Using only stray capacitance for MCU oscillator circuits

Question

I've included a diagram of typical Pierce oscillator circuit taken from ST Application Note AN2867:

The note suggests that C_L1 and C_L2 should be chosen such that the following equation holds:

Since C_L1 and C_L2 are typically equal, this equation can be rewritten to solve for their value: C_Lx = 2(C_L-C_S).

As a rule of thumb, the stray capacitance is usually taken to be 2pF - 5pF, although I've seen a datasheet suggest 10pF as a rough estimate.

The question: By choosing an oscillator with a low load capacitance C_L, e.g. equal to the expected stray capacitance, C_Lx goes to zero. Is there any problem with choosing an oscillator with a small load capacitance and eliminating the need for the two capacitors C_L1 and C_L2 from a design by depending solely on the stray capacitance?

Answer 1

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 – Schematic created using CircuitLab}