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 -----
From: David VanHorn <dvanhorn@CEDAR.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, April 12, 2000 11:09 PM
Subject: Re: [PICLIST] oscillator strangeness


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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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