Specification of Reactive Hardware/Software Systems - Electronic ...

6.7 Real-Time Modelling 225
in the system. Accuracy is the aberration of a local clock from the global time. Timing
requirements can be specified with a maximum, a minimum or both, depending on the
needs.
The requirements on events in the system can be defined relative or absolute in global
or local time. Events can be the starting or stopping of an internal action or an execution
in the system (process), the deliverance of a result, the appearance of the arrival of a
stimulus or the appearance of a response at an output. The time between events can be
specified as a duration (time interval). A time between an input and an output is often
called a response time. Further the time between two input events or between two output
events may be specified as a time interval. A special type of time interval is the time
between the same type of events. This type of time interval can be defined as a repetition
rate [HP88]. A repetition rate is a useful concept for inputs. A repetition rate for outputs
can be specified as a reprocessing rate. In addition the character of the communication
such as bursty traffic must be defined.
Periodical processing can be specified as a reception rate on a processing request message.
A processing time may be specified as a net processing time or as an elapse time, which
is the maximum processing time including the delay caused by communication and
waiting for resources etcetera.
There are several options for adding timing requirements to a specification of a system.
Some preliminary ideas about alternatives are the adding of requirements to:
system boundaries;
architecture module structure;
implementation structure;
message flows;
object interfaces;
cluster interfaces.
For the moment we foresee the mapping of timing requirements onto clusters. Clusters
already carry requirements as they represent various types of boundaries (see Section
5.8). Mapping timing requirements onto clusters can be adequate because of the relation
of timing and architecture. Boundaries formalise architecture and implementation
structure design using clusters.
The problems that designers in practice meet are very complex. Determination of
response-times of processes is hard. The communication between processes, the switching
between processes and the waiting time for resources make timing calculations very
difficult. This makes it very hard to design a complex real-time system adequately without
special tools that support verification of the timing constraints. In addition the use
of real-time operating systems will be useful. This implies that the properties of such a
system part must be incorporated in a model.