Datasheet
3. Start DFLL close loop
This procedure will reduce DFLL Lock time to DFLL Fine lock time.
Related Links
15. GCLK - Generic Clock Controller
10.3.2 NVM Software Calibration Area Mapping
17.6.7.1.3 Frequency Locking
The locking of the frequency in closed-loop mode is divided into two stages. In the first, coarse stage, the
control logic quickly finds the correct value for DFLLVAL.COARSE and sets the output frequency to a
value close to the correct frequency. On coarse lock, the DFLL Locked on Coarse Value bit
(PCLKSR.DFLLLOCKC) in the Power and Clocks Status register will be set.
In the second, fine stage, the control logic tunes the value in DFLLVAL.FINE so that the output frequency
is very close to the desired frequency. On fine lock, the DFLL Locked on Fine Value bit
(PCLKSR.DFLLLOCKF) in the Power and Clocks Status register will be set.
Interrupts are generated by both PCLKSR.DFLLLOCKC and PCLKSR.DFLLLOCKF if
INTENSET.DFLLOCKC or INTENSET.DFLLOCKF are written to one.
CLK_DFLL48M is ready to be used when the DFLL Ready bit (PCLKSR.DFLLRDY) in the Power and
Clocks Status register is set, but the accuracy of the output frequency depends on which locks are set.
For lock times, refer to the Electrical Characteristics.
Related Links
37. Electrical Characteristics
17.6.7.1.4 Frequency Error Measurement
The ratio between CLK_DFLL48M_REF and CLK48M_DFLL is measured automatically when the
DFLL48M is in closed-loop mode. The difference between this ratio and the value in DFLLMUL.MUL is
stored in the DFLL Multiplication Ratio Difference bit group(DFLLVAL.DIFF) in the DFLL Value register.
The relative error on CLK_DFLL48M compared to the target frequency is calculated as follows:
ERROR =
DIFF
MUL
17.6.7.1.5 Drift Compensation
If the Stable DFLL Frequency bit (DFLLCTRL.STABLE) in the DFLL Control register is zero, the
frequency tuner will automatically compensate for drift in the CLK_DFLL48M without losing either of the
locks. This means that DFLLVAL.FINE can change after every measurement of CLK_DFLL48M.
The DFLLVAL.FINE value overflows or underflows can occur in close loop mode when the clock source
reference drifts or is unstable. This will set the DFLL Out Of Bounds bit (PCLKSR.DFLLOOB) in the
Power and Clocks Status register.
To avoid this error, the reference clock in close loop mode must be stable, an external oscillator is
recommended and internal oscillator forbidden. The better choice is to use an XOSC32K.
17.6.7.1.6 Reference Clock Stop Detection
If CLK_DFLL48M_REF stops or is running at a very low frequency (slower than CLK_DFLL48M/(2 *
MUL
MAX
)), the DFLL Reference Clock Stopped bit (PCLKSR.DFLLRCS) in the Power and Clocks Status
register will be set. Detecting a stopped reference clock can take a long time, on the order of 217
CLK_DFLL48M cycles. When the reference clock is stopped, the DFLL48M will operate as if in open-loop
mode. Closed-loop mode operation will automatically resume if the CLK_DFLL48M_REF is restarted. An
SAM D21 Family
SYSCTRL – System Controller
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Datasheet Complete
DS40001882D-page 182