Motorola MPC533 Reference Manual page 490

Digital Subsystem
The software initiated continuous-scan mode is a useful choice with queue 2 for converting
channels that do not need to be synchronized to anything, or for the slow-to-change analog
channels. Interrupts are normally not used with the software initiated continuous-scan
mode. Rather, the software reads the latest conversion result from the result table at any
time. Once initiated, software action is not needed to sustain conversions of channel.
13.5.4.4.2 External Trigger Continuous-Scan Mode
The QADC64E provides external trigger signals for both queues. When the external trigger
software initiated continuous-scan mode is selected, a transition on the associated external
trigger signal initiates queue execution. The polarity of the external trigger signal is
programmable, so that the software can select a mode which begins queue execution on the
rising or falling edge. Each CCW is read and the indicated conversions are performed until
an end-of-queue condition is encountered. When the next external trigger edge is detected,
the queue execution begins again automatically. Software initialization is not needed
between trigger events.
When a pause bit is encountered in external trigger continuous-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.
Some applications need to synchronize the sampling of analog channels to external events.
There are cases when it is not possible to use software initiation of the queue scan sequence,
since interrupt response times vary.
13.5.4.4.3 External Gated Continuous-Scan Mode
The QADC64E provides external gating for queue 1 only. When external gated
continuous-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
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. When the gate
opens again, the queue execution automatically begins again from the beginning of the
queue. Software initialization is not needed between trigger events. If a pause in a CCW is
encountered, the pause flag will not set, and execution continues without pausing.
The purpose of external gated continuous-scan mode is to continuously collect digitized
samples while the gate is open and to have the most recent samples available. It is up to the
programmer to ensure that the queue is large enough so that a maximum gate open time will
not reach an end-of-queue. However it is useful to take advantage of a smaller queue in the
manner described in the next paragraph.
In the event that the queue completes before the gate closes, a completion flag will be set
and the queue will roll over to the beginning and continue conversions until the gate closes.
If the gate remains open and the completion flag is not cleared, when the queue completes
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PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE