crystal capacitors, digital inputs

mynameisdan said:

I'm getting into electronics after a few years away (and I was only really a novice then), and I thought that a simple circuit involving a PIC would be a good place to start.

So first off, I'm using a 40Mhz crystal for a PIC18F4550 and I'm wondering what value capacitors to use. I've searched and found nothing on determining the actual value of these, just that they exist.

The digital inputs are theoretically ∞ impedance, so would it matter if the voltage applied to it was slightly above the rated logic voltage and just with a current limiting resistor (12v)?

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ericgibbs said:

hi,
I would use 12pF caps on a 40MHz crystal.
The digital inputs have internal clamp diodes to 0V and +Vs , rated at < 20mA, so I would use a series resistor as a limiter or resistor potential divider.

EDIT:
Note the 18F4550 has an internal PLL, so you can use a lower frequency crystal if required.

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mynameisdan said:

Thanks very much, is there anywhere I can find this information? For example a lookup up reference or a general formula? Or is it based on experience?

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