A technique for specifying interface modules for real-time systems

Documentation plays a key role as a component of design process, and a preview of a task before it comes to be executed. A well-specified task might not take less implementation time than one without documents, but one of the obvious advantages is that misunderstandings are avoided and readable specification makes it easy for the successive developers to exploit or modify the software or hardware design. Such merit is particularly useful for aviation and military applications in which reliability and maintainability are very important aspects for judging the success of a project. Interface Modules (IM) are modules that encapsulate input or output device hardware and the related software, so that the application software can be written without specific knowledge of the particular devices used. Replacing or modifying an interface device will only lead to changes in the IM, rather than changing the other modules in the whole system. In real-time and embedded systems, an IM will often relate real-valued external quantities (e.g., time, positions in space) with discrete valued software quantities. An IM specification must therefore use a combination of notations and formalisms. In this paper, we present a technique for IM specification that is an extension of the System Requirements Documentation technique presented in ill, which is based on the Software Cost Reduction (SCR) method. An IM is specified as a "sub-system" that interacts with both the external environment and other software modules in the larger system. ne interface quantities are modeled as functions of time and the behaviour is described in terms of conditions, events and mode classes. This technique facilitates concise and formal description of the module behaviour, including tolerances and delays.