Configure the input option by writing the Source Select (SEL) bit in the XOSC32K Control A (CLKCTRL.XOSC32KCTRLA) register.
The XOSC32K is enabled by writing a ‘1’ to the ENABLE bit in
CLKCTRL.XOSC32KCTRLA. When enabled, the configuration of the general purpose
input/output (GPIO) pins used by the XOSC32K is overridden as XTAL32K1 and XTAL32K2
pins. The oscillator needs to be enabled 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) bit field in XOSC32KCTRLA.
When XOSC32K is configured to use an external clock on XTAL32K1, the start-up time is fixed to two cycles.