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

224 Modelling of Concurrent Reactive Behaviour
6.7.2 Timing Requirements and Properties
Timing requirements play a role in almost all hardware/software systems. However
the nature of these requirements can be very different for various system parts and for
various systems. A violation of timing requirements can have serious consequences in
control systems. Next to system (parts) with strict time demands there exist system
parts with requirements that can be so weak that they can be characterised as desired
properties. We define real-time as a property of a system (part) to fulfil timing requirements
for reactions on external events and internal events.
Two sorts of real-time requirements can be distinguished:
Hard real-time (HRT) requirements that must absolutely be met. Violation causes
severe failures, damage, malfunctioning or in general dangerous or otherwise
unacceptable behaviour of the system.
Soft real-time (SRT) requirements that are desired to be met but not at any cost.
Violation does not cause severe problems, however may lead to degradation of the
system performance.
The design process of a real-time system is directed to realising real-time properties that
match the real-time requirements. Our method aims at an integration of analysis, specification
and design. Real-time properties can in general not be known before the
implementation of a system. Real-time requirements are in general given on the level of
a system boundary. It is hard to distribute these requirements over the internal modules
of a complex system model. Simulation of the properties of a model can only be performed
realistically when timing properties can be estimated properly. In our method
we pay attention to design and possible prescribed technologies and topologies in an
early phase. We foresee that this will enable to pay attention to estimation of some of
the timing properties as early as possible.
6.7.3 Preliminary Approach towards Requirements
System level requirements are textually described in so-called Architecture Response
Time Requirements. The requirements must be related to the modules of an architecture
design of a system. In a second phase Implementation Response Time Requirements
are added. Both sorts of requirements can only be described in relation to events and
responses that are related to communication channels.
An overview of definitions and descriptions of timing aspects of software systems are
found in [Bon92] (in Dutch). These concepts can be redefined for distributed mixed
hardware/software systems. Responses of the system may be defined in terms of
absolute time or relative to other responses or stimuli. Distributed systems can distinguish
between local and global time. Global time is the virtual time given by the synchronised
local clocks of the distributed system parts. Local time is the result of inspection of
a local clock. Concepts that describe aberrances of time are precision and accuracy.
Precision is the upper limit of the aberration at some time between arbitrary local clocks