SAM4L Datasheet, PDF (207/1185 Pages) ATMEL Corporation – ATSAM ARM-based Flash MCU

ATSAM4L4/L2
After a Power-On Reset (POR), the Calibration field (RCFASTCFG.CALIB) is loaded with a fac-
tory-defined value stored in the Flash fuses. The Flash Calibration Done bit (RCFASTCFG.FCD)
is set when RCFASTCFG.CALIB has been loaded from flash.
For testing purposes, it is possible to override the default fuse values by writing to the
RCFASTCFG.CALIB field. To prevent unexpected frequency change, the RCFAST must be dis-
abled before modifying the CALIB field.
To prevent unexpected writes due to software bugs, write access to the RCFASTCFG register is
protected by a locking mechanism. For details refer to Section 13.7.7 ”Unlock Register” on page
222. The RCFAST is automatically switched off in certain sleep modes to reduce power con-
sumption, as described in Section 10. ”Power Manager (PM)” on page 102.
The RCFAST is an internal oscillator that can, when operating in open loop mode, output a 4-, 8-
or 12-MHz frequency, factory-calibrated under typical voltage and temperature. Alternatively, the
frequency can be further tuned to a multiplication ratio relative to the 32 kHz reference clock.
Figure 13-7. RCFAST Block Diagram
CALIB
0
1
4/8/12MHz
TRIMMABLE
RC OSCILLATOR
RCFAST
Clock output
TUNEEN
FRANGE
COUNT
COMPARATOR
COUNTER
32kHz
Clock Input
13.6.5.1
13.6.5.2
13.6.5.3
LOCK
LOCKLOST
JITMODE
LOCKMARGIN
NBPERIODS
Product Dependencies
The tuner uses a 32 kHz clock as reference clock and should only be enabled when the 32 kHz
clock is running. Refer to the 32 kHz Clock section for details.
General Use
The RCFAST consists of two parts, one oscillator and one tuner. The oscillator is enabled by
writing a one to the Oscillator Enable bit (EN) in the 4/8/12MHz RC Oscillator Configuration Reg-
ister (RCFASTCFG). The tuner is enabled by writing a one to the Tuner Enable bit (TUNEEN) in
RCFASTCFG. The oscillator has to be enabled for the tuner to work.
Open Loop Mode
If the tuner is disabled (RCFASTCFG.TUNEEN is zero), the RCFAST is in open loop mode. In
open loop mode, the user can control the oscillation frequency by writing to the Frequency
Range (FRANGE) and Calibration (CALIB) fields of the 4/8/12MHz RC Oscillator Configuration
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