DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

fixed identifier and hence a fixed priority, it can be assumed that each given message must
contain a bounded number of bytes. Given a bounded size, and a bounded rate at which the
message is sent, this effectively bounds the peak load on the bus. This allows scheduling
analysis to be applied to obtain a latency bound for each message on the bus [67].
The queuing of a hard real-time message can occur with jitter (variability in queuing
times) as shown in Figure 4.8. The shaded boxes in Figure 4.8 represent the ‘windows’ in which
a node can queue a message. Jitter is important because to ignore it would lead to insufficient
analysis. For example, ignoring jitter would lead to the assumption that message m could be
queued at most once in an interval of duration (b – a). In fact, during the interval (a … b] the
message could be queued twice: once at a (as late as possible in the first queuing window), and
once at b (as early as possible in the next queuing window). Queuing jitter can be defined as the
difference between the earliest and latest possible times a given message can be queued [67].
Figure 4.8 Message Queuing Jitter [67].
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