4.5.1 Connection for XTAL32K (External 32.768 kHz Crystal Oscillator)

Ultra-low power 32.768 kHz oscillators typically dissipate significantly below 1 μW, and the current flowing in the circuit is, therefore, extremely small. The crystal frequency is highly dependent on the capacitive load.

A series resistor R_S may be required to prevent overdriving the oscillator. The gain from the oscillator driver may sometimes be too high for low-frequency oscillators, and adding impedance with R_S can decrease the gain. The overdrive causes the oscillator to not swing properly, as the signal will be saturated (clipped or “squashed”). Overdriving the crystal can also lead to the circuit jumping to a higher harmonic.

The following figure shows how to connect an external 32.768 kHz crystal oscillator:

Figure 4-7. Recommended External 32.768 kHz Oscillator Connection Circuit Schematic