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Digital Subsystem
though the software is not interested in data taken from every edge. The software can start
the external trigger single-scan mode and get one set of data, and at a later time, start the
queue again for the next set of samples.
When a pause bit is encountered during external trigger single-scan mode, another trigger
event is required for queue execution to continue. Software involvement is not needed to
enable queue execution to continue from the paused state.
13.5.4.3.3 External Gated Single-Scan Mode
The QADC64E provides external gating for queue 1 only. When external gated single-scan
mode is selected, the input level on the associated external trigger signal enables and
disables queue execution. The polarity of the external gated signal is fixed so only a high
level opens the gate and a low level closes the gate. Once the gate is open, each CCW is
read and the indicated conversions are performed until the gate is closed. Software must
enable the scan to occur by setting the single-scan enable bit for queue 1. If a pause in a
CCW is encountered, the pause flag will not set, and execution continues without pausing.
While the gate is open, queue 1 executes one time. Each CCW is read and the indicated
conversions are performed until an end-of-queue condition is encountered. When queue 1
completes, the QADC64E sets the completion flag (CF1) and clears the single-scan enable
bit. Software may set the single-scan enable bit again to allow another scan of queue 1 to
be initiated during the next open gate.
If the gate closes before queue 1 completes execution, the current CCW completes,
execution of queue 1 stops, the single-scan enable bit is cleared, and the PF1 bit is set.
Software can read the CWPQ1 to determine the last valid conversion in the queue. Software
must set the single-scan enable bit again and should clear the PF1 bit before another scan
of queue 1 is initiated during the next open gate. The start of queue 1 is always the first
CCW in the CCW table.
Since the condition of the gate is only sampled after each conversion during queue
execution, closing the gate for a period less than a conversion time interval does not
guarantee the closure will be captured.
13.5.4.3.4 Periodic/Interval Timer Single-Scan Mode
Both queues can use the periodic/interval timer in a single-scan queue operating mode. The
timer interval can range from 128- to 128-Kbyte QCLK cycles in binary multiples. When
the periodic/ interval timer single-scan mode is selected and the software sets the
single-scan enable bit in QACR1 or QACR2, the timer begins counting. When the time
interval elapses, an internal trigger event is created to start the queue and the QADC64E
begins execution with the first CCW.
The QADC64E automatically performs the conversions in the queue until a pause or an
end-of-queue condition is encountered. When a pause occurs, queue execution stops until
MOTOROLA Chapter 13. Queued Analog-to-Digital Converter Legacy Mode Operation
13-43
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
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