Queue Priority Timing Examples - Motorola ColdFire MCF5281 User Manual

Digital Control Subsystem

27.8.1 Queue Priority Timing Examples

This subsection describes the QADC priority scheme when trigger events on two queues
overlap or conflict.
27.8.1.1 Queue Priority
Queue 1 has priority over queue 2 execution. These cases show the conditions under which
queue 1 asserts its priority:
• When a queue is not active, a trigger event for queue 1 or queue 2 causes the
corresponding queue execution to begin.
• When queue 1 is active and a trigger event occurs for queue 2, queue 2 cannot begin
execution until queue 1 reaches completion or the paused state. The status register
records the trigger event by reporting the queue 2 status as trigger pending.
Additional trigger events for queue 2, which occur before execution can begin, are
flagged as trigger overruns.
• When queue 2 is active and a trigger event occurs for queue 1, the current queue 2
conversion is aborted. The status register reports the queue 2 status as suspended.
Any trigger events occurring for queue 2 while it is suspended are flagged as trigger
overruns. Once queue 1 reaches the completion or the paused state, queue 2 begins
executing again. The programming of the RESUME bit in QACR2 determines
which CCW is executed in queue 2.
• When simultaneous trigger events occur for queue 1 and queue 2, queue 1 begins
execution and the queue 2 status is changed to trigger pending.
• When subqueues are paused
The pause feature can be used to divide queue 1 and/or queue 2 into multiple subqueues. A
subqueue is defined by setting the pause bit in the last CCW of the subqueue.
Figure 27-22 shows the CCW format and an example of using pause to create subqueues.
Queue 1 is shown with four CCWs in each subqueue and queue 2 has two CCWs in each
subqueue.
The operating mode selected for queue 1 determines what type of trigger event causes the
execution of each of the subqueues within queue 1. Similarly, the operating mode for queue
2 determines the type of trigger event required to execute each of the subqueues within
queue 2.
For example, when the external trigger rising edge continuous-scan mode is selected for
queue 1, and there are six subqueues within queue 1, a separate rising edge is required on
the external trigger signal after every pause to begin the execution of each subqueue (refer
to Figure 27-22).
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