Embedded and Real Time Systems Lecture

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Embedded and Real Time SystemsLecture 4

• David Andrews
• dandrews_at_eecs.ukans.edu

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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

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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

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Processing Attributes (Continued)
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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

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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

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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