After reset, the Main RC oscillator is enabled with the 12 MHz frequency selected. This oscillator is selected as the source of MAINCK. MAINCK is the default clock selected to start the system.
Only the 8/12 MHz RC oscillator frequencies are calibrated in production. Refer to the section “Electrical Characteristics”.
The software can disable or enable the Main RC oscillator with the MOSCRCEN bit in the Clock Generator Main Oscillator Register (CKGR_MOR).
The output frequency of the Main RC oscillator can be selected among 4, 8 or 12 MHz. Selection is done by configuring the field MOSCRCF in CKGR_MOR. When changing the frequency selection, the MOSCRCS bit in the Power Management Controller Status Register (PMC_SR) is automatically cleared and MAINCK is stopped until the oscillator is stabilized. Once the oscillator is stabilized, MAINCK restarts and PMC_SR.MOSCRCS is set. Note that enabling the Main RC oscillator (MOSCRCEN = 1) and changing its frequency (MOSCRCF) at the same time is not allowed.
This oscillator must be enabled first and its frequency changed in a second step.
When disabling the Main RC oscillator by clearing the CKGR_MOR.MOSCRCEN bit, the PMC_SR.MOSCRCS bit is automatically cleared, indicating that the oscillator is OFF.
Setting the MOSCRCS bit in the Power Management Controller Interrupt Enable Register (PMC_IER) triggers an interrupt to the processor.