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23.3.4 Conversions
The VDAC consists of two channels (channel 0 and 1) with separate 12-bit data registers (VDACn_CH0DATA and VDACn_CH1DATA).
These can be used to produce two independent single ended outputs or the channel 0 register can be used to drive both outputs in
differential mode. The VDAC supports two conversion modes: continuous and sample/off.
23.3.4.1 Continuous Mode
In continuous mode the VDAC channels will drive their outputs continuously with the data in the VDACn_CHxDATA registers. A channel
is configured in continuous mode by programming the CONVMODE bitfield in VDACn_CHxCTRL to CONTINUOUS. This mode will
maintain the output voltage and no manual refresh is needed.
In continuous mode the SETTLETIME field in VDACn_OPAxTIMER should be programmed to zero to achive the maximum update rate.
23.3.4.2 Sample/Off Mode
In sample/off mode the VDAC will only drive the output for a limited time per conversion. A channel is configured in sample/off mode by
programming the CONVMODE bitfield in VDACn_CHxCTRL to SAMPLEOFF. How long the channel should drive the output can be
controlled by programming the SETTLETIME field in the VDACn_OPAxTIMER register. The VDAC will drive the output for SETTLE-
TIME f
DAC_CLK
cycles before tristating the output again (and therefore if SETTLETIME is set to zero, the output will never be driven
when using sample/off mode).
23.3.4.3 Conversion Trigger
Conversions can only be done while a channel is enabled, see
23.3.3 Enabling and Disabling a Channel
.
If TRIGMODE is programmed to SW, SWPRS or SWREFRESH a conversion can be started by writing to the VDACn_CHxDATA regis-
ter. The data registers are also mapped to a combined data register, VDACn_COMBDATA, where the data values for both channels can
be written simultaneously. Writing to this register will trigger all enabled channels.
If TRIGMODE is programmed to PRS or SWPRS, a conversion can be started by an incoming pulse on the PRS channel selected in
PRSSEL in VDACn_CHxCTRL. The PRSASYNC bit in VDACn_CHxCTRL determines if the VDAC expects a PRS pulse coming from a
synchronous or asynchronous PRS producer.
If TRIGMODE is programmed to REFRESH or SWREFRESH a conversion will start on an overflow of the internal refresh timer. See
If TRIGMODE is programmed to LESENSE a conversion will start when the LESENSE block sends a request. This setting needs to be
selected whenever the channel is under LESENE control.
23.3.4.4 PRS Triggers
PRS triggers can be used to set a constant sample frequency, for instance by using a TIMER. In order to get a jitter-free sample rate,
set DACCLKMODE to SYNC, set the CH0PRESCRST bit and clear the PRSASYNC bit. Note that this is only possible for channel 0.
The PRSASYNC bit tells whether the VDAC expects a synchronous PRS producer or not. When this bit is cleared, the PRS pulse must
come from a synchronous producer and HFPERCLK must be running (this clock is turned off in EM2 and below). When PRSASYNC is
set, the corresponding PRS channels should also bet configued as asynchronous (see the PRS chapter).
When either DACCLKMODE is set to ASYNC or the PRSASYNC bit is set, the sample frequency cannot be guaranteed to be jitter-free
with respect to the PRS pulses.
The PRS frequency should never be higher than 0.5 MHz (the fastest possible sample rate). In addition the PRS frequency should not
be higher than f
HFPERCLK
/12 (in synchronous mode). If the PRS frequency is set too high, some PRS pulses will be dropped and the
output can jitter.
23.3.5 Reference Selection
These voltage references are available and are selected by programming the REFSEL field in VDACn_CTRL.
• Internal 1.25 V Low Noise Bandgap Reference
• Internal 2.5 V Low Noise Bandgap Reference
• Internal 1.25 V Low Power Bandgap Reference
• Internal 2.5 V Low Power Bandgap Reference
• AVDD
Reference Manual
VDAC - Digital to Analog Converter
silabs.com
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