Motorola MPC533 Reference Manual page 486

Digital Subsystem
immediately. In the other single-scan queue operating modes, once the single-scan enable
bit is written, the selected trigger event must occur before the queue can start. The
single-scan enable bit allows the entire queue to be scanned once. A trigger overrun is
captured if a trigger event occurs during queue execution in an edge-sensitive external
trigger mode or a periodic/interval timer mode.
In the periodic/interval timer single-scan mode, the next expiration of the timer is the
trigger event for the queue. After the queue execution is complete, the queue status is shown
as idle. The software can restart the queue by setting the single-scan enable bit to a one.
Queue execution begins with the first CCW in the queue.
13.5.4.3.1 Software Initiated Single-Scan Mode
Software can initiate the execution of a scan sequence for queue 1 or 2 by selecting the
software initiated single-scan mode, and writing the single-scan enable bit in QACR1 or
QACR2. A trigger event is generated internally and the QADC64E immediately begins
execution of the first CCW in the queue. If a pause occurs, another trigger event is
generated internally, and then execution continues without pausing.
The QADC64E automatically performs the conversions in the queue until an end-of-queue
condition is encountered. The queue remains idle until the software again sets the
single-scan enable bit. While the time to internally generate and act on a trigger event is
very short, software can momentarily read the status conditions, indicating that the queue
is paused. The trigger overrun flag is never set while in the software initiated single-scan
mode.
The software initiated single-scan mode is useful in the following applications:
• Allows software complete control of the queue execution
• Allows the software to easily alternate between several queue sequences.
13.5.4.3.2 External Trigger Single-Scan Mode
The external trigger single-scan mode is available on both queue 1 and queue 2. The
software programs the polarity of the external trigger edge that is to be detected, either a
rising or a falling edge. The software must enable the scan to occur by setting the
single-scan enable bit for the queue.
The first external trigger edge causes the queue to be executed one time. Each CCW is read
and the indicated conversions are performed until an end-of-queue condition is
encountered. After the queue is completed, the QADC64E clears the single-scan enable bit.
Software may set the single-scan enable bit again to allow another scan of the queue to be
initiated by the next external trigger edge.
The external trigger single-scan mode is useful when the input trigger rate can exceed the
queue execution rate. Analog samples can be taken in sync with an external event, even
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PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
MPC533 Reference Manual MOTOROLA