Motorola MPC533 Reference Manual page 489

operating modes, the selected trigger event must occur before the queue can start. A trigger
overrun is captured if a trigger event occurs during queue execution in the external trigger
continuous-scan mode and the periodic/interval timer continuous-scan mode.
After the queue execution is complete, the queue status is shown as idle. Since the
continuous-scan queue operating modes allow the entire queue to be scanned multiple
times, software involvement is not needed to enable queue execution to continue from the
idle state. The next trigger event causes queue execution to begin again, starting with the
first CCW in the queue.
Coherent samples are guaranteed. The time between
consecutive conversions has been designed to be consistent.
However, there is one exception. For queues that end with a
CCW containing EOQ code (channel 63), the last queue
conversion to the first queue conversion requires 1 additional
CCW fetch cycle. Therefore continuous samples are not
coherent at this boundary.
In addition, the time from trigger to first conversion cannot be guaranteed since it is a
function of clock synchronization, programmable trigger events, queue priorities, and so
on.
13.5.4.4.1 Software Initiated Continuous-Scan Mode
When the software initiated continuous-scan mode is programmed, the trigger event is
generated automatically by the QADC64E. Queue execution begins immediately. If a pause
is encountered, another trigger event is generated internally, and then execution continues
without pausing. When the end-of-queue is reached, another internal trigger event is
generated, and queue execution begins again from the beginning of the queue.
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 idle. The trigger
overrun flag is never set while in the 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 software can always read the
result table to get the latest converted value for each channel. The channels scanned are kept
up to date by the QADC64E without software involvement. Software can read a result value
at any time.
The software initiated continuous-scan mode may be chosen for either queue, but is
normally used only with queue 2. When the 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.
MOTOROLA Chapter 13. Queued Analog-to-Digital Converter Legacy Mode Operation
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
NOTE
Digital Subsystem
13-45