MC68336/376
QUEUED ANALOG-TO-DIGITAL CONVERTER MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
8-30
tion begins when software sets the single-scan enable bit. When a continuous-scan
mode is selected, the queue remains active in the selected queue operating mode
after the QADC completes each queue scan sequence.
During queue execution, the QADC reads each CCW from the active queue and exe-
cutes conversions in four stages:
1. Initial sample
2. Transfer
3. Final sample
4. Resolution
During initial sample, the selected input channel is connected to the sample capacitor
at the input of the sample buffer amplifier.
During the transfer period, the sample capacitor is disconnected from the multiplexer,
and the stored voltage is buffered and transferred to the RC DAC array.
During the final sample period, the sample capacitor and amplifier are bypassed, and
the multiplexer input charges the RC DAC array directly. Each CCW specifies a final
input sample time of 2, 4, 8, or 16 QCLK cycles. When an analog-to-digital conversion
is complete, the result is written to the corresponding location in the result word table.
The QADC continues to sequentially execute each CCW in the queue until the end of
the queue is detected or a pause bit is found in a CCW.
When the pause bit is set in the current CCW, the QADC stops execution of the queue
until a new trigger event occurs. The pause status flag bit is set, which may generate
an interrupt request to notify software that the queue has reached the pause state.
When the next trigger event occurs, the paused state ends, and the QADC continues
to execute each CCW in the queue until another pause is encountered or the end of
the queue is detected.
An end-of-queue condition is indicated as follows:
The CCW channel field is programmed with 63 ($3F) to specify the end of the
queue.
The end of queue 1 is implied by the beginning of queue 2, which is specified in
the BQ2 field in QACR2.
The physical end of the queue RAM space defines the end of either queue.
When any of the end-of-queue conditions is recognized, a queue completion flag is
set, and if enabled, an interrupt request is generated. The following situations prema-
turely terminate queue execution:
Since queue 1 is higher in priority than queue 2, when a trigger event occurs on
queue 1 during queue 2 execution, the execution of queue 2 is suspended by
aborting execution of the CCW in progress, and queue 1 execution begins. When
queue 1 execution is complete, queue 2 conversions restart with the first CCW
entry in queue 2 or the first CCW of the queue 2 subqueue being executed when
queue 2 was suspended. Alternately, conversions can restart with the aborted
queue 2 CCW entry. The resume RES bit in QACR2 allows software to select