Chapter 2
Theory of Operation
2-22
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National Instruments Corporation
can sustain for waveform generation. It can also be used to store a complete
waveform which can be output repetitively without any further data transfer
to the FIFO.
Analog Output Timing Circuitry
This section describes the different methods of setting the analog output
voltage, including single-point updating and waveform generation. The
DAQ-STC and DAQ-PnP provide the timing signals necessary to write to
the DACs and update them.
The DACs are double buffered and can be individually configured for
immediate update or timed update mode. In immediate update mode, the
double buffering of the DAC is disabled and the value written to the DAC
will appear immediately on the output after the write completes. In the
timed update mode, the double buffering is enabled. When double
buffering is enabled, writing the digital value loads that value into the buffer
stage of the DAC. When an update signal is received, the analog output of
the DAC changes.
Single-Point Output
The data values can be written directly to the DACs under software control
without the use of the timing engine provided in the DAQ-STC. This is
typically useful for setting the analog outputs to DC levels, where precise
timing of the output change is not important. Writing directly to the DACs
is accomplished by writing the desired value to the DAC<0..1> Direct Data
Register. This action will store the value in the first buffer of the DAC. If
the DAQ-STC is programmed for immediate updating mode, the value will
also be applied to the analog output. If the DAQ-STC is programmed for
timed updating mode, the appropriate update signal, LDAC0* or LDAC1*,
must be asserted by writing to the appropriate DAQ-STC register.
Waveform Generation
The timing engine in the DAQ-STC can be used for waveform generation,
where the update signals connected to the DACs can be generated at precise
intervals. In this model, the DAQ-STC will generate the update signals and
then transfer the data values for the next update to the first buffer of
the DACs.
The update interval counter (UI) is 24 bits wide and generates the update
pulses. These update pulses can be routed to the DACs. The update counter
(UC) is 24 bits wide and counts the number of updates. The value loaded
into this counter defines a buffer. The buffer counter (BC) determines how
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