Motorola ColdFire MCF5281 - Manual (Page 588)

Queued Analog-to-Digital Converter (QADC)
28-50 Freescale Semiconductor
In the case of software-initiated continuous-scan mode, the trigger event is generated internally and queue
execution begins immediately. In the other continuous-scan queue operating modes, the selected trigger
event must occur before the queue can start. A trigger overrun is captured if a trigger event occurs during
queue execution in the externally triggered continuous-scan mode or the periodic timer continuous-scan
mode.
After queue execution is complete, the queue status is shown as idle. Because the continuous-scan queue
operating modes allow the entire queue to be scanned multiple times, software involvement is not needed
for queue execution to continue from the idle state. The next trigger event causes queue execution to begin
again, starting with the first CCW in the queue.
NOTE
In continuous-scan modes, all samples are guaranteed to be taken during
one pass through the queue (coherently), except when a queue 1 trigger
event halts queue 2 execution. The time between consecutive conversions
has been designed to be consistent. However, for queues that end with a
CCW containing the EOQ code (channel 63), the time between the last
queue conversion and the first queue conversion requires one additional
CCW fetch cycle. Continuous samples are not coherent at this boundary.
In addition, the time from trigger to first conversion cannot be guaranteed,
because it is a function of clock synchronization, programmable trigger
events, queue priorities, and so on.
28.8.7.1 Software-Initiated Continuous-Scan Mode
When software-initiated continuous-scan mode is selected, the trigger event is generated automatically by
the QADC. Queue execution begins immediately. If a pause is encountered, another trigger event is
generated internally, and execution continues without pausing. When the end-of-queue is reached, another
internal trigger event is generated and queue execution restarts at the beginning of the queue.
While the time to internally generate and act on a trigger event is very short, the queue status field can be
read as momentarily indicating that the queue is idle. The trigger overrun flag is never set while in
software-initiated continuous-scan mode.
The software-initiated continuous-scan mode keeps the result registers updated more frequently than any
of the other queue operating modes. The result table can always be read to get the latest converted value
for each channel. The channels scanned are kept up to date by the QADC without software involvement.
The software-initiated continuous-scan mode may be chosen for either queue, but is normally used only
with queue 2. When software-initiated continuous-scan mode is chosen for queue 1, that queue operates
continuously and queue 2, being lower in priority, never gets executed. The short interval of time between
a queue 1 completion and the subsequent trigger event is not sufficient to allow queue 2 execution to begin.
The software-initiated continuous-scan mode is a useful choice with queue 2 for converting channels that
do not need to be synchronized to anything or for slow-to-change analog channels. Interrupts are normally
not used with the software-initiated continuous-scan mode. Rather, the latest conversion results can be read
from the result table at any time. Once initiated, software action is not needed to sustain conversions of
channel.
28.8.7.2 Externally Triggered Continuous-Scan Mode
The QADC provides external trigger signals for both queues. When externally triggered continuous-scan
mode is selected, a transition on the associated external trigger signal initiates queue execution. The
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3