22.5.3 Main Crystal Oscillator

After reset, the Main crystal oscillator is disabled and is not selected as the source of MAINCK.

As the source of MAINCK, the Main crystal oscillator provides a very precise frequency. The software enables or disables this oscillator in order to reduce power consumption via CKGR_MOR.MOSCXTEN.

When disabling this oscillator by clearing the CKGR_MOR.MOSCXTEN, PMC_SR.MOSCXTS is automatically cleared, indicating the oscillator is off.

When enabling this oscillator, the user must initiate the startup time counter. The startup time depends on the characteristics of the external device connected to this oscillator.

When CKGR_MOR.MOSCXTEN and CKGR_MOR.MOSCXTST are written to enable this oscillator, the PIO lines multiplexed with XIN and XOUT are driven by the Main crystal oscillator. PMC_SR.MOSCXTS is cleared and the counter starts counting down on SLOW_CLK divided by 8 from the CKGR_MOR.MOSCXTST value. Since the CKGR_MOR.MOSCXTST value is coded with 8 bits, the startup time can be programmed up to 2048 SLOW_CLK periods, corresponding to about 62 ms when running at 32.768 kHz.

When the startup time counter reaches ‘0’, PMC_SR.MOSCXTS is set, indicating that the oscillator is stabilized. Setting the MOSCXTS bit in the Interrupt Mask Register (PMC_IMR) can trigger an interrupt to the processor.