32.17 Main Crystal Oscillator Failure Detection

The Main crystal oscillator failure detector monitors the 8 to 24 MHz crystal oscillator or ceramic resonator-based oscillator to identify a possible failure of this oscillator.

The clock failure detector can be enabled or disabled by configuring CKGR_MOR.CFDEN. The detector is also disabled in either of the following cases:

  • after a VDDCORE reset
  • when the oscillator is disabled (CKGR_MOR.MOSCXTEN = 0)

A failure is detected by means of a counter incrementing on the main oscillator clock edge and detection logic is triggered by the 32 kHz generated by the 64 kHz (typical) RC oscillator. This oscillator is automatically enabled when CKGR_MOR.CFDEN = 1.

The counter is cleared when the 32 kHz generated by the 64 kHz (typical) RC oscillator clock signal is low, and enabled when the signal is high. Thus, the failure detection time is one RC oscillator period. If, during the high level period of the 32 kHz generated by the 64 kHz (typical) RC oscillator clock signal, less than eight 8 to 24 MHz crystal oscillator clock periods have been counted, then a failure is reported.

If a failure of the Main clock is detected, PMC_SR.CFDEV indicates a failure event and generates an interrupt if the corresponding interrupt source is enabled. The interrupt remains active until a read occurs in PMC_SR. The user can know the status of the clock failure detection at any time by reading PMC_SR.CFDS.

Figure 32-11. Clock Failure Detection (Example)

If the 8 to 24 MHz crystal oscillator or ceramic resonator-based oscillator is selected as the source clock of MAINCK (CKGR_MOR.MOSCSEL = 1), and if MCK source is PLLACK or UPLLCK (PMC_MCKR.CSS = 2 or 3), a clock failure detection automatically forces MAINCK to be the source clock for the Main System Bus clock (MCK). Then, regardless of the PMC configuration, a clock failure detection automatically forces the 12 MHz RC oscillator to be the source clock for MAINCK. If this oscillator is disabled when a clock failure detection occurs, it is automatically re-enabled by the clock failure detection mechanism.

It takes two 32 kHz (typical) clock cycles to detect and switch from the 8 to 24 MHz crystal oscillator to the 12 MHz RC oscillator if the source Main System Bus clock (MCK) is Main clock (MAINCK), or three 32 kHz (typical) cycles if the source of MCK is PLLACK or UPLLCK.

A clock failure detection activates a fault output that is connected to the Pulse Width Modulator (PWM) Controller. With this connection, the PWM controller is able to force its outputs and to protect the driven device, if a clock failure is detected.

The user can know the status of the clock failure detector at any time by reading PMC_SR.FOS.

This fault output remains active until the defect is detected and until it is cleared by FOCLR in the PMC Fault Output Clear register (PMC_FOCR).