I made a write-up about my experiments the STM8L LSE (Low Speed External) oscillator that uses a 32.768kHz crystal to clock peripherals like the RTC.

For my experiments I used the nrf24stm8l board that I made some time ago but a chip on a standard "green TSSOP20 breakout PCB" for $0.10 with a crystal soldered to pins 1 and 2 also works (caps were not needed when I tried it):

The write-up gets started with the LSE and the beeper peripheral. Using the beeper is a good test for a crystal oscillator since one can hear the crystal start up! The next experiment introduces the RTC peripheral with a basic example for configuration, initialization and use.