15.6.2 32.768 kHz External Crystal Oscillator (XOSC32K) Operation

The XOSC32K can operate in these modes:

• External clock, with an external clock signal connected to XIN32
• Crystal oscillator, with an external 32.768 kHz crystal connected between XIN32 and XOUT32

At reset, the XOSC32K is disabled, and the XIN32/XOUT32 pins can either be used as General Purpose I/O (GPIO) pins or by other peripherals in the system.

When XOSC32K is enabled, the operating mode determines the GPIO usage. When in crystal oscillator mode, the XIN32 and XOUT32 pins are controlled by the OSC32KCTRL, and GPIO functions are overridden on both pins. When in external clock mode, the only XIN32 pin will be overridden and controlled by the OSC32KCTRL, while the XOUT32 pin can still be used as a GPIO pin.

The XOSC32K is enabled by writing a '1' to the Enable bit in the 32.768 kHz External Crystal Oscillator Control register (XOSC32K.ENABLE = 1). The XOSC32K is disabled by writing a '0' to the Enable bit in the 32.768 kHz External Crystal Oscillator Control register (XOSC32K.ENABLE = 0). When disabling the XOSC32K it is important only to write the Enable bit to '0' before additional changes are made to the XOSC32K register.

To enable the XOSC32K as a crystal oscillator, the XTALEN bit in the 32.768 kHz External Crystal Oscillator Control register must be set (XOSC32K.XTALEN = 1). If XOSC32K.XTALEN is '0', the external clock input will be enabled.

The XOSC32K 32.768 kHz output is enabled by setting the 32.768 kHz Output Enable bit in the 32.768 kHz External Crystal Oscillator Control register (XOSC32K.EN32K = 1). The XOSC32K also has a 1.024 kHz clock output. This is enabled by setting the 1.024 kHz Output Enable bit in the 32.768 kHz External Crystal Oscillator Control register (XOSC32K.EN1K = 1).

It is also possible to lock the XOSC32K configuration by setting the Write Lock bit in the 32.768 kHz External Crystal Oscillator Control register (XOSC32K.WRTLOCK = 1). If set, the XOSC32K configuration is locked until a Power-On Reset (POR) is detected.

The XOSC32K will behave differently in different sleep modes based on the settings of XOSC32K.RUNSTDBY, XOSC32K.ONDEMAND, and XOSC32K.ENABLE. If XOSC32KCTRL.ENABLE = 0, the XOSC32K will be always stopped. For XOS32KCTRL.ENABLE = 1, the table below is valid:

As a crystal oscillator usually requires a very long start-up time, the 32.768 kHz External Crystal Oscillator will keep running across resets when XOSC32K.ONDEMAND = 0, except for Power-on Reset (POR). After a reset or when waking up from a sleep mode where the XOSC32K was disabled, the XOSC32K will need time to stabilize on the correct frequency, depending on the external crystal specification. This start-up time can be configured by changing the Oscillator Start-Up Time bit group (XOSC32K.STARTUP) in the 32.768 kHz External Crystal Oscillator Control register. During the start-up time, the oscillator output is masked to ensure that no unstable clock propagates to the digital logic.

Once the external clock or crystal oscillator is stable and ready to be used as a clock source, the XOSC32K Ready bit in the Status register is set (STATUS.XOSC32KRDY = 1). The transition of STATUS.XOSC32KRDY from '0' to '1' generates an interrupt if the XOSC32K Ready bit in the Interrupt Enable Set register is set (INTENSET.XOSC32KRDY = 1).

The XOSC32K can be used as a source for Generic Clock Generators (GCLK) or for the Real-Time Counter (RTC). Before enabling the GCLK or the RTC module, the corresponding oscillator output must be enabled (XOSC32K.EN32K or XOSC32K.EN1K) in order to ensure proper operation. In the same way, the GCLK or RTC modules must be disabled before the clock selection is changed. For details on RTC clock configuration, refer also to 15.6.7 Real-Time Counter Clock Selection.

When the XOSC32K 1024 Hz clock output is used to clock the RTC (RTCCTRL.RTCSEL=4), ensure the XOSC32K is in the 'Always run' ( as shown above in Table 15-1 XOSC32K Sleep Behavior) configuration.