MC68336/376
QUEUED ANALOG-TO-DIGITAL CONVERTER MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
8-22
When a queue is programmed for a continuous-scan mode, the single-scan enable bit
in the queue control register does not have any meaning or effect. As soon as the
queue operating mode is programmed, the selected trigger event can initiate queue
execution.
In the case of the software initiated continuous-scan mode, the trigger event is gener-
ated internally and queue execution begins immediately. In the other continuous-scan
queue operating modes, the selected trigger event must occur before the queue can
start. A trigger overrun is recorded if a trigger event occurs during queue execution in
the external trigger continuous-scan mode and the periodic timer continuous-scan
mode. When a pause is encountered during a scan, another trigger event is required
for queue execution to continue. Software involvement is not required for queue
execution to continue from the paused state.
After queue execution is complete, the queue status is shown as idle. Since the con-
tinuous-scan queue operating modes allow an entire queue to be scanned multiple
times, software involvement is not required for 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 when using
the continuous-scan queue operating modes since the relationship
between any two conversions may be variable due to programmable
trigger events and queue priorities.
By programming the MQ1 field in QACR1 or the MQ2 field in QACR2, the following
modes can be selected for queue 1 and/or 2:
Software initiated continuous-scan mode
— When this mode is programmed, the trigger event is generated automatically
by the QADC, and queue execution begins immediately. If a pause is encoun-
tered, queue execution ceases for two QCLKs, while another trigger event is
generated internally; execution then continues. When the end-of-queue is
reached, another internal trigger event is generated, and queue execution be-
gins 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 ini-
tiated continuous-scan mode.
— This mode keeps the result registers updated more frequently than any of the
other queue operating modes. 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.
— This 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