
MOTOROLA
7-8
DIGITAL CONTROL
QADC
REFERENCE MANUAL
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 multi-
ple 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.
NOTE
It may not be possible to guarantee coherent samples since the rela-
tionship between any conversion in time and any other conversion
may be variable (due to programmable trigger events, queue priori-
ties, and so on).
7.3.3.1 Software Initiated Continuous-Scan Mode
When the software initiated continuous-scan mode is programmed, the trigger event
is generated automatically by the QADC. Queue execution begins immediately. If a
pause is encountered, queue execution ceases for two QCLKs, while another trigger
event is generated internally, and then execution continues. 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 paused or 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 QADC without software involvement. Software can
read a result value at any time.
NOTE
It may not be possible to guarantee coherent samples since the rela-
tionship between any conversion in time and any other conversion
may be variable (due to programmable trigger events, queue priori-
ties, and so on).
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 pause and
the internally generated trigger event, or between a queue 1 completion and the sub-
sequent 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 con-
verting 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
F
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