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'Oscillator startup trouble - where to look?'
1999\10\02@083848 by Quentin

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I think somebody here complained before about build in caps that does
not work.
Never used resonators with that build in myself. Try use resonators with
external 15pF caps.
Or swap the resonator with one from the working boards.

How far away is your resonator from the OSC. pins? Best to have it as
close as possible with direct tracks to the pins.

Normally when I get intermitted startups it, mostly it's noisy supply
lines. put some cap filters in. And since 90% of the time my PICS is
used in the industrial field, I always use external Power On Reset.
Sometimes as long as a one second delay.

Also, are your selection of oscillator type correct in the Config
register?

Hope this helps
Quentin

1999\10\02@092512 by Dave Johnson

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Quentin wrote:

>Or swap the resonator with one from the working boards.
I swapped in a new one with the same result.

>How far away is your resonator from the OSC. pins?
Very close, a couple mm.

>Normally when I get intermitted startups it, mostly it's noisy supply
>lines. put some cap filters in.
Hmm, the power is actually coming from a well-regulated handheld computer
that my device plugs into, and I have plenty of bypass. Plus the other 3
boards work flawlessly plugged into the same device.

>Also, are your selection of oscillator type correct in the Config
>register?
Yes, XT. I did notice that if I change it to HS (more drive, right?) it
comes up more reliably, but still not 100%

Dave Johnson

1999\10\02@095715 by Quentin

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Another thing I've come across before:
Do you have an external signal from a seperate supply (common ground)
that goes into the PIC and is it on or switching when you power up? Such
as a PWM signal or serial comms. This has stopped my PICS from starting
up.

Quentin

1999\10\02@103116 by Reginald Neale

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>I have four identical prototype boards with PIC16F877s and 4MHz ceramic
>resonators (with built-in caps). Three of the four boards work great, but
>the fourth has startup problems. And since oscillators and crystals are
>still black magic to me, I'm a little clueless what to look for.
>
>Symptoms: sometimes it never comes to life at all when powered up, and
>scoping the resonator shows me a flatline. Sometimes it does come up, but
>usually after a delay. Sometimes just scoping the resonator, or touching
>pins around the resonator, kicks it into life, at least momentarily.
>(There's an led startup sequence so I can see it's progress, and
>sometimes it gets part way through then freezes, other times it starts up
>and keeps going.) Once it ran for a minute or so then just stopped in its
>tracks.
>
>Thanks for any debugging suggestions, this is an area I need to learn
>more about, obviously :-)
>
>Dave Johnson

 Dave:

 1. Make sure you have CONFIG set to XT osc.

 2. Put a 1 megohm resistor across the OSC pins and see if that
    makes a difference.

 3. You DO have the middle (GND) pin of the resonator connected,
    right? It should go straight to the Vss pin of the device.

 4. Look at your power supply filter cap size and its effect on
    Vdd ramp up time.

 Reg Neale

1999\10\02@121236 by Dave Johnson
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Reginald Neale wrote:

>  1. Make sure you have CONFIG set to XT osc.
Yep.

>  2. Put a 1 megohm resistor across the OSC pins and see if that
>     makes a difference.
I'll try that. Pardon my naivete, but for my own knowledge, why would
this make a difference?

>  3. You DO have the middle (GND) pin of the resonator connected,
>     right? It should go straight to the Vss pin of the device.
Yes, it punches directly into my ground plane right next to the resonator.

>  4. Look at your power supply filter cap size and its effect on
>     Vdd ramp up time.
The power is coming from a handheld computer, and is very clean and well
defined, so I don't think that's the trouble.

Again, 3 boards work fine in exactly the same conditions, so it must be
something on this particular board. This one was reworked a little more
than the others, so maybe an iffy solder joint or a stray bit of grunge?
Where should I look, on the OSC pins?

Dave Johnson

1999\10\02@122109 by Dave VanHorn

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> >  4. Look at your power supply filter cap size and its effect on
> >     Vdd ramp up time.
> The power is coming from a handheld computer, and is very clean and well
> defined, so I don't think that's the trouble.

Taking regged power across a cable is generally a mistake.
Measure it at the chip, you may be suprised.

> Again, 3 boards work fine in exactly the same conditions, so it must be
> something on this particular board.

Or just a marginal design. Happens all the time.
I've seen 10 protos working fine, and when the design was put into
production, 50% failed. (A PHD engineer put external loads on a modem chip's
osc that wasn't specified to drive any external load.. "But all the protos
are working!"..... )

>This one was reworked a little more
> than the others, so maybe an iffy solder joint or a stray bit of grunge?
> Where should I look, on the OSC pins?

You might try lifting the xtal/resonator up a bit. Depending on the package,
you can get a little crud caught under the package, which could be causing
the problem.

1999\10\02@153319 by Dave Johnson

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Dave VanHorn wrote:
>> The power is coming from a handheld computer, and is very clean and well
>> defined, so I don't think that's the trouble.
>Taking regged power across a cable is generally a mistake.
>Measure it at the chip, you may be suprised.
No cable involved, my board plugs directly into the computer (similar to
a PCMCIA card, and it's the same connector), but I'll double-check it.

>> Again, 3 boards work fine in exactly the same conditions, so it must be
>> something on this particular board.
>Or just a marginal design. Happens all the time.
Quite possible :-) But if so, I'd sure like to un-marginalize it...

So aside from power, what else could screw me up here? The resonator is
as close to the PIC as I could get it (less than 2mm to the pins), the
trace lengths are minimal, and the ground connection is solid, punched
right down into my ground plane next to the resonator.

>You might try lifting the xtal/resonator up a bit. Depending on the package,
>you can get a little crud caught under the package, which could be causing
>the problem.
It's a surface mount resonator (Murata CSTCC4.00MG if you care), and I
did replace it once, I'll re-solder and clean it up to make sure.

Thanks to all for the suggestions.

Dave Johnson

1999\10\02@155428 by Phillip Vogel

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Dave Johnson wrote:

> It's a surface mount resonator (Murata CSTCC4.00MG if you care), and I
> did replace it once, I'll re-solder and clean it up to make sure.

There is absolutly no substitute for cleanliness. Especially in high impedence
circuits (like this oscillator). Wash with whatever solvent is appropriate for
your flux. Then wash it again, with 99% isopropyl alcohol. Then follow up with
a wash in distilled water, and don't forget to dry thoroughly.

There's a coralarry to Nurphy's law which states: "Amplifiers oscillate,
oscillators amplify."

good luck,
Phillip

--
Phillip M. Vogel, President   | "It's not what you've been taught,
Bartal Design Group, Inc.     |  it's what you've learned." (me)
318 Marlboro Road             | +1-201-567-1343 FAX:+1-201-568-2891
Englewood, NJ 07631  USA      | spam_OUTphillipTakeThisOuTspambartal.com

1999\10\02@172214 by William K. Borsum

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At 02:38 PM 10/2/99 +0200, you wrote:
>I think somebody here complained before about build in caps that does
>not work.
>Never used resonators with that build in myself. Try use resonators with
>external 15pF caps.
>Or swap the resonator with one from the working boards.
>
>How far away is your resonator from the OSC. pins? Best to have it as
>close as possible with direct tracks to the pins.
>
>Normally when I get intermitted startups it, mostly it's noisy supply
>lines. put some cap filters in. And since 90% of the time my PICS is
>used in the industrial field, I always use external Power On Reset.
>Sometimes as long as a one second delay.
>
>Also, are your selection of oscillator type correct in the Config
>register?

Hi
Been following the thread, and one thing comes to mind.
Most of my problems have been with watch crystals in LP mode.
Power was very clean, caps were all the right values straight out of the
PIC data books, etc., and it still wouldn't kick into oscillation.
Also worked with OLD revisions of the '54s, but not the new die shrinks.
Solution cost less than two cents.  Add a CARBON composition resistor in
series with the oscillator as shown in the pic data book.
Experiment with the values, but I found 100K to be about right.
The problem is caused (in my case) by having too clean of a system--the
oscillator needs some noise to get it going, and the carbon composition is
inherently noisy enough to get things going.

Pick a value that keeps the P-P oscillations just within the power supply
rail voltages.

Let me know if this helps with the resonators too.

Kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<.....borsumKILLspamspam@spam@dascor.com> & <http://www.dascor.com>San Diego, California, USA

1999\10\02@200228 by Harold M Hallikainen

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I had a lot of trouble with various resonators (16 MHz on 16c74x).  Each
company said the resonator was fine if it worked in their CMOS inverter
circuit and they made no claims regarding operation with a PIC.  The
exception is muRata.  Go to their website, then follow the link to their
site in Japan.  They have a search engine where you can put in the number
of the chip you want to use with the resonator and it'll return the
muRata part number that WILL WORK.  They've actually characterized the
resonators with the various chips.

Thousands of resonators later, I've had good luck...

Harold


On Fri, 1 Oct 1999 20:29:42 -0700 Dave Johnson <djohnsonspamKILLspamSIRIUS.COM>
writes:
{Quote hidden}

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1999\10\03@032336 by David Duffy

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<x-flowed>I have always used the 4MHz 3 pin resonators made by AVX-Kyorcera.
They have always worked 100% with all of the Pic's I have ever used.
I have heard of other people having trouble with other brands of resonator.
Try a totally different type - that may be all that's wrong.
Regards...

</x-flowed>

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