From Time-Triggered to Time-Deterministic Real-Time Systems

With the increased use of powerful, performance-optimized hardware components in embedded systems, timing prediction is getting more and more complex. Thus while the execution speed of software is generally increasing, it is getting more and more difficult (if not infeasible) to perform an accurate and safe timing analysis of software that runs on those high-end embedded computer systems. This paper presents a very rigid software execution model for building distributed hard real-time subsystems that are time predictable. The software model is based on the time-triggered communication model. It uses a purely timetriggered input-output interface and relies on single-path code (code that is free from input-data dependent control flow) in both the operating system and application software. Tasks are only preempted at pre-planned task preemption points and a simple clock synchronization keeps the operations of the hard realtime subsystem in synchrony with the real-time environment. The proposed execution model yields software that is time-predictable by construction. Verifying temporal correctness and tracing the timing behavior of this software is trivial.