In embedded systems with severe resource constraints, a `time-triggered hybrid' (TTH) scheduler can be employed as an alternative to a fully pre-emptive design. A TTH approach allows the system designer to create a static schedule made up of 1) a collection of tasks that operate co-operatively and 2) a single — short — pre-emptive task. Although well matched to the needs of an important group of embedded systems (including control and data acquisition systems), the predictable performance of a TTH scheduler can be jeopardized if tasks exceed their predicted worst-case execution times. To address this problem, a novel TTH framework is described in this paper, which has `Task Guardians' associated with both the co-operative tasks and the pre-emptive task. The resulting framework is shown to have very limited resource requirements but highly predictable behaviour, even in the event of task overruns. Finally, a simplified design (supporting only co-operative tasks) is also described.
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