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Digital Control Subsystem
27.8.1.2 Queue 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.
Each situation in Figure 27-23 through Figure 27-33 is 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.
Trigger
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 27-23 describes the status bits.
Bit
CF flag
PF flag
Trigger overrun
error (TOR)
27-40
NOTE
Table 27-22. Trigger Events
Table 27-23. Status Bits
Set when the end of the queue is reached
Set when a queue completes execution up through a pause bit
Set when a new trigger event occurs before the queue is finished servicing the
previous trigger event
MCF5282 User's Manual
Events
Function
MOTOROLA
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