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Digital Control Subsystem
execution of the first CCW in the queue. If a pause occurs, another trigger event is
generated internally, and then execution continues without pausing.
The QADC automatically performs the conversions in the queue until an end-of-queue
condition is encountered. The queue remains idle until the single-scan enable bit is again
set. 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 paused. The trigger
overrun flag is never set while in software-initiated single-scan mode.
The software-initiated single-scan mode is useful when:
• Complete control of queue execution is required
• There is a need to easily alternate between several queue sequences
27.8.6.2 Externally Triggered Single-Scan Mode
The externally triggered single-scan mode is available on both queue 1 and queue 2. Both
rising and falling edge triggered modes are available. A scan must be enabled by setting the
single-scan enable bit for the queue.
The first external trigger edge causes the queue to be executed one time. Each CCW is read
and the indicated conversions are performed until an end-of-queue condition is
encountered. After the queue is completed, the QADC clears the single-scan enable bit. The
single-scan enable bit can be written again to allow another scan of the queue to be initiated
by the next external trigger edge.
The externally triggered single-scan mode is useful when the input trigger rate can exceed
the queue execution rate. Analog samples can be taken in sync with an external event, even
though application software does not require data taken from every edge. Externally
triggered single-scan mode can be enabled to get one set of data and, at a later time, be
enabled again for the next set of samples.
When a pause bit is encountered during externally triggered single-scan mode, another
trigger event is required for queue execution to continue. Software involvement is not
required for queue execution to continue from the paused state.
27.8.6.3 Externally Gated Single-Scan Mode
The QADC provides external gating for queue 1 only. When externally gated single-scan
mode is selected, the input level on the associated external trigger signal enables and
disables queue execution. The polarity of the external gate signal is fixed so that only a high
level opens the gate and a low level closes the gate. Once the gate is open, each CCW is
read and the indicated conversions are performed until the gate is closed. Queue scan must
be enabled by setting the single-scan enable bit for queue 1. If a pause is encountered, the
pause flag does not set, and execution continues without pausing.
27-52
MCF5282 User's Manual
MOTOROLA
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