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14.5 Digital Subsystem
The digital control subsystem includes the control logic to sequence the conversion activity,
the clock and periodic/interval timer, control and status registers, the conversion command
word table RAM, and the result word table RAM.
The central element for control of the QADC64E conversions is the 64-entry CCW table.
Each CCW specifies the conversion of one input channel. Depending on the application,
one or two queues can be established in the CCW table. A queue is a scan sequence of one
or more input channels. By using a pause mechanism, sub queues can be created in the two
queues. Each queue can be operated using one of several different scan modes. The scan
modes for queue 1 and queue 2 are programmed in QACR1 and QACR2 (control registers
1 and 2). Once a queue has been started by a trigger event (any of the ways to cause the
QADC64E to begin executing the CCWs in a queue or sub-queue), the QADC64E
performs a sequence of conversions and places the results in the result word table.
14.5.1
Queue Priority
Queue 1 has priority over queue 2 execution. The following 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
captured 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 queue 2 is suspended are captured 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. Refer to Section 14.3.7, "Control
Register 2" for more information.
• 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.
14.5.2
Sub-Queues That are Paused
The pause feature can be used to divide queue 1 and/or queue 2 into multiple sub-queues.
A sub-queue is defined by setting the pause bit in the last CCW of the sub-queue.
MOTOROLAChapter 14. Queued Analog-to-Digital Converter Enhanced Mode Operation
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
Digital Subsystem
14-39
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