Tra c Lights An AutoFocus Case Study

In this paper we present a case study on Auto Focus a tool prototype for the development of dis tributed and concurrent systems based on the con cepts of the formal method Focus We develop spec ify consistency check and simulate the controller of a pedestrian tra c light using di erent graphical de scription techniques to illustrate an engineering pro cess for concurrent systems Introduction The importance of software in embedded systems is rapidly increasing More and more functionality of such systems formerly realized using specialized hard ware solutions is now being implemented by software This tendency will even increase in the years to come Due to the very limited computing resources avail able in embedded systems controllers in the very early years software development for embedded systems very much concentrated on the e ort to tweak and optimize the code in order to cope with these circum stances Today high performance micro controllers allow increasingly complex software solutions for em bedded systems Thus it is obvious that the main fo cus in software development for these systems is shift ing towards a task of managing the complexity and inter relationsships of software components It is widely recognized that this increasing complex ity of embedded software development can be man aged only by adequate means of structuring both from the point of view of the techniques used to describe di erent aspects of such systems and from the devel opment process point of view Both of these aspects notations and process should be supported by ade quate tools Due to the complexity of today s embed The authors of this paper were funded by DFG Sonderforschungsbereich Werkzeuge und Methoden f ur die Nutzung paralleler Rechnerarchitekturen the project Sys Lab supported by DFG Leibnitz and Siemens Nixdorf the project KorSys nanced by the German Federal Ministry of Education and Research BMBF and the Forschungsverbund Software Engineering FORSOFT ded software facilities for veri cation and validation of properties of the software become a critical issue for e cient use of such tools in development AutoFocus is a tool prototype for development of concurrent embedded systems Having its origins and its formal background in the formal development method Focus it o ers a structured approach to modeling embedded systems using graphical notations and a re nement based development process Using the example of a pedestrian tra c lights con troller we demonstrate how a simple embedded sys tem can be developed with AutoFocus Related work Obviously tool support for the de velopment of distributed systems is not new The spectrum ranges from tools for mainly veri cation ori ented approaches to those focussing on simulation and code generation like or and In general however these approaches do not try to combine the aspects of intuitive description techniques a strong semantical basis and veri cation support as well as simulation and code generation For example the UML method includes several description techniques and o ers tool support for the speci cation of systems using those techniques How ever UML lacks a precise semantical basis and there fore does not allow formal veri cation of vital system properties AutoFocus supports a lean subset of description techniques based on a common mathematical model allowing both pragmatically oriented development and integration of formal techniques Description Techniques To form a comprehensive and structured picture of a system it should be described from di erent points of view and on di erent levels of abstraction There fore AutoFocus o ers four hierarchically structured description techniques system structure diagrams SSDs data type de nitions DTDs state transition diagrams STDs and extended event traces EETs each one covering di erent views on the system The integration of the views on a common semantic basis leads to one integrated formal system speci cation of the system AutoFocus supports the hierarchical development of systems Depending on the granularity components or views can be atomic or consist of sub components or sub views themselves Therefore AutoFocus al lows the user to switch between di erent levels of gran ularity by using the hierarchical description techniques described in the following sections System Structure Diagrams SSDs A distributed system consists of its components and the communication channels among them An embedded system communicates with its environment To describe the static aspects of distributed systems viewing it as a network of interconnected components with the ability to exchange messages over channels we use system structure diagrams SSDs Graphi cally SSDs as shown in Fig are similar to data ow diagrams represented by graphs with labelled rect angular nodes symbolizing components arrow shaped labelled edges symbolizing channels and circles at both ends symbolizing ports