Prescaler
The CFD has an internal configurable prescaler to generate the safe clock from the OSC16M oscillator.
The prescaler size allows to scale down the OSC16M oscillator so the safe clock frequency is not higher
than the XOSC clock frequency monitored by the CFD. The division factor is 2^P, with P being the value
of the CFD Prescaler bits in the CFD Prescaler Register (CFDPRESC.CFDPRESC).
Example
For an external crystal oscillator at 0.4MHz and the OSC16M frequency at 16MHz, the
CFDPRESC.CFDPRESC value should be set scale down by more than factor 16/0.4=80,
e.g. to 128, for a safe clock of adequate 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.
Sleep Mode
The CFD is halted depending on configuration of the XOSC and the peripheral clock request. For further
details, refer to the Sleep Behavior table above. The CFD interrupt can be used to wake up the device
from sleep modes.
21.6.4. 16MHz Internal Oscillator (OSC16M) Operation
The OSC16M is an internal oscillator operating in open-loop mode and generating 4, 8, 12, or 16MHz
frequency. The OSC16M frequency is selected by writing to the Frequency Select field in the OSC16M
register (OSC16MCTRL.FSEL). OSC16M is enabled by writing '1' to the Oscillator Enable bit in the
OSC16M Control register (OSC16MCTRL.ENABLE), and disabled by writing a '0' to this bit. Frequency
selection must be done when OSC16M is disabled.
After enabling OSC16M, the OSC16M clock is output as soon as the oscillator is ready
(STATUS.OSC16MRDY=1). User must ensure that the OSC16M is fully disabled before enabling it by
reading STATUS.OSC16MRDY=0.
After reset, OSC16M is enabled and serves as the default clock source at 4MHz.
OSC16M will behave differently in different sleep modes based on the settings of
OSC16MCTRL.RUNSTDBY, OSC16MCTRL.ONDEMAND, and OSC16MCTRL.ENABLE. If
OSC16MCTRL.ENABLE=0, the OSC16M will be always stopped. For OSC16MCTRL.ENABLE=1, this
table is valid:
Table 21-2. OSC16M Sleep Behavior
CPU Mode
OSC16MCTRL.RUN
STDBY
OSC16MCTRL.OND
EMAND
Sleep Behavior
Active or Idle
-
0
Always run
Active or Idle
-
1
Run if requested by peripheral
Standby
1
0
Always run
Standby
1
1
Run if requested by peripheral
Standby
0
-
Run if requested by peripheral
Atmel SAM L22G / L22J / L22N [DATASHEET]
Atmel-42402E-SAM L22G / L22J / L22N_Datasheet_Complete-07/2016
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