Motorola MPC533 Reference Manual page 570

Digital Subsystem
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.
14.5.4.3.4 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
the timer interval elapses again, and then queue execution continues. When the queue
execution reaches an end-of-queue situation, the single-scan enable bit is cleared. Software
may set the single-scan enable bit again, allowing another scan of the queue to be initiated
by the periodic/interval timer.
The periodic/interval timer generates a trigger event whenever the time interval elapses.
The trigger event may cause the queue execution to continue following a pause, or may be
considered a trigger overrun. Once the queue execution is completed, the single-scan
enable bit must be set again to enable the timer to count again.
Normally only one queue will be enabled for periodic/interval timer single-scan mode and
the timer will reset at the end-of-queue. However, if both queues are enabled for either
single-scan or continuous periodic/interval timer mode, the end-of-queue condition will not
reset the timer while the other queue is active. In this case, the timer will reset when both
queues have reached end-of-queue. See Section 14.5.6, "Periodic/Interval Timer" for a
definition of periodic/interval timer reset conditions.
The periodic/interval timer single-scan mode can be used in applications which need
coherent results, for example:
• When it is necessary that all samples are guaranteed to be taken during the same scan
of the analog signals
• When the interrupt rate in the periodic/interval timer continuous-scan mode would
be too high
• In sensitive battery applications, where the single-scan mode uses less power than
the software initiated continuous-scan mode
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