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PICList Thread
'oscillator strangeness'
2000\04\12@213705 by Reginald Neale

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<x-flowed>I am adding an A/D capability to an existing controller design. This
involves migrating from a 16C54 to a 16C710.

There seems to be a big difference in the robustness of the
oscillator. In both cases, I'm using a  2 MHz Matsushita resonator
with built-in caps and the config choice set to XT. The 16C54 is
pretty bulletproof. You can hook a scope probe or a logic probe to
OSC1 or OSC2 and it keeps right on going. The 16C710 quits at the
slightest loading. That makes me worry about possible stability
problems and frequent unwanted resets.

Just to see what would happen, I programmed a 16C710 to use the
config choice HS. Big improvement. Now it acts like the 16C54. But HS
is not normally the config setting for a 2 MHz part.

Anyone have any comments/suggestions/guidance?

Reg Neale

</x-flowed>

2000\04\12@220515 by Erik Reikes

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I found the same to be true with the 16F876.  I have a 3.6864MHz Crystal
with 18pF loading caps tied to ground.  If I program it to XT (recommended
in the data sheet) it will run about 25% of the time.  HS works like a
charm.  Anyone else have this experience too?  I thought I might have the
wrong loading caps, but they seem correct for the osciallator according to
the app note.

Good luck!

At 09:37 PM 4/12/00 -0700, you wrote:
{Quote hidden}

Erik Reikes
Senior Software Engineer
Xsilogy, Inc.

spam_OUTereikesTakeThisOuTspamxsilogy.com
ph : (858) 535-5113
fax : (858) 535-5163
cell : (858) 663-1206

2000\04\12@223932 by David VanHorn

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At 07:07 PM 4/12/00 -0700, Erik Reikes wrote:
>I found the same to be true with the 16F876.  I have a 3.6864MHz Crystal
>with 18pF loading caps tied to ground.  If I program it to XT (recommended
>in the data sheet) it will run about 25% of the time.  HS works like a
>charm.  Anyone else have this experience too?  I thought I might have the
>wrong loading caps, but they seem correct for the osciallator according to
>the app note.

The app note can't tell you what value of caps YOUR crystals need in YOUR
circuit.
It's probably within the ballpark, but the correct aproach is to start with
the crystal specs.
It's got almost nothing to do with the chip, except that the chip
contributes a little
loading capacitance as well.

1: is the xtal paralell resonant?
2: What is the specified loading value?
3: Multiply by two.
4: Subtract 4-5 pF (guesstimate of circuit loading)
5: Put one cap of this value to the uP's ground pin, on each xtal lead.
6: Measure xtal frequency, adjust caps to get on frequency.

If the xtal is series resonant:
It can be made to run.
You have to guess at the loading values.
It will never run at the "nameplate" frequency.
ie: an 8.000000 MHz series xtal will never run at 8.000000 in a paralell
circuit,
it can't go there in that mode.

Dont' add extra caps and resistors unless the data sheet tells you to, and
only under
the conditions where they tell you to use them.

A possible exception is 32kHz fork crystals, which take very little drive,
and can be snapped
by a circuit designed for an AT cut crystal. These may need a series
resistance to cut the
drive level down, but then you need to know their impedance at resonance,
and the drive
level of the uP to get it right.


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2000\04\12@234118 by Giles

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David,
On your chip, do you have RB3 pulled high on power up?  If so, be sure you
have the low power program bit turned off.  This only applied to the flash
parts and if you have a problem with the chip starting to run all the time
on power up.  Atleast that is what happened to me.

Best regards,
Giles


{Original Message removed}

2000\04\13@025242 by William K. Borsum

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<x-flowed>HS mode ups the gain of the oscillator--or so I have been told.  So more
drive to the crystal.
I've found that OSC1 usually can handle a scope probe, and OSC2
can't--again supposedly one is drive output and the other is feedback to
the oscillator circuit.

I've seen MAJOR oscillator problems in the 16C54 and 57 parts as they went
through the die shrinks--A 54LP worked fine, but the 54A, B, & C revs would
not oscillate without a carbon comp resistor in series to introduce enough
noise to start the crystal wiggling (watch chip type).  Hadn't thought to
try anything other than LP mode.

Would like to hear your experiences with this.

Kelly


At 09:37 PM 4/12/00 , you wrote:
{Quote hidden}

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

</x-flowed>

2000\04\13@052244 by Alan B Pearce

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>2: What is the specified loading value?
>3: Multiply by two.
>4: Subtract 4-5 pF (guesstimate of circuit loading)
>5: Put one cap of this value to the uP's ground pin, on each xtal lead.

You may actually find that using unequal capacitors with the series equivalent
capacitance being correct for the crystal loading may work better. This is then
similar to a colpitts oscillator where you may have unequal capacitors. If I
remember correctly the larger value capacitor is put on the driven side of the
crystal (i.e. the osc output pin). this will also make attaching a probe to this
pin less critical.

2000\04\13@114059 by David VanHorn

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At 10:21 AM 4/13/00 +0100, Alan B Pearce wrote:
>>2: What is the specified loading value?
>>3: Multiply by two.
>>4: Subtract 4-5 pF (guesstimate of circuit loading)
>>5: Put one cap of this value to the uP's ground pin, on each xtal lead.
>
>You may actually find that using unequal capacitors with the series
equivalent
>capacitance being correct for the crystal loading may work better. This is
then
>similar to a colpitts oscillator where you may have unequal capacitors. If I
>remember correctly the larger value capacitor is put on the driven side of
the
>crystal (i.e. the osc output pin). this will also make attaching a probe
to this
>pin less critical.

This complicates the brew.  The two caps appear to the xtal in series, and
you're
trying to get to XpF using two 2XpF caps in series.  If you make them
unequal, then
you have to make one larger than 2X, and the other smaller.

I'd want to know more about the exact drive circuit before attempting that
trick.

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2000\04\14@195716 by Robert A. LaBudde

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<x-flowed>At 07:07 PM 4/12/00 -0700, Erik wrote:
>I found the same to be true with the 16F876.  I have a 3.6864MHz Crystal
>with 18pF loading caps tied to ground.  If I program it to XT (recommended
>in the data sheet) it will run about 25% of the time.  HS works like a
>charm.  Anyone else have this experience too?  I thought I might have the
>wrong loading caps, but they seem correct for the osciallator according to
>the app note.

Try using 33 pF. Higher capacitance helps oscillator stability. I use this
value for 3.5 - 4.0 MHz. 18 pF seems low.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ralspamKILLspamlcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================

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