Datasheet

136
Atmel | SMART SAM D20 [DATASHEET]
Atmel-42129K–SAM-D20_datasheet–06/2014
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. If the DFLLVAL.FINE value overflows or
underflows due to large drift in temperature and/or voltage, the DFLL Out Of Bounds bit (PCLKSR.DFLLOOB) in the
Power and Clocks 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
PCLKSR.DFLLOOB if the DFLL Out Of Bounds bit (INTENSET.DFLLOOB) in the Interrupt Enable Set register is set.
This interrupt will also be triggered if the tuner is not able to lock on the correct Coarse value.
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 2
17
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 PCLKSR.DFLLRCS if the
DFLL Reference Clock Stopped bit (INTENSET.DFLLRCS) in the Interrupt Enable Set register is set.
16.6.7.2 Additional Features
Dealing with Delay in the DFLL in Closed-Loop Mode
The time from selecting a new CLK_DFLL48M frequency until this frequency is output by the DFLL48M can be up to
several microseconds. If the value in DFLLMUL.MUL is small, this can lead to instability in the DFLL48M locking
mechanism, which can prevent the DFLL48M from achieving locks. To avoid this, a chill cycle, during which the
CLK_DFLL48M frequency is not measured, can be enabled. The chill cycle is enabled by default, but can be disabled by
writing a one to the DFLL Chill Cycle Disable bit (DFLLCTRL.CCDIS) in the DFLL Control register. Enabling chill cycles
might double the lock time.
Another solution to this problem consists of using less strict lock requirements. This is called Quick Lock (QL), which is
also enabled by default, but it can be disabled by writing a one to the Quick Lock Disable bit (DFLLCTRL.QLDIS) in the
DFLL Control register. The Quick Lock might lead to a larger spread in the output frequency than chill cycles, but the
average output frequency is the same.
Wake from Sleep Modes
DFLL48M can optionally reset its lock bits when it is disabled. This is configured by the Lose Lock After Wake bit
(DFLLCTRL.LLAW) in the DFLL Control register. If DFLLCTRL.LLAW is zero, the DFLL48M will be re-enabled and start
running with the same configuration as before being disabled, even if the reference clock is not available. The locks will
not be lost. When the reference clock has restarted, the Fine tracking will quickly compensate for any frequency drift
during sleep if DFLLCTRL.STABLE is zero. If DFLLCTRL.LLAW is one when the DFLL is turned off, the DFLL48M will
lose all its locks, and needs to regain these through the full lock sequence.
Accuracy
There are three main factors that determine the accuracy of F
clkdfll48m
. These can be tuned to obtain maximum accuracy
when fine lock is achieved.
z Fine resolution: The frequency step between two Fine values. This is relatively smaller for high output frequencies.
z Resolution of the measurement: If the resolution of the measured F
clkdfll48m
is low, i.e., the ratio between the
CLK_DFLL48M frequency and the CLK_DFLL48M_REF frequency is small, then the DFLL48M 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.
z The accuracy of the reference clock.
16.6.8 3.3V Brown-Out Detector Operation
The 3.3V BOD monitors the 3.3V VDDANA supply (BOD33). It supports continuous or sampling modes.