Lpc2000

> The oscillator requires quite large value capacitors - try 33 pF.

It really depends on the Xtal, rather than the clock oscillator itself.

Normally you would load the crystal with its actual load capacitance.

Ultimately the caps also affect start up time and reliability of oscillation.

It depends on whether the (assumed Pierce) oscillator is implemented as

a NOT gate, or with a FET. (generally you can tell whether the oscillator

needs a bias resistor or not)

Sometimes the oscillator's drive can be a bit low, depending on silicon.
For some Xtals, the load caps might need optimising a bit, but with most

MCUs you don't have that luxuary since you need a 50% dutycycle

(unless you're using the PLL, then it's OK)

Ideally, the input cap dictates the loading of the crystal, and the output cap

sets the phase shift for the oscillator.

A good rule of thumb would be to use double the load value of the Xtal

on either side, so it depends on the Xtal that you're using.

A typical CL=12 pF Xtal would eg. use 27 pF on either side.

You can get other problems too, like the Xtal can go into Third Overtone etc.

(Anyone ever used the P300 of Intellon ?? 3OT problems galore :-)

Best is to use a good quality, fundamental Xtal with low ESR and low load capacitance

for good startup.

This area is too often overlooked.

Xtal oscillators aren't just "throw it in" kind of circuits.

The industry tends to refer to "Computer Grade" Xtals, which have the higher

ESR and load capacitance, a dangerous combination at times !

-- Kris