> I am trying to run a PIC18C452 from an ECS HC-49US crystal
> (http://www.ecsxtal.com/pdf2/hc-49us.pdf) It is a 4.000 MHz crystal and
the
> package says 20pF on the outside.  According to the datasheet for the PIC,
> it says that a series resistor may be needed for the crystal to function
> properly.  I have tried a 50K pot to vary this resistance,

That sounds way to high.  Start with 0 ohms and 22pF caps to ground at
either side of the crystal.  Keep the crystal close to the PIC, the leads
short, and observe good grounding techniques.  Then use the right oscillator
mode for that speed crystal.


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