-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

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.


-----BEGIN PGP SIGNATURE-----
Version: PGPfreeware 6.5.2 for non-commercial use <http://www.pgp.com>

iQA/AwUBOPVIioFlGDz1l6VWEQL9OwCg6RrZ6WLLzWEuUZAaSjbI4RKCY/0AnRDw
fWQrI6bRX1PvNBbEzKLWNRUe
=xq9+
-----END PGP SIGNATURE-----