Abstract: Both the function and finish time of an embedded real-time task affect the physical behavior of an embedded control system. Traditional process/thread model deals with timing indirectly with priorities, lacking timing semantics, so that the finish time of a task is of no predictability. Henzinger has come up with the LET（Logical Execution Time） model, which describes timing requirements clearly using coordinate timing semantic, however, it’s still based on process/thread model, which may introduce timing-unpredictability. By combining Servant/Exe-Flow Model and LET model. This paper presents a new time-predictable programming model, Timed SEFM, which uses refined SEFM model to deal with functionality and uses LET model to deal with timing. An implementation of intelligent car system is presented as a case study.

Key words: hard real-time, control system, programming model, time predictability, Logical Execution Time（LET）, servant/exe-flow

摘要： 实时任务的功能和其完成时间共同影响嵌入式控制系统的物理行为。传统的进程/线程模型缺乏时间语义，时间属性只能用优先级间接表达，任务的实际完成时间不具有确定性。Henzinger提出的LET（Logical Execution Time）编程模型用协作式的时间触发语义明确描述时间需求，但其所基于的操作系统仍沿用进程/线程模型，仍会引入时间不确定性。结合服务体/执行流模型SEFM（Servant/Exe-Flow Model）和LET模型，定义了一种时间确定的编程模型Timed SEFM，用改进的SEFM模型描述系统的功能行为，用LET描述系统的时间行为。把智能小车控制系统的实现作为研究实例。

关键词: 硬实时, 控制系统, 编程模型, 时间可预测性, 逻辑执行时间, 服务体/执行流