Motorola MCF5281, MCF5282 user manual Software-Initiated Continuous-Scan Mode

Models: MCF5282 MCF5281

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Digital Control Subsystem

NOTE

In continuous-scan modes, all samples are guaranteed to be taken during one pass through the queue (coherently), except when a queue 1 trigger event halts queue 2 execution. The time between consecutive conversions has been designed to be consistent. However, for queues that end with a CCW containing the EOQ code (channel 63), the time between the last queue conversion and the first queue conversion requires one additional CCW fetch cycle. Continuous samples are not coherent at this boundary.

In addition, the time from trigger to first conversion cannot be guaranteed, because it is a function of clock synchronization, programmable trigger events, queue priorities, and so on.

27.8.7.1Software-Initiated Continuous-Scan Mode

When software-initiated continuous-scan mode is selected, the trigger event is generated automatically by the QADC. Queue execution begins immediately. If a pause is encountered, another trigger event is generated internally, and execution continues without pausing. When the end-of-queue is reached, another internal trigger event is generated and queue execution restarts at the beginning of the queue.

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 idle. The trigger overrun flag is never set while in 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 result table can always be read to get the latest converted value for each channel. The channels scanned are kept up to date by the QADC without software involvement.

The software-initiated continuous-scan mode may be chosen for either queue, but is normally used only with queue 2. When 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 completion and the subsequent 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 converting channels that do not need to be synchronized to anything or for slow-to-change analog channels. Interrupts are normally not used with the software-initiated continuous-scan mode. Rather, the latest conversion results can be read from the result table at any time. Once initiated, software action is not needed to sustain conversions of channel.

MOTOROLA

Chapter 27. Queued Analog-to-Digital Converter (QADC)

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Motorola MCF5281, MCF5282 user manual Software-Initiated Continuous-Scan Mode