Motorola MPC533 Reference Manual page 574

Digital Subsystem
again, then continues with the queue. Once end-of-queue has been detected, the next trigger
event causes queue execution to begin again with the first CCW in the queue.
The periodic/interval timer generates a trigger event whenever the time interval elapses.
The trigger event may cause the queue execution to continue following a pause or queue
completion, or may be considered a trigger overrun. As with all continuous-scan queue
operating modes, software action is not needed between trigger events. Since both queues
may be triggered by the periodic/interval timer, see Section 14.5.6, "Periodic/Interval
Timer" for a summary of periodic/interval timer reset conditions.
Software enables the completion interrupt when using the periodic/interval timer
continuous-scan mode. When the interrupt occurs, the software knows that the periodically
collected analog results have just been taken. The software can use the periodic interrupt to
obtain non-analog inputs as well, such as contact closures, as part of a periodic look at all
inputs.
14.5.5 QADC64E Clock (QCLK) Generation
Figure 14-22 is a block diagram of the clock subsystem. The QCLK provides the timing for
the A/D converter state machine which controls the timing of the conversion. The QCLK
is also the input to a 17-stage binary divider which implements the periodic/interval timer.
To retain the specified analog conversion accuracy, the QCLK frequency (F
within the tolerance specified in Appendix E, "Electrical Characteristics."
Before using the QADC64E, the software must initialize the prescaler with values that put
the QCLK within the specified range. Though most software applications initialize the
prescaler once and do not change it, write operations to the prescaler fields are permitted.
A change in the prescaler value while a conversion is in
progress is likely to corrupt the result from any conversion in
progress. Therefore, any prescaler write operation should be
done only when both queues are in the disabled modes.
14-50
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
WARNING
MPC533 Reference Manual
) must be
QCLK
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