Scheduling The Can Bus With Earliest Deadline Techniques

Controller area networks (CANs) are widely used in real-time automobile control and are gaining wider acceptance as a standard for factory automation. This paper discusses the applicability of earliest-deadline-first (EDF) techniques to the scheduling of CAN messages. EDF can guarantee higher network utilization than fixed-priority schemes like deadline- or rate-monotonic (DM, RM), but it is difficult to implement in local area networks or local buses. The reason is the need for updating the deadlines (priorities) at each scheduling round and the limited number of priority levels offered by the arbitration protocol. This deadline encoding problem results in an additional priority inversion factor when considering the schedulability analysis of hard real-time messages. This paper describes an effective deadline encoding method and discusses its implementation and its effects on the guarantee analysis. In spite of a limited processor overhead (less than 5% of CPU time), the proposed EDF implementation allows an increase (up to 20%) in the feasible network workload. This tradeoff will be made more convenient as controller technology evolves.