start-up time has elapsed. The start-up time is configured by the Oscillator Start-Up Time in the External
Multipurpose Crystal Oscillator Control register (XOSC32K.STARTUP). Once the XOSC32K Start-Up
Time is elapsed, the XOSC32K clock is constantly monitored.
During a period of 4 safe clocks (monitor period), the CFD watches for a clock activity from the
XOSC32K. There must be at least one rising and one falling XOSC32K clock edge during 4 safe clock
periods to meet non-failure conditions. If no or insufficient activity is detected, the failure status is
asserted: The Clock Failure Detector status bit in the Status register (STATUS.CLKFAIL) and the Clock
Failure Detector interrupt flag bit in the Interrupt Flag register (INTFLAG.CLKFAIL) are set. If the CLKFAIL
bit in the Interrupt Enable Set register (INTENSET.CLKFAIL) is set, an interrupt is generated as well. If the
Event Output enable bit in the Event Control register (EVCTRL.CFDEO) is set, an output event is
generated, too.
After a clock failure was issued the monitoring of the XOSC32K clock is continued, and the Clock Failure
Detector status bit in the Status register (STATUS.CLKFAIL) reflects the current XOSC32K activity.
Clock Switch
When a clock failure is detected, the XOSC32K clock is replaced by the safe clock in order to maintain an
active clock during the XOSC32K clock failure. The safe clock source is the OSCULP32K oscillator clock.
Both 32KHz and 1KHz outputs of the XOSC32K are replaced by the respective OSCULP32K 32KHz and
1KHz outputs. The safe clock source can be scaled down by a configurable prescaler to ensure that the
safe clock frequency does not exceed the operating conditions selected by the application. When the
XOSC32K clock is switched to the safe clock, the Clock Switch bit in the Status register
(STATUS.CLKSW) is set.
When the CFD has switched to the safe clock, the XOSC32K is not disabled. If desired, the application
must take the necessary actions to disable the oscillator. The application must also take the necessary
actions to configure the system clocks to continue normal operations. In the case the application can
recover the XOSC32K, the application can switch back to the XOSC32K clock by writing a '1' to Switch
Back Enable bit in the Clock Failure Control register (CFDCTRL.SWBACK). Once the XOSC32K clock is
switched back, the Switch Back bit (CFDCTRL.SWBACK) is cleared by hardware.
Prescaler
The CFD has an internal configurable prescaler to generate the safe clock from the OSCULP32K
oscillator. The prescaler size allows to scale down the OSCULP32K oscillator so the safe clock frequency
is not higher than the XOSC32K clock frequency monitored by the CFD. The maximum division factor is
2.
The prescaler is applied on both outputs (32KHz and 1KHz) of the safe clock.
Example
For an external crystal oscillator at 32KHz and the OSCULP32K frequency is 32KHz, the
XOSC32K.CFDPRESC should be set to 0 for a safe clock of equal frequency.
Event
If the Event Output Enable bit in the Event Control register (EVCTRL.CFDEO) is set, the CFD clock
failure will be output on the Event Output. When the CFD is switched to the safe clock, the CFD clock
failure will not be output on the Event Output.
Atmel SAM L22G / L22J / L22N [DATASHEET]
Atmel-42402E-SAM L22G / L22J / L22N_Datasheet_Complete-07/2016
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