Embedded and Real Time Systems Lecture #4 David Andrews

What We Will Cover Today Processing Timing Attributes –Purely Cyclic –Mostly Cyclic –Asynchronous and somewhat predictable –Asynchronous and unpredictable Mapping Into Real Time Systems –Hard Real Time Systems –Soft Real Time Systems

Processing Time Attributes Processing Time Attributes From Controlled Object Define a “Task” as a Control Thread –Can have 1 or many independent threads being processed by computer –Scheduling Theory Developed To Host Multiple Threads on Common Hardware –Resource Allocation Depends on Attributes Purely Cyclic –Every task executes periodically. –I/O Operations Performed by Polling –All Resources Statically Allocated (CPU Cycles, Memory, I/O) –Typically Non Networked Systems

Processing Attributes (Continued) Mostly Cyclic –Most Tasks Operate Periodically –System Must Also Respond To Asynchronous External Events External Commands, Network Requests –Modern Avionics In This Class Asynchronous and Somewhat Predictable –Most Tasks Not Periodic, Time Between Invocations of a Task Can Very Considerably –Resource Needs May Very During Different “Periods” –Variations Can Be Bounded Or With Known Statistically Properties

Processing Attributes (Continued) Asynchronous and Unpredictable –Tasks Respond to Asynchronous Events –System Usually Modeled As Stochastic System Can Give Statistical Guidelines –Hard To Make Guarantees

Hard/Soft Real Time Systems Hard Real Time Systems –System Must Guarantee Deadlines or Catastrophic Events Happen –Tasks Modeled As Purely Cyclic Resources Statically Assigned To Task Known Worst Case Execution Times Still Meet Timeliness Needs Built In Redundancies (Hot Spares) Can Still Work With Modifying Schedules Soft Real Time Systems –System Makes Best Attempt to Meet Deadlines Some Partial Benefit Exists if Task Partially Completes, or Completes Late Quality of Service Defined and Studied to Optimize Soft Real Time Systems

Embedded Architecture Requirements Underlying Hardware Not Aware of Real Time Requirements –Scheduling Is Software Abstraction for Time Keeping On Top of Timer and Interrupt –Time Triggered –Event Triggered Priority Scheduling Periodic Scheduling QoS Methods –Resource Control Polling Interrupts