Embedded and Real Time Systems Lecture #2 David Andrews

What We Will Cover Today Fundamental Concepts and Definitions –Functional and Temporal Requirements –What Is A Distributed System Things That Matter Most In Embedded Systems –A Look At Tradeoffs –Examples Where We Are Going Next –Example System –Modeling/Requirements/System Architecture

Concepts To Keep In Mind Elements of Real-Time Systems –Centralized vs Distributed –What Does Real Time Mean Control Loop Issues –Latencies Various Embedded Systems Constraints –Fail Safe vs. Fail Operational –Guarantees vs. Best Effort –Resource Adequacy –Event Triggered vs. Time Triggered

Real Time “System” Organization System = 1.Computer (Centralized or Distributed) 2.+Controlled Plant (Bigger System Computer Is Part of) 3.+Plant I/O (Sensors, Actuators) 4.+Operator (Systems without are closed) 5.+Operator Interface 6.+Physical Environment Real time computer system has correctness of behavior dependant on both logical results of computations and also physical instant when results are produced

Real Time System Design Engineers Job 1.Make It Work Correctly (Functionality + Real Time) and Safely 2.Make It Meet Real World Constraints (Size, Power, Weight) 3.Optimize ( Cost Performance, Performance, Usability, etc.)

Functional Requirements Values at a particular instant of time are state variables –Moving Car Has State Variables of position, speed, etc.. –State Variables of interest are real-time entities –Only Valid For Small Time Instance Observability (Computer Input/ Data Collection) Sources 1.Environment (Observed State) 2.Controlled Plant (Existing State) 3.Operator/Desired Intent (Desired Next State/Set Point/”Sweat Spot”) Controlability (Computer Output) Changes State Variables (Sphere of Control) 1.Plants Effect On Environment (Change Speed, Direction, Mixture, etc) 2.State of Plant Itself 3.Guiding Actions of Human Operator

Simple Example

Real Time Entities Real Time Entity is a state variable of interest –Each RT entity exists within a sphere of control Can observe outside sphere, but cannot alter –Each RT entity is only valid for a short period of time Observation occurs at a specific time instant and valid until next observation List Some RTE’s and Sphere of Control For Car Example –RTE’s: Light, Presence/Absence of Other Cars Car Dynamics: Speed, Direction, Brakes, Pistons, –Sphere of Control Car Can Only Observe Traffic Light, Presence/Absence of Other Cars Car’s Speed Within Sphere of Control of Car but Not Other Observers