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Hierarchy in Object-Oriented Petri Nets in the Specification of Digital Systems.

Machado, Ricardo Jorge

MSc Thesis, pages 1-120. Dep. Informática, Universidade do Minho, November 1996. In Portuguese.

Abstract: The design of complex digital systems is conceptually divided in two parts: the control unit and the data path. The control unit may, at times, present strong parallel behavior, which has been modeled with synchronous interpreted Petri nets. This approach has some advantages in relation to the traditional one in the use of several local state machines working simultaneously using common signals for mutual synchronization. The existing solutions for modeling the global system specify both parts almost independently; so they cannot incorporate in one formalism both the representations of the global system. An extension to the synchronous interpreted Petri nets was developed, shobi-PN, supporting hierarchy in Petri net models and the use of objects to model the data path. A full digital system can be specified with a structured and incremental approach. Essentialy, the shobi-PN model substitutes the traditional tokens by objects. The invocation of methods corresponds to the reading of input signals and to the generation of control signals, which permits simultaneously the modeling of the data path behavior. The modeling of three parallel controllers, as well as the respective data paths, permitted the verification of the shobi-PN model adequability to specify digital systems in an hierarchical, modular and incremental way.

The majority of the available electronic CAD tools for digital system design make available HDLs as a specification format, but may not allow the direct modeling of concurrent and cooperative activities. A computer application was defined to directly support the proposed Petri net model, permitting graphical system specifications. As a solution for the implementation of the application, one can use the SCBA environment, which initially required the mapping of the shobi-PN model in a computational model based on agents, the MCBA. Since this application also uses the CONPAR environment, it is possible to obtain a control unit VHDL description of the specified system.

Keywords: Petri Nets; Digital Systems Specification; Electronic CAD Tools.


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