13-50
MPC561/MPC563 Reference Manual
MOTOROLA
Digital Subsystem
a high level opens the gate and a low level closes the gate. Once the gate is open, each CCW
is read and the indicated conversions are performed until the gate is closed. When the gate
opens again, the queue execution automatically begins again from the beginning of the
queue. Software initialization is not needed between trigger events. If a pause in a CCW is
encountered, the pause flag will not set, and execution continues without pausing.
The purpose of external gated continuous-scan mode is to continuously collect digitized
samples while the gate is open and to have the most recent samples available. It is up to the
programmer to ensure that the queue is large enough so that a maximum gate open time will
not reach an end-of-queue. However it is useful to take advantage of a smaller queue in the
manner described in the next paragraph.
In the event that the queue completes before the gate closes, a completion flag will be set
and the queue will roll over to the beginning and continue conversions until the gate closes.
If the gate remains open and the completion flag is not cleared, when the queue completes
a second time the trigger overrun flag will be set and the queue will roll-over again. The
queue will continue to execute until the gate closes or the mode is disabled.
If the gate closes before queue 1 completes execution, the current CCW completes
execution of queue 1 stops and QADC64E sets the PF1 bit to indicate an incomplete queue.
Software can read the CWPQ1 to determine the last valid conversion in the queue. In this
mode, if the gate opens again, execution of queue 1 begins again. The start of queue 1 is
always the first CCW in the CCW table.
Since the condition of the gate is only sampled after each conversion during queue
execution, closing the gate for a period less than a conversion time interval does not
guarantee the closure will be captured.
13.5.4.4.4 Periodic/Interval Timer Continuous-Scan Mode
The QADC64E includes a dedicated periodic/interval timer for initiating a scan sequence
on queue 1 and/or queue 2. Software selects a programmable timer interval ranging from
128 to 128 Kbytes times the QCLK period in binary multiples. The QCLK period is
prescaled down from the IMB3 MCU clock.
When a periodic/interval timer continuous-scan mode is selected for queue 1 and/or queue
2, the timer begins counting. After the programmed interval elapses, the timer generated
trigger event starts the appropriate queue. Meanwhile, the QADC64E automatically
performs the conversions in the queue until an end-of-queue condition or a pause is
encountered. When a pause occurs, the QADC64E waits for the periodic interval to expire
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