The model represents that information which is relevant and abstracts away from irrelevant detail. The implication of this is that there may be several models of the same system, since different information is relevant in different contexts. One kind of model might concentrate on representing the function of the system, whereas another kind might represent the way in which the system is composed of subsystems.
We will come back to this idea in more detail in the next section. There are a number of important motivations for formalizing this idea of a model. First, when a digital system is needed, the requirements of the system must be specified. The job of the engineers is to design a system that meets these requirements.
To do that, they must be given an understanding of the requirements, hopefully in a way that leaves them free to explore alternative implementations and to choose the best according to some criteria. One of the problems that often arises is that requirements are incompletely and ambiguously spelled out, and the customer and the design engineers disagree on what is meant by the requirements document. This problem can be avoided by using a formal model to communicate requirements.
A second reason for using formal models is to communicate understanding of the function of a system to a user. The designer cannot always predict every possible way in which a system may be used, and so is not able to enumerate all possible behaviors.
If the designer provides a model, the user can check it against any given set of inputs and determine how the system behaves in that context. Thus a formal model is an invaluable tool for documenting a system. A third motivation for modeling is to allow testing and verification of a design using simulation.
If we start with a requirements model that defines the behavior of a system, we can simulate the behavior using test inputs and note the resultant outputs of the system. According to our design methodology, we can then design a circuit from subsystems, each with its own model of behavior. We can simulate this- composite system with the same test inputs and compare the outputs with those of the previous simulation. If they are the same, we know that the composite system meets the requirements for the cases tested. Otherwise we know that some revision of the design is needed.
We can continue this process until we reach the bottom level in our design hierarchy, where the components are real devices whose behavior we know.
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Subsequently, when the design is manufactured, the test inputs and outputs from simulation can be used to verify that the physical circuit functions correctly. This approach to testing and verification of course assumes that the test inputs cover all of the circumstances in which the final circuit will be used. The issue of test coverage is a complex problem in itself and is an active area of research. A fourth motivation for modeling is to allow formal verification of the correctness of a design. Formal verification requires a mathematical statement of the required function of a system.
This statement may be expressed in the notation of a formal logic system, such as temporal logic.
The Designer's Guide to VHDL, third edition
Formal verification also requires a mathematical definition of the meaning of the modeling language or notation used to describe a design. The process of verification involves application of the rules of inference of the logic system to prove that the design implies the required function. While formal verification is not yet in everyday use, it is an active area of research.
There have already been significant demonstrations of formal verification techniques in real design projects, and the promise for the future is bright. One final, but equally important, motivation for modeling is to allow automatic synthesis of circuits. If we can formally specify the function required of a system, it is in theory possible to translate that specification into a circuit that performs the function. The advantage of this approach is that the human cost of design is reduced, and engineers are free to explore alternatives rather than being bogged down in design detail.
The designer's guide to VHDL
Also, there is less scope for errors being introduced into a design and not being detected. If we automate the translation from specification to implementation, we can be more confident that the resulting circuit is correct. The unifying factor behind all of these arguments is that we want to achieve maximum reliability in the design process for minimum cost and design time.
We need to ensure that requirements are clearly specified and understood, that subsystems are used correctly and that designs meet the requirements. A major contributor to excessive cost is having to revise a design after manufacture to correct errors.
The Designer's Guide to VHDL, third edition, Volume 3 by Peter Ashenden | | Booktopia
By avoiding errors, and by providing better tools for the design process, costs and delays can be contained. We can classify these models into three domains: function, structure and geometry. The functional domain is concerned with the operations performed by the system. In a sense, this is the most abstract domain of description, since it does not indicate how the function is implemented. Brand new Book.
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antulepi.cf Publisher: Morgan Kaufmann , This specific ISBN edition is currently not available. He was previously a senior lecturer in computer science and is now a Visiting Research Fellow at the University of Adelaide. His research interests are computer organization and electronic design automation. Ashenden is also an independent consultant specializing in electronic design automation EDA.
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