10.3.4.2.2 32.768 kHz Crystal Oscillator (XOSC32K)

This oscillator supports two input options: Either a crystal is connected to the pins TOSC1 and TOSC2, or an external clock running at 32.768 kHz is connected to TOSC1. The input option must be configured by writing the Source Select (SEL) bit in the XOSC32K Control A (CLKCTRL.XOSC32KCTRLA) register.

The XOSC32K is enabled by writing a '1' to its ENABLE bit in CLKCTRL.XOSC32KCTRLA. When enabled, the configuration of the GPIO pins used by the XOSC32K is overridden as TOSC1, TOSC2 pins. The ENABLE bit needs to be set for the oscillator to start running when requested.

The start-up time of a given crystal oscillator can be accommodated by writing to the Crystal Start-up Time (CSUT) bits in CLKCTRL.XOSC32KCTRLA.

When XOSC32K is configured to use an external clock on TOSC1, the start-up time is fixed to two cycles.