ESP32 External 32.768kHZ Crystal for RTC

[dalbert]

Something to be aware of:
The ESP32 r1 and r3 require different loading for the crystal oscillator.
I use a 6pF 3.2x1.5mm 2-SMD crystal with two loading caps, all located close to the XO pins and following good layout practices.
My older boards built with ESP32 r1 require at least 10pF load capacitors to start reliably
My newer boards built with ESP32 r3 *require* a parallel R (2-10M) and won't start without it; they work best (closest to nominal freq) with 6pF or slightly lower load caps.

Support for an external 32kHz xtal is very important for many embedded applications; it would be great if Espressif would provide more documentation and design guidance for the low speed XO.