On Thu, 11 Jul 2002 07:11:30 -0700, John Dammeyer wrote:
>Hi Matt,
>
>>You can tell if you are overdriving the crystal by looking at=
 the OSC
>>waveform. It should be a nice clean sine wave, slightly less=
 than
>>rail-to-rail. If you are getting a clipped sine wave then you=
 are
>>overdriving the crystal.
>
>Which waveform? =A0The OSC2 or the OSC1? =A0Or both?

I normally look at OSC2 (CLKOUT), since this is the low impedance=

output drive for the oscillator circuit. Normally it will not
significantly effect the oscillator, but the 'scope probe you use=
 has
to be considered. A good 10x probe will put about a 10MOhm/10pf=
 load to
ground on the PIC output. A cheap 1x probe will many times kill=
 the
oscillator.

>For a 20MHz PIC what sort of resistor value do you find you end=
 up
>using? =A01K, 10K, 100K or do you select with a particular batch=
 of
>crystals?

It really depends on the crystal I'm going to use. If I'm using a=

typical HC49/U can (1 mW drive level), then no resistor is=
 generally
required. If I am using a HC49/US (0.1 mW drive level), then I=
 will go
with something around 470 Ohms. For most 32KHz watch crystal=
 circuits,
I'll start around 180K Ohms and have gone as high as 330K. 32KHz=

circuits are extremely difficult to measure as they are very high=

impedance all around and loading of any kind (even board moisture=
 and
dust!) can have a significant effect on the oscillator.

>Cheers,
>
>John Dammeyer

P.S. Some interesting (I hope) crystal experiences in an OT post=
 to
follow.

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