Using only stray capacitance for MCU oscillator circuits
The ESD diodes are part of the stray capacitance. These, being installed on the silicon SUBSTRATE, inject all the MCU trash into the XTAL's energy storage loop.

You'll have lower jitter (phasenoise) if the ESD diodes are swamped out by external capacitors. The oscillator will build up the stored energy until cycle-by-cycle losses equal the cycle-by-cycle newly-injected energy. 

Note you have the same resonant current circulating in both capacitors of the PI network. This can be a reason to make the Cinput (OSCin, CL1) be substantially smaller, so the voltage across Cinput is quite LARGE. Of course, smaller caps are more vulnerable to MCU trash injected thru the ESD diodes/structures. Experiment with this tradeoff.







simulate this circuit &ndash; Schematic created using CircuitLab