odriew wrote:Is there a simple way to test the oscillator with a DMM?
Well... If the DC voltage at XTAL2 (output) is within .5V of either rail, you can be fairly sure the oscillator is
not running. If the DC voltage is somewhere between 25 and 75% of Vcc, it might be running.
odriew wrote:What exactly do the load caps do for the oscillator circuit?
I'm afraid I have to fall back on the old weight-and-spring / resonant-circuit analogy here. At the normal operating frequency, the xtal could be modelled as an inductor. The resonant frequency is determined by the values of the effective inductance and the load caps.
So, how can it run at all with only the board capacitance? I'm out of my depth here, but... Above the normal operating frequency, you approach
series resonance and the model of the xtal as an inductor is begins to fall apart. With a 20% frequency increase (I'm making these numbers up) the
effective inductance might go up by a factor of 8. So, in this made-up example, if the board capacitance were 12% of the expected total capacitance, the crystal would still resonate at only 120% of the nominal frequency. Not 800% as you'd expect from my original claim that the xtal can be modelled as an inductor.
There are layers and layers (and layers) to crystal oscillator design, and I didn't find a link that seemed just right. Search for crystal oscillator [design | analysis | model | tutorial]