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a combined data register, DACn_COMBDATA, where the data values for both channels can be written
simultaneously. Writing to this register will start all enabled channels.
If the PRSEN bit in DACn_CHxCTRL is set, a DAC conversion on channel x will not be started by data
write, but when a positive one HFPERCLK cycle pulse is received on the PRS input selected by PRSSEL
in DACn_CHxCTRL.
The CH0DV and CH1DV bits in DACn_STATUS indicate that the corresponding channel contains data
that has not yet been converted.
When entering Energy Modes 2,3 or 4, both DAC channels must be stopped. If the DAC is enabled for
the first time after entering Energy Mode 2,3 or 4 the output of the DAC will be undefined. This can be
worked around by enabling the DAC before entering a lower energy mode. The DAC channel can be
enabled and the data registers written to even though the output is disabled.
26.3.1.5 Clock Prescaling
The DAC has an internal clock prescaler, which can divide the HFPERCLK by any factor between 1 and
128, by setting the PRESC bits in DACnCTRL. The resulting DAC_CLK is used by the converter core
and the frequency is given by Equation 26.1 (p. 367) :
DAC Clock Prescaling
f DAC_CLK = f HFPERCLK / 2 ^ PRESC
(26.1)
where f HFPERCLK is the HFPERCLK frequency. One conversion takes 2 DAC_CLK cycles and the
DAC_CLK should not be set higher than 1 MHz.
Normally the PRESCALER runs continuously when either of the channels are enabled. When running
with a prescaler setting higher than 0, there will be an unpredictable delay from the time the conversion
was triggered to the time the actual conversion takes place. This is because the conversions is controlled
by the prescaled clock and the conversion can arrive at any time during a prescaled clock (DAC_CLK)
period. However, if the CH0PRESCRST bit in DACn_CTRL is set, the prescaler will be reset every time
a conversion is triggered on channel 0. This leads to a predictable latency between channel 0 trigger
and conversion.
26.3.2 Reference Selection
Three internal voltage references are available and are selected by setting the REFSEL bits in
DACn_CTRL:
? Internal 2.5V
? Internal 1.25V
? V DD
The reference selection can only be changed while both channels are disabled. The references for the
DAC need to be enabled for some time before they can be used. This is called the warm-up period, and
starts when one of the channels is enabled. For a bandgap reference, this period is 5 DAC_CLK cycles
while the V DD reference needs 1 DAC_CLK cycle. The DAC will time this period automatically(given that
the prescaler is set correctly) and delay any conversion triggers received during the warm-up until the
references have stabilized.
26.3.3 Programming of Bias Current
The bias current of the bandgap reference and the DAC output buffer can be scaled by the BIASPROG
and HALFBIAS bit fields of the DACn_BIASPROG register as illustrated in Figure 26.2 (p. 368) .
2011-04-12 - d0001_Rev1.10
367
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