31.5.2 8 to 24 MHz Crystal Oscillator

After reset, the 8 to 24 MHz crystal oscillator is disabled and is not selected as the source of MAINCK.

As the source of MAINCK, the 8 to 24 MHz crystal oscillator provides an accurate frequency. 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, PMC_SR.MOSCXTS is automatically cleared, indicating the 8 to 24 MHz crystal oscillator is off.

When enabling this oscillator, the user must initiate the start-up time counter. This start-up time depends on the characteristics of the external device connected to this oscillator. Refer to the section “Electrical Characteristics” for the start-up time.

When CKGR_MOR.MOSCXTEN and CKGR_MOR.MOSCXTST are written to enable this oscillator, PMC_SR.MOSCXTS is cleared and the counter starts counting down on the Slow clock divided by 8 from the MOSCXTST value. When the counter reaches 0, PMC_SR.MOSCXTS is set, indicating that the 8 to 24 MHz crystal oscillator is stabilized. Setting MOSCXTS in the PMC Interrupt Mask register (PMC_IMR) triggers an interrupt to the processor.