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In continuous-scan modes, all samples are guaranteed to be
taken during one pass through the queue (coherently), except
when a queue 1 trigger event halts queue 2 execution. The time
between consecutive conversions has been designed to be
consistent. However, for queues that end with a CCW
containing the EOQ code (channel 63), the time between the
last queue conversion and the first queue conversion requires
one additional CCW fetch cycle. Continuous samples are not
coherent at this boundary.
In addition, the time from trigger to first conversion cannot be
guaranteed, because it is a function of clock synchronization,
programmable trigger events, queue priorities, and so on.
27.8.7.1 Software-Initiated Continuous-Scan Mode
When software-initiated continuous-scan mode is selected, the trigger event is generated
automatically by the QADC. Queue execution begins immediately. If a pause is
encountered, another trigger event is generated internally, and execution continues without
pausing. When the end-of-queue is reached, another internal trigger event is generated and
queue execution restarts at the beginning of the queue.
While the time to internally generate and act on a trigger event is very short, the queue
status field can be read as momentarily indicating that the queue is idle. The trigger overrun
flag is never set while in software-initiated continuous-scan mode.
The software-initiated continuous-scan mode keeps the result registers updated more
frequently than any of the other queue operating modes. The result table can always be read
to get the latest converted value for each channel. The channels scanned are kept up to date
by the QADC without software involvement.
The software-initiated continuous-scan mode may be chosen for either queue, but is
normally used only with queue 2. When software-initiated continuous-scan mode is chosen
for queue 1, that queue operates continuously and queue 2, being lower in priority, never
gets executed. The short interval of time between a queue 1 completion and the subsequent
trigger event is not sufficient to allow queue 2 execution to begin.
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 slow-to-change analog
channels. Interrupts are normally not used with the software-initiated continuous-scan
mode. Rather, the latest conversion results can be read from the result table at any time.
Once initiated, software action is not needed to sustain conversions of channel.
MOTOROLA
Chapter 27. Queued Analog-to-Digital Converter (QADC)
NOTE
Digital Control Subsystem
27-55
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