Motorola MCF5282, MCF5281 user manual Queue Priority Schemes, Trigger Events, Status Bits

Digital Control Subsystem

27.8.1.2Queue Priority Schemes

Because there are two conversion command queues and only one A/D converter, a priority scheme determines which conversion occurs. Each queue has a variety of trigger events that are intended to initiate conversions, and they can occur asynchronously in relation to each other and other conversions in progress. For example, a queue can be idle awaiting a trigger event; a trigger event can have occurred, but the first conversion has not started; a conversion can be in progress; a pause condition can exist awaiting another trigger event to continue the queue; and so on.

The following paragraphs and figures outline the prioritizing criteria used to determine which conversion occurs in each overlap situation.

NOTE

Each situation in Figure 27-23through Figure 27-33is labeled S1 through S19. In each diagram, time is shown increasing from left to right. The execution of queue 1 and queue 2 (Q1 and Q2) is shown as a string of rectangles representing the execution time of each CCW in the queue. In most of the situations, there are four CCWs (labeled C1 to C4) in both queue 1 and queue 2. In some of the situations, CCW C2 is presumed to have the pause bit set, to show the similarities of pause and end-of-queue as terminations of queue execution.

Trigger events are described in Table 27-22.

	Table 27-22. Trigger Events
Trigger	Events
T1	Events that trigger queue 1 execution (external trigger, software-initiated single-scan enable
	bit, or completion of the previous continuous loop)
T2	Events that trigger queue 2 execution (external trigger, software-initiated single-scan enable
	bit, timer period/interval expired, or completion of the previous continuous loop)

When a trigger event causes a CCW execution in progress to be aborted, the aborted conversion is shown as a ragged end of a shortened CCW rectangle.

The situation diagrams also show when key status bits are set. Table 27-23describes the status bits.

	Table 27-23. Status Bits
Bit	Function
CF flag	Set when the end of the queue is reached
PF flag	Set when a queue completes execution up through a pause bit
Trigger overrun	Set when a new trigger event occurs before the queue is finished servicing the
error (TOR)	previous trigger event

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