15.6.5 32.768 kHz Ultra-Low Power Internal Oscillator (OSCULP32K) Operation

The OSCULP32K provides a tunable, low-speed, and ultra-low power clock source. The OSCULP32K is factory-calibrated under typical voltage and temperature conditions. The OSCULP32K should be preferred to the OSC32K whenever the power requirements are prevalent over frequency stability and accuracy.

The OSCULP32K is enabled by default after a Power-on Reset (POR) and will always run except during POR. The frequency of the OSCULP32K Oscillator is controlled by the value in the Calibration bits in the 32.768 kHz Ultra-Low Power Internal Oscillator Control register (OSCULP32K.CALIB). This data is used to compensate for process variations.

OSCULP32K.CALIB is automatically loaded from Flash Factory Calibration during start-up. The calibration value can be overridden by the user by writing to OSCULP32K.CALIB.

It is also possible to lock the OSCULP32K configuration by setting the Write Lock bit in the 32.768 kHz Ultra-Low Power Internal Oscillator Control register (OSCULP32K.WRTLOCK = 1). If set, the OSCULP32K configuration is locked until a Power-on Reset (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.