30.5.6 Main Frequency Counter
The Main frequency counter measures the Main RC oscillator and the Main crystal oscillator against the SLCK and is managed by CKGR_MCFR.
During the measurement period, the Main frequency counter increments at the speed of the clock defined by the bit CKGR_MCFR.CCSS.
A measurement is started in the following cases:
- When CKGR_MCFR.RCMEAS is written to ‘1’.
- When the Main RC oscillator is selected as the source of MAINCK and when this oscillator is stable (i.e., when the MOSCRCS bit is set)
- When the Main crystal oscillator is selected as the source of MAINCK and when this oscillator is stable (i.e., when the MOSCXTS bit is set)
- When MAINCK source selection is modified
The measurement period ends at the 16th falling edge of SLCK, the MAINFRDY bit in CKGR_MCFR is set and the counter stops counting. Its value can be read in the MAINF field of CKGR_MCFR and gives the number of clock cycles during 16 periods of SLCK, so that the frequency of the Main RC oscillator or Main crystal oscillator can be determined.
If switching the source of MAINCK to the Main crystal oscillator from the Main RC oscillator, follow the programming sequence below to ensure that the oscillator is present and that its frequency is valid:
- Enable the Main crystal oscillator by setting CKGR_MOR.MOSCXTEN. Configure the CKGR_MOR. MOSCXTST field with the Main crystal oscillator startup time as defined in the section “Electrical Characteristics”.
- Wait for PMC_SR.MOSCXTS flag to rise, indicating the end of a startup period of the Main crystal oscillator.
- Select the Main crystal oscillator as the source clock of the Main frequency counter by setting CKGR_MCFR.CCSS.
- Initiate a frequency measurement by setting CKGR_MCFR.RCMEAS.
- Read CKGR_MCFR.MAINFRDY until its value equals 1.
- Read CKGR_MCFR.MAINF and compute the value of the Main crystal frequency.
If the MAINF value is valid, software can switch MAINCK to the Main crystal oscillator. Refer to "Main Clock Source Selection".
