6. MRICDF: A Polychronous Model for Embedded Software Synthesis

Safety critical applications require embedded software that can guarantee deterministic output and on time results during execution. Synchronous programming languages are by nature focused on providing the programmer with deterministic output at fixed points in time [1, 2]. Synchronous programming languages such as Esterel [3], LUSTRE [4], SIGNAL [5], etc., have successfully generated sequential software for embedded applications targeting avionics, rail transport, etc. The underlying formalism of these languages is robust and the associated software tools provide correctness preserving refinement from the specification to implementation. But even within the class of synchronous programming languages, there are very distinctive Models of Computation (MoC) [6]. Esterel and LUSTRE use an external global tick (or global clock) as reference for events that occur in the system. SIGNAL has an entirely different Model of Computation, where no assumptions are made on the global tick while designing the system. Each event has its own tick associated with it and for compilation the analysis of the specification would have to yield a totally ordered tick from the specification. In its absence, a variable (also known as signal) is constructed which has an event happening in synchrony with every event occurring in the system. This signal can be viewed as a root clock of the system with a hierarchical tree structure relating it with every other signal. Since events can occur at each signal at different rates, this MoC is also known as polychronous or multi-rate.