14-52
MPC561/MPC563 Reference Manual
MOTOROLA
Digital Subsystem
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.
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.
14.5.4.4.2 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.
14.5.4.4.3 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