The specification of timing requirements for real-time systems using timed petri nets
Jr. James Edward Coolahan · 1984
Real-time systems are considered successful by correctly executing their intended functions within critical time constraints. Thus, their requirements specifications must address the execution time requirements for system processes. This dissertation develops a formal methodology for specifying the timing requirements for a real-time system in terms of process execution times. A Petri net model that uses places to represent system processes is extended by augmenting these places with time variables representing process execution times. The concept of time-safeness of places is introduced to specify how places not inherently safe by the net structure may be safe in the presence of timing information. The modular modeling methodology permits the hierarchical specification of sequential, alternative, iterative, and parallel activities, as well as the mutually exclusive sharing of system resources. Mathematical representations of the modular transformations are given corresponding to the graphical representations, so that a mathematical model is maintained throughout the graphical modeling process. The modularity of the transformations ensures that the net remains analyzable for timing requirements purposes and guarantees freedom from deadlock. The ability to integrate graphical data flow specifications with the control and timing specifications is also demonstrated. System timing requirements are stated as the conditions under which all places in the net will be time-safe. The modifications to system timing requirements caused by each transformation are precisely stated and rigorously proven, so that a formal specification of timing requirements that is derived from user-stated system level requirements evolves with the system model. The concept of a time-driven system is introduced that includes a special cyclic construction that acts as a master system clock. The utility of the timing requirements specification in the feasibility testing of target process execution times and in the refinement of these targets as some actual execution times become available is illustrated. Several examples of the development of timing requirements are given, including communication protocol and process control system applications. A comprehensive detailed example for a scientific data acquisition system is presented that demonstrates the practicality of the formal methodology for a real-world system.