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2.3 FIELDS OF APPLICATION 11
Specification System
Abstraction Module 1
Submodule 1
Time
Figure 2.3 Top-down design sequence
• The implementable part of the specification can be validated by simulations.
• The implementable part of the specification is available as a precisely defined
reference for the verification of the design.
• The functional part of the specification is unambiguous and complete (in con-
trast to a specification in natural language). In the event of doubt, a simulation
is run.
• The implementable specification and the models of the individual design stages
mean that full documentation is available, which however still remains to be
supplemented by comprehensive commentary.
In the case of mixed-signal design, the implementable specification can be made
available to the test engineers at an early stage as part of a ‘simultaneous engi-
neering’ approach. Using a model for the testing machine a virtual test is created,
in which test programmes can be developed on the workstation. This removes the
fixed sequence of design → production → test development and also saves a great
deal of time on test development.
However, the disadvantage of the use of implementable specifications is that
some technical content can be expressed in a simpler, more compact and more
easily understood form in natural language than in a formal modelling language.
In addition, there is the question of adhering to the formally correct description
of the desired semantics, which incurs an additional cost in relation to a paper
specification. Finally, problems in the physical realisation, such as excessive delay
times for certain blocks, are not recognised until a relatively late stage.
For mechanics the top-down design sequence is still in the development stage.
A significant reason for this is that unified and standardised description methods
for mechanical behaviour, with which a design can be taken incrementally from an
abstract specification to a detailed implementation, are only now being developed.
