24.6.4 32 kHz Ultra Low-Power Internal Oscillator (OSCULP32K) Operation
The OSCULP32K provides a tunable, low-speed, and ultra low-power clock source.
The OSCULP32K is enabled by default after a Power-on Reset (POR), and will always run except during POR.
Users can lock the OSCULP32K configuration by setting the Write Lock bit in the 32 kHz Ultra Low-Power Internal Oscillator Control register (OSCULP32K.WRTLOCK = 1). If set, the OSCULP32K configuration is locked until POR is detected.
The OSCULP32K can be used as a source for Generic Clock Generators (GCLK) or for the Real-Time Counter (RTC). To ensure proper operation, the GCLK or RTC modules must be disabled before the clock selection is changed.
OSCULP32K Clock Switch
The Clock switch operation requires the XOSC32K to be enabled (XOSC32K.ENABLE=1 and STATUS.XOSC32KRDY = 1). When the OSCULP32K Clock Switch Enable bit (OSCULP32K.ULP32KSW) is set, the CLK_OSCULP32K clock is switched to the XOSC32K Clock Oscillator. When the clock switch process is complete, the OSCULP32K Clock Switch bit in Status register (STATUS.ULP32KSW) is set. The OSCULP32K oscillator is shut off, and the XOSC32K oscillator becomes always running. The CFD feature is also disabled by hardware. When set, the OSCULP32K.ULP32KSW can be reset only by POR operation.