A Flexible Ada Framework for Jitter-Sensitive Mixed-Criticality Real-Time Systems

Real-time systems often require combining time-triggered (TT) and event-triggered (ET) execution models to balance predictability and flexibility. Previous work proposed a unified framework supporting both paradigms, enabling the integration of jitter-sensitive activities within a static TT schedule while preserving the responsiveness of ET execution. However, that framework did not address the requirements of mixed-criticality systems (MCS), where tasks may exhibit different execution-time assumptions depending on the system criticality level. This paper extends the original framework, designed for Ada under the Ravenscar profile, to support mixed-criticality workloads. First, the task model is enhanced to incorporate multiple execution-time estimates per job, allowing tasks to adapt their behaviour across criticality levels, including the possibility of selectively disabling jobs. Second, the TT scheduling model is extended to support criticality-aware execution, introducing adaptive slot durations, application-level overrun handling, and mechanisms to control the system criticality level at run time. Third, the framework preserves the semantic consistency of multi-frame tasks under criticality-level changes by defining a clear separation between system-wide and task-local criticality. Additionally, the mode-change mechanism is extended to support timed mode changes, enabling precise control over plan transitions and facilitating synchronisation across distributed schedules. The proposed approach maintains the predictability of TT execution while providing the flexibility required to support mixed-criticality behaviour. An execution example illustrates the applicability of the framework and the interaction between its main components.