Delays in Esterel

The timing details in many embedded applications are inseparable from other behavioural aspects. Time is also a resource; a physical constraint on system design that introduces limitations and costs. Design and implementation choices are often explored and decided simultaneously, complicating both tasks and encouraging platform specific programs where the meaning of a specification is mixed with the mechanisms of implementation. The Esterel programming language is ideal for describing complex reactive behaviours. But, perhaps surprisingly, timing details cannot be expressed without making significant implementation choices at early stages of design. We illustrate this point with an example application where reactive behaviour and physical time are intertwined. A simple solution is proposed: add a statement for expressing delays in physical time. While there are similar statements or library calls in many programming languages, the novelty of our proposal is that the delay statements are later replaced with standard Esterel statements when platform details become available. Delays are thus expressed directly in terms of physical time, but later implemented as a discrete controller using existing techniques. This approach is familiar in control system design where analytical models are constructed in continuous time and then later discretized to produce implementations. We present some ideas for performing the translation and outline some of the remaining challenges and uncertainties.