Motorola MPC533 Reference Manual page 488

Digital Subsystem
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 13.5.6, "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
13.5.4.4 Continuous-Scan Modes
When the application software wants to execute multiple passes through a sequence of
conversions defined by a queue, a continuous-scan queue operating mode is selected. By
programming the MQ1 field in QACR1 or the MQ2 field in QACR2, the following
software initiated modes can be selected:
• Software initiated continuous-scan mode
• External trigger continuous-scan mode
• External gated continuous-scan mode
• Periodic/interval timer continuous-scan mode
When a queue is programmed for a continuous-scan mode, the single-scan enable bit in the
queue control register does not have any meaning or effect. As soon as the queue operating
mode is programmed, the selected trigger event can initiate queue execution.
In the case of the software-initiated continuous-scan mode, the trigger event is generated
internally and queue execution begins immediately. In the other continuous-scan queue
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