Motorola MCF5282 Externally Triggered Single-Scan Mode, Externally Gated Single-Scan Mode

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

27.8.6.2Externally 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.

27.8.6.3Externally 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