Message Queue Overrun - STMicroelectronics SPC572L series Reference Manual

e200z215An3 Nexus 3 Module
If more than three messages attempt to enter the queue in the same cycle, the highest
priority messages are stored and the remaining message(s) will be dropped due to a
collision. Collision events are expected to be rare.
The Overrun Control register (OVCR) controls the Nexus behavior as the message queue
fills. The Nexus block may be programmed to:
•
Allow the queue to overflow, drain the contents, queue an overrun error message and
resume tracing.
•
Stall the processor when the queue utilization reaches the selected threshold.
•
Suppress selected message types when the queue utilization reaches the selected
threshold.
66.8.1

Message Queue Overrun

In this mode, the message queue will stop accepting messages when an overrun condition
is detected. The contents of the queues will be allowed to drain until empty. Incoming
messages are discarded until the queue is emptied. Once empty, an overrun error message
is enqueued that contains information about the types of messages that were discarded due
to the overrun condition.
66.8.2 CPU Stall
In this mode, processor instruction issue is stalled when the queue utilization reaches the
selected threshold. The processor is stalled long enough to drop one threshold level below
the level that triggered the stall. For example, if stalling the processor is triggered at 1/4 full,
the stall will stay in effect until the queue utilization drops to empty. There may be significant
skid from the time that the stall request is made until the processor is able to stop
completing instructions. This skid should be taken into consideration when programming the
threshold. Refer to
programming options.
66.8.3 Message Suppression
In this mode, the message queue will disable selected message types when the queue
utilization reaches the selected threshold. This allows lower bandwidth tracing to continue
and possibly avoid an overrun condition. If an overrun condition occurs despite this
message suppression, the queue will respond according to the behavior described in
Section 66.8.1, Message Queue
error message if they occur after overrun in addition to other discarded message types.
Once triggered, message suppression will remain in effect until queue utilization drops to
the threshold below the level selected to trigger suppression.
66.8.4 Nexus Message Priority
Nexus messages may be lost due to contention with other message types under the
following circumstances:
•
More than three messages are generated in the same cycle
Table 1098
Up to three message requests can be queued into the message buffer in a given cycle. If
more than three message requests exist in a given cycle, the three highest priority message
classes are queued into the message buffer. The remaining messages that did not
1946/2058
Section 66.4.9, Nexus Overrun Control (OVCR)
Overrun. Suppressed message types are reported in the
lists the various message types and their relative priority from highest to lowest.
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