28.5.2 Main Crystal Oscillator

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

The software enables or disables this oscillator in order to reduce power consumption via CKGR_MOR.MOSCXTEN.

When disabling this oscillator by clearing CKGR_MOR.MOSCXTEN, the PMC_SR.MOSCXTS status bit is automatically cleared, indicating the oscillator is off. To activate the Main Crystal Oscillator Bypass mode, see Bypassing the Main Crystal Oscillator.

When enabling this oscillator, the user must initiate the start-up time counter. The start-up 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, XIN and XOUT are driven by the main crystal oscillator. PMC_SR.MOSCXTS is cleared and the counter starts counting down on MD_SLCK divided by 8 from the CKGR_MOR.MOSCXTST value. Since the CKGR_MOR.MOSCXTST value is coded with 8 bits, the start-up time can be programmed up to 2048 MD_SLCK periods, corresponding to about 62 ms when running at 32.768 kHz.

When the start-up 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.