Datasheet

135
Atmel | SMART SAM D20 [DATASHEET]
Atmel-42129K–SAM-D20_datasheet–06/2014
Closed-Loop Operation
In closed-loop operation, the output frequency is continuously regulated against a reference clock. Once the
multiplication factor is set, the oscillator fine tuning is automatically adjusted. The DFLL48M must be correctly configured
before closed-loop operation can be enabled. After enabling the DFLL48M, it must be configured in the following way:
1. Enable and select a reference clock (CLK_DFLL48M_REF). CLK_DFLL48M_REF is Generic Clock Channel 0
(DFLL48M_Reference). Refer to “GCLK – Generic Clock Controller” on page 79 for details.
2. Select the maximum step size allowed in finding the Coarse and Fine values by writing the appropriate values to
the DFLL Coarse Maximum Step and DFLL Fine Maximum Step bit groups (DFLLMUL.CSTEP and DFLL-
MUL.FSTEP) in the DFLL Multiplier register. A small step size will ensure low overshoot on the output frequency,
but will typically result in longer lock times. A high value might give a large overshoot, but will typically provide
faster locking. DFLLMUL.CSTEP and DFLLMUL.FSTEP should not be higher than 50% of the maximum value of
DFLLVAL.COARSE and DFLLVAL.FINE, respectively.
3. Select the multiplication factor in the DFLL Multiply Factor bit group (DFLLMUL.MUL) in the DFLL Multiplier regis-
ter. Care must be taken when choosing DFLLMUL.MUL so that the output frequency does not exceed the
maximum frequency of the DFLL. If the target frequency is below the minimum frequency of the DFLL48M, the out-
put frequency will be equal to the DFLL minimum frequency.
4. Start the closed loop mode by writing a one to the DFLL Mode Selection bit (DFLLCTRL.MODE) in the DFLL Con-
trol register.
The frequency of CLK_DFLL48M (F
clkdfll48m
) is given by:
where F
clkdfll48mref
is the frequency of the reference clock (CLK_DFLL48M_REF). DFLLVAL.COARSE and
DFLLVAL.FINE are read-only in closed-loop mode, and are controlled by the frequency tuner to meet user specified
frequency. In closed-loop mode, the value in DFLLVAL.COARSE is used by the frequency tuner as a starting point for
Coarse. Writing DFLLVAL.COARSE to a value close to the final value before entering closed-loop mode will reduce the
time needed to get a lock on Coarse.
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” on page 563.
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:
F
clkdfll48m
DFLLMUL MUL F
clkdfll48mref
×⋅=
ERROR
DIFF
MUL
--------------
=