Atmel SAM4L Xplained Pro Evaluation Kit Atmel ATSAM4L-XPRO ATSAM4L-XPRO Data Sheet

42023E–SAM–07/2013

DFLLxCONF.LLAW is one when going to a Power Save Mode where the DFLLx is turned off,
the DFLLx will lose all its locks when waking up, and needs to regain these through the full lock
sequence.

There are mainly three factors that decide the accuracy of the f

CLK_DFLLx

. These can be tuned to

obtain maximum accuracy when fine lock is achieved.

• FINE resolution: The frequency step between two FINE values. This is relatively smaller for 

high output frequencies.

• Resolution of the measurement: If the resolution of the measured f

CLK_DFLLx

 is low, i.e. the 

ratio between the CLK_DFLLx frequency and the CLK_DFLLx_REF frequency is small, then 
the DFLLx might lock at a frequency that is lower than the targeted frequency. It is 
recommended to use a reference clock frequency of 32KHz or lower to avoid this issue for 
low target frequencies.

• The accuracy of the reference clock.

Rev: 1.1.4.0

The system RC oscillator has a startup time of three cycles, and is always available except in
some sleep modes. Refer to 

 for details. The

system RC oscillator operates at a nominal frequency of 115 kHz, and is calibrated using the
Calibration Value field (CALIB) in the RC Oscillator Calibration Register (RCCR). After a Power-
on Reset (POR), the RCCR.CALIB field is loaded with a factory defined value stored in the Flash
fuses. Refer to the Fuse setting chapter for more details about RCCR fuses and how to program
the fuses.

If the Flash Calibration Done (FCD) bit in the RCCR is zero at any reset, the flash calibration will
be redone and the RCCR.FCD bit will be set before program execution starts in the CPU. If the
RCCR.FCD is one, the flash calibration will only be redone after a Power-on Reset.

To prevent unexpected writes to RCCR due to software bugs, write access to this register is pro-
tected by a locking mechanism. For details refer to 

.

Although it is not recommended to override default factory settings, it is still possible to override
the default values by writing to RCCR.CALIB.

Rev: 2.0.02

The RCFAST can be used as the main clock in the device, as described in 

. The RCFAST can also be used as source for the generic clocks,

as described in the “Generic Clocks” section of the SCIF. 

The RCFAST is enabled by writing a one to the Oscillator Enable bit in the RCFAST Configura-
tion Register (RCFASTCFG.EN), and disabled by writing a zero to this bit. When enabling the
RCFAST, RCFASTCFG.EN must be read back until it reads one. The user must ensure that the
RCFAST is fully disabled before enabling, and that the RCFAST is fully enabled before disabling
by reading back RCFASTCFG.EN.