28.6.4.1 Basic Operation

Operating modes

The DFLL48M will behave differently in different sleep modes based on the settings of DFLLCTRLA.RUNSTDBY, DFLLCTRLA.ONDEMAND and DFLLCTRLA.ENABLE, as shown in the following table.

The DFLL48M is used as a clock source for the generic clock generators, as described in the GCLK chapter.

The DFLL48M is factory-calibrated to provide 48MHz in open loop mode. Registers DFLLVAL.COARSE and DFLLVAL.FINE store the factory frequency calibration values after reset.

Open-Loop Operation

After any reset, the open-loop mode is selected. When operating in open-loop mode, the output frequency of the DFLL48M will be determined by the values written to the DFLL Coarse Value bit group and the DFLL Fine Value bit group (DFLLVAL.COARSE and DFLLVAL.FINE) in the DFLL Value register.

It is possible to change the values of DFLLVAL.COARSE and DFLLVAL.FINE and thereby the output frequency of the DFLL48M output clock, CLK_DFLL48M, while the DFLL48M is enabled and in use. CLK_DFLL48M is ready to be used when STATUS.DFLLRDY is set after enabling the DFLL48M.

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. The output of Peripheral Channel 0 provides the input reference clock to DFLL48M (GCLK_DFLL48M_REF). Set GCLK.PCHCTRL0.GEN = 0 to select Generic Clock Channel 0 output as the input to this peripheral channel. Select the clock input to GCLK0 via GCLK.GENCTRL0.SRC
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 DFLLMUL. 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 register. Care must be taken when choosing DFLLMUL.MUL so that the output frequency does not exceed the maximum frequency of the device.
4. Start the closed loop mode by writing a one to the DFLL Mode Selection bit (DFLLCTRLA.MODE) in the DFLL Control register.

The frequency of CLK_DFLL48M (Fclkdfll48m) is given by:

where Fclkdfll48mref 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 (STATUS.DFLLLCKC) in the 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 (STATUS.DFLLLCKF) in the Status register will be set.

If the the ByPass Lock bit (DFLLCTRLB.BPLCKC) in the DFLL Control register is set, the coarse stage is by-passed, the DFLLVAL.COARSE keeps it’s value and the DFLL Coarse Value bit (STATUS.DFLLLCKC) is immediately set.

Interrupts are generated by both STATUS.DFLLLCKC and STATUS.DFLLLCKF if INTENSET.DFLLCKC or INTENSET.DFLLCKF are written to '1'.

CLK_DFLL48M is ready to be used when the DFLL Ready bit (STATUS.DFLLRDY) in the 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.

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:

Drift Compensation

If the Stable DFLL Frequency bit (DFLLCTRLB.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. If the DFLLVAL.FINE value overflows or underflows due to large drift in temperature and/or voltage, the DFLL Out Of Bounds bit (STATUS.DFLLOOB) in the Status register will be set. After an Out of Bounds error condition, the user must rewrite DFLLMUL.MUL to ensure correct CLK_DFLL48M frequency. An interrupt is generated on a zero-to-one transition on STATUS.DFLLOOB if the DFLL Out Of Bounds bit (INTENSET.DFLLOOB) in the Interrupt Enable Set register is set. This interrupt will also be set if the tuner is not able to lock on the correct Coarse value. If the Stable DFLL Frequency bit (DFLLCTRLB.STABLE) in the DFLL Control register is one, the DFLLVAL.COARSE and DFLLVAL.FINE values will stay constant after the lock. The user can check for a possible drift by reading the frequency error in the DFLL Multiplication Ratio Difference bit group (DFLLVAL.DIFF).

Reference Clock Stop Detection

If CLK_DFLL48M_REF stops or is running at a very low frequency (slower than CLK_DFLL48M/(2 * MULMAX)), the DFLL Reference Clock Stopped bit (STATUS.DFLLRCS) in the 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 interrupt is generated on a zero-to-one transition on STATUS.DFLLRCS if the DFLL Reference Clock Stopped bit (INTENSET.DFLLRCS) in the Interrupt Enable Set register is set.