Motorola ColdFire MCF5281 - Manual (Page 586)

Queued Analog-to-Digital Converter (QADC)
28-48 Freescale Semiconductor
28.8.6.1 Software-Initiated Single-Scan Mode
Software can initiate the execution of a scan sequence for queue 1 or 2 by selecting software-initiated
single-scan mode and writing the single-scan enable bit in QACR1 or QACR2. A trigger event is generated
internally and the QADC immediately begins 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
28.8.6.2 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.
28.8.6.3 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.
While the gate is open, queue 1 executes one time. Each CCW is read and the indicated conversions are
performed until an end-of-queue condition is encountered. When queue 1 completes, the QADC sets the
completion flag (CF1) and clears the single-scan enable bit. Set the single-scan enable bit again to allow
another scan of queue 1 to be initiated during the next open gate.
If the gate closes before queue 1 completes execution, the current CCW completes, execution of queue 1
stops, the single-scan enable bit is cleared, and the PF1 bit is set. The CWPQ1 field can be read to
determine the last valid conversion in the queue. The single-scan enable bit must be set again and the PF1
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3