1. 1. Introduction 1.1 Background 1.2 Real-time applications 1.3 Misconceptions 1.4 Issues in real-time computing 1.5 Structure of a real-time system

2. Spring 2002Real-Time Systems (Shin) 2 1.1 Background  Definitions – Real-time systems correctness of system operation depends on temporal characteristics as well as logical and functional characteristics – Timing constraints deadline, period, execution time, etc. – Real-time applications those that must satisfy timing constraints, typically, hard real-time

3. Spring 2002Real-Time Systems (Shin) 3  Embedded computer systems – All embedded systems are real-time systems, but not all real-time systems are embedded systems. – real-time vs. embedded: sometimes, interchangeable (cf. rectangles vs. squares)  Real-time vs. General-purpose – Real-time computer systems differ from their general-purpose counterparts in two important ways. (1) They are much more specific in their applications. (2) The consequences of their failure are more drastic.

4. Spring 2002Real-Time Systems (Shin) 4 Response time requirements for real-time applications 1 s 100 ms 10 ms 1 ms 100  s 10  s  s 1 ns response time applications Speech and audio systems Process control systems and industrial automation Fire alarm Medical diagnosis Robot controllers Flight simulation Telemetry control Network control

5. Spring 2002Real-Time Systems (Shin) 5  Brief history – 1950’s 1956: computerized process control in a Texaco refinery – 1960’s chemical industry, NASA – 1970’s minicomputers, real-time executives – 1980’s microcomputers, distributed control – 1990’s parallel computers, open systems

6. Spring 2002Real-Time Systems (Shin) 6  Task classes – hard vs. soft vs. firm real-time tasks task value functions – periodic vs. aperiodic tasks cf. sporadic tasks: aperiodic tasks with a bounded interarrival time – critical vs. noncritical tasks value time deadline 0 soft hard firm

7. Spring 2002Real-Time Systems (Shin) 7 1.2 Real-Time Applications  Industry, defense, weapons – Nuclear plants – Process control – Patient monitoring – Fly-by-wire avionics, Spacecraft – Guided missile control – SCADA – Signal processing (e.g. radar)  Business – Real-time databases, OLTP – Multimedia applications (e.g. VOD)

8. Spring 2002Real-Time Systems (Shin) 8  Simple digital control: Typical example – For sampled data systems under PID control the mth output f m for the mth sampled input f m = f m-1 + Δf m where Δf m is the sum of the P(proportional), I(integral) and D(derivative) terms. – For example, Δf m = k p (ε m – ε m-1 ) + k i ε m + k d (ε m-2 ε m-1 + ε m-2 ) where k p, k i and k d are proportional coefficients for P, I and D, respectively, and ε m = input_value – ref_value – Sampling is a periodic behavior. sampling period  periodic task In multirate systems, the state is defined by multiple state variables whose periods may be different.

9. Spring 2002Real-Time Systems (Shin) 9 DAC Computation (Control algorithm implementation) ADC Plant ActuatorSensor input value output value reference value Implementation with an infinite loop: An example initialize I/O ports, internal control variables; set timer to interrupt periodically with period T; at each timer interrupt, do obtain input; compute control output; send output to the plant; end do;

10. Spring 2002Real-Time Systems (Shin) 10  Conveyer belt – An example electric motor conveyer speed counter interface control ‘ speed ’ ‘ adjust ’ alarm-detector actuator sensor computer

11. Spring 2002Real-Time Systems (Shin) 11 An example: Software control structure of a flight controller Do the following in each 1/180-second cycle Validate sensor data and select data source: in the presence of failure, reconfigure the system Do the following 30-Hz avionics tasks, each once every six cycles: keyboard input and mode selection data normalization and coordinate transformation tracking reference update Do the following 30-Hz computation, each once every six cycles: control laws of the outer pitch-control loop control laws of the outer roll-control loop control laws of the outer yaw- and collective-control loop Do each of the following 90-Hz computations once every two cycles, using outputs produced by 30-Hz computations and avionics tasks as input control laws of the inner pitch-control loop control laws of the inner roll- and collective-control loop Compute the control laws of the inner yaw-control loop, using outputs produced by 90-Hz control-law computations as input Output commands. Carry out built-in test. Wait until the beginning of the next cycle.

12. Spring 2002Real-Time Systems (Shin) 12 1.3 Misconceptions (Stankovic)  There is no science in real-time system design.  Advances in supercomputer hardware will take care of real-time requirements.  Real-time computing is equivalent to fast computing.  Real-time programming is assembly coding, priority interrupt programming, and device driver writing.  Real-time systems research is performance engineering.

13. Spring 2002Real-Time Systems (Shin) 13  The problem in real-time system design have all been solved in other areas of computer science or operations research.  It is not meaningful to talk about guarantee-ing real-time performance because of imperfect software/hardware/environment.  Real-time systems function in a static environment.

14. Spring 2002Real-Time Systems (Shin) 14 1.4 Issues in Real-Time Computing  Specification and verification  Task scheduling  Real-time operating systems  Real-time programming languages  Distributed real-time databases  Fault tolerance  Real-time system architectures  Real-time communication  Clock synchronization

15. Spring 2002Real-Time Systems (Shin) 15 1.5 Structure of a Real-Time System  Logical view controlled process sensorsjob list clock actuators trigger generator execution displayoperator Environ- ment

16. Spring 2002Real-Time Systems (Shin) 16  Schematic decomposition Central cluster computing unit Peripheral cluster data converters Low-rate cluster sensor and actuators