson FC-135 series  
> 32.768KHz 12.5pF crystal good to 85C. I am hanging 27pF caps off the  
>  crystal. After 2 years of successful builds we are suddenly having  
> this circuit fail at high temp. The oscillator fails at 60-70C and a  
>  few even do not work at room temperature. I am having 40% fallout  
> at  temp. If you lower the temperature to <40C it starts to work  
> while  power is applied. The problem is made more difficult since  
> the units  are potted. We cannot get at the components to  
> troubleshoot. We are  trying to test unpotted units over  
> temperature. In the meantime I  have tried to find a threshold of  
> failure on a working unpotted unit  by increasing and decreasing the  
> 27pF caps - amazingly the circuit  works at room temperature with  
> 47pF caps on the high end to 6pF on  the low end. With 6pF I can  
> make it fail if I raise the temperature  to 70C, but as it cools it  
> begins to wor!
>  k. I have wrote Epson for crystal ideas and sent them my layout -  
> any ideas from you guys? What have you all used? Maybe someone has a  
>  successful crystal oscillator design I could use.

I suspect that the system problem may be more related to the PIC in
this case than the crystal. There are a couple of (IMO) very serious  
errata that apply to that micro for temperatures above 70°C.  
Experience suggests that finding one or two cockroaches may be a good  
indication that there are more of them about.

Suggest you see if you can get any info out of Microchip.

In the meantime, try another type of PIC that has the Timer1  
oscillator, but does not have the errata items I mentioned, and see if  
you can prove (or at least get an indication) it's tied to this  
silicon. 22pF should be a bit closer to optimum, but it's doubtful  
(confirmed by your testing) that cap values would result in much  
change other than a slight error in frequency.

Best regards,
Spehro Pefhany
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[Andre Abelian-3]

Hi Shawn,

1. buy higher temp part PIC18LF4320E/ML 125C it should help.
2. use Fail-Safe Clock Monitor feature to switch internal 31k clock
instead
3. use different Brand name crystals to test.

 
Andre Abelian




-----Original Message-----
From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf
Of Leslie, Shawn J
Sent: Friday, June 27, 2008 10:13 AM
To: 'piclist@...'
Subject: [PIC] Timer 1 Oscillator Failure at high temperature

I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as
an RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz
12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal.
After 2 years of successful builds we are suddenly having this circuit
fail at high temp. The oscillator fails at 60-70C and a few even do not
work at room temperature. I am having 40% fallout at temp. If you lower
the temperature to <40C it starts to work while power is applied. The
problem is made more difficult since the units are potted. We cannot get
at the components to troubleshoot. We are trying to test unpotted units
over temperature. In the meantime I have tried to find a threshold of
failure on a working unpotted unit by increasing and decreasing the 27pF
caps - amazingly the circuit works at room temperature with 47pF caps on
the high end to 6pF on the low end. With 6pF I can make it fail if I
raise the temperature to 70C, but as it cools it begins to wor!
 k. I have wrote Epson for crystal ideas and sent them my layout - any
ideas from you guys? What have you all used? Maybe someone has a
successful crystal oscillator design I could use.

Cheers,
Shawn



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[Kenneth Lumia-2]

Hi,

1.  Get a crystal impedence meter and measure "good"
and "bad" crystals.  Compare against spec.  Verify that
the parameters don't shift when heat is applied (such
as frequency jumps, no oscillation, capacitance
changes, etc).

2. Get network analyzer and plot crystal response.
Look for spurs.  Heat crsytal and see if anything
shifts.

3. Create loop and measure crystal current.

4. Use small series resistors and calculate safety
factor.


- Ken

----- Original Message -----
From: "Leslie, Shawn J" <sjlesli@...>
To: <piclist@...>
Sent: Friday, June 27, 2008 1:12 PM
Subject: [PIC] Timer 1 Oscillator Failure at high
temperature

>I am using a PIC18LF4320I/ML. I am using Timer 1 and
>the oscillator as an RTC. I am driving the oscillator
>with a Epson FC-135 series 32.768KHz 12.5pF crystal
>good to 85C. I am hanging 27pF caps off the crystal.
>After 2 years of successful builds we are suddenly
>having this circuit fail at high temp. The oscillator
>fails at 60-70C and a few even do not work at room
>temperature. I am having 40% fallout at temp. If you
>lower the temperature to <40C it starts to work while
>power is applied. The problem is made more difficult
>since the units are potted. We cannot get at the
>components to troubleshoot. We are trying to test
>unpotted units over temperature. In the meantime I
>have tried to find a threshold of failure on a working
>unpotted unit by increasing and decreasing the 27pF
>caps - amazingly the circuit works at room temperature
>with 47pF caps on the high end to 6pF on the low end.
>With 6pF I can make it fail if I raise the temperature
>to 70C, but as it cools it begins to wor!
> k. I have wrote Epson for crystal ideas and sent them
> my layout - any ideas from you guys? What have you
> all used? Maybe someone has a successful crystal
> oscillator design I could use.
>
> Cheers,
> Shawn
>
>
>
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[Bob Axtell-3]

A few years ago I had a similar problem with potting compound causing
oscillator problems. My fix was to place a glob of RTV (clear bathtub
caulk) around the crystal and oscillator circuit, and allow it to cure fully
before you pot the circuit with other stuff.

One other possible problem is that while the 27pF caps can TAKE 80C,
they might lose their capacitance badly at 80C.

--Bob

On 6/27/08, Scott Touchton <STouchton@...> wrote:

> I had almost this exact problem several years ago.  Turns out the
> increased permeativity of the potting compound caused too much parasitic
> capacitance.  The low power oscillator design was marginal at best, and
> this extra capacitance pushed it over the edge.
>
> Removed the potting and all was well.  You might want to try that.
>
>
> Also, I have had problems with Epson crystals damaged during reflow (in
> a Quad oven running the correct profile).  Seems some crystals were
> marginal and would not sustain the profile spec'd by Epson.  Changed to
> another manufacturer and that problem went away.
>
> Scott F. Touchton
>
>
>
> -----Original Message-----
> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf
> Of Leslie, Shawn J
> Sent: Friday, June 27, 2008 1:13 PM
> To: 'piclist@...'
> Subject: [PIC] Timer 1 Oscillator Failure at high temperature
>
> I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as
> an RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz
> 12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal.
> After 2 years of successful builds we are suddenly having this circuit
> fail at high temp. The oscillator fails at 60-70C and a few even do not
> work at room temperature. I am having 40% fallout at temp. If you lower
> the temperature to <40C it starts to work while power is applied. The
> problem is made more difficult since the units are potted. We cannot get
> at the components to troubleshoot. We are trying to test unpotted units
> over temperature. In the meantime I have tried to find a threshold of
> failure on a working unpotted unit by increasing and decreasing the 27pF
> caps - amazingly the circuit works at room temperature with 47pF caps on
> the high end to 6pF on the low end. With 6pF I can make it fail if I
> raise the temperature to 70C, but as it cools it begins to wor!
>  k. I have wrote Epson for crystal ideas and sent them my layout - any
> ideas from you guys? What have you all used? Maybe someone has a
> successful crystal oscillator design I could use.
>
> Cheers,
> Shawn
>
>
>
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