Inroduction to Real time systems

Real Time System-- Subra Ganesan 1

Real Time SystemsA brief introduction for ECE 573 students

Subra Ganesan

Professor, Oakland University, Rochester, MI 48309

Rochester, MI 48309


Real time embedded systems are used in many applications

widely.

In this lecture you learn:

Introduction to RTS (Real Time Systems)

Definition

Example Car and Driver

Characteristics of RTS


Introduction

Any system where a timely response by the computer to

external input/condition is vital is a real-time system.

Timely: Meet the deadline.

Deadline: Hard and soft

Complete task: Accurately or estimate within deadline.

Hard Real-Time Systems: Aircraft,Nuclear Reactor

control.

Soft Real-time Systems: Multimedia,Internet access.


Automotive DSP Applications

Navigation Cellular Phone Comfort/Security Equipment

Powertrain Anti-Lock Brakes Voice Command Electronic Steering

Global PositioningDigital MapAdaptative cruiseParking aid

Digital RadioCompact DiskGraphic EqualizerSound Enhancement

Voice DialingFiltering

Electrical seatsWindow lifterAirbagAnti theft

Electrical brakes (BBW) Speech Recognition(E.G. lights, window)Speaker Verificationfor Secure Lock

Electrical powerSteering (EPS)4-Wheel steering

Hydraulic or Air Pressure ControlDynamic Control

Audio Processing

Which are the Hard/Soft Real Time Systems ?


Driver is the real time controller

Car is the controlled process

Road is the operating environment

Mission is to reach the destination

Performance Measure:

1)Time to reach the destination under various road conditions

2)Safety of the driver even if the destination is not reached

Task deadlines are not constants ,varies with the operating environment.

Writing formal specification and relating them is difficult.

A Car and Driver Example


Issues in Real-time Computing

Research areas cover: Computer architecture, fault tolerant

computers, networks, embedded systems, standards, digital

communication, operating system clock synchronization,..

Example:

Task-Scheduling:For a normal system the goal is fairness to all

tasks. Round-robin scheduling.

For a RT system: Meet the deadlines for critical and high priority

tasks is the goal.

Task-Execution time should be predictable in RT system. For a

cache based RT system memory access time varies.


Structure of a Real-time System

Controlled

processSensors

Job

ListClock

Trigger

generator

ActuatorsExecution

Display Operator

Environment

Figure: A Schematic Diagram of a real-time system


Task classes

Periodic and aperiodic tasks

Aperiodic tasks with a bounded inter-arrival time are

called sporadic tasks

Critical and non-critical tasks.


OPERATING SYSTEM ISSUES

Task assignment and scheduling

Communications protocols

Failure management and recovery

Clock synchronization algorithms

Other issues

Programming languages

Databases

Performance measure


Characteristics of RTS

Performance :

validation process includes

(1) Checking design correctness using formal and informal

methods,

(2) Characterizing performance

(3) Reliability

Choosing performance measure is crucial.

e.g : average time, peak time or variance.

Clock cycle may be misleading.

For soft RT, degraded response time is acceptable.


Traditional Performance Measures

Reliability is the probability the system will not fail in an

interval

Availability is the fraction of time for which the system is up.

Throughput is the average number of instructions per unit time

the RTS can process.

Capacity Reliability(CR): Interval time when reliability is high.

Expected capacity survival time is the expected time for CR to

drop to specific value.

Mean computation before failure

Computational reliability

Computational availability

Per formability

Accomplishment levels

Cost functions for hard deadlines: Cost Vs Response times


Definition of Real Time System

There are many interpretation of a real time system; however, they

all consider the notion of response time the time take for the

system to generate output from some associated input.

Any information processing activity or system which has to

respond to externally generated input stimuli within a finite and

specified period.

A real-time system is a system that is required to react to stimuli

from the environment (including the passage of physical time)

within time intervals dictated by the environment.

The correctness of a real-time system depends not only on the

logical result of the computation, but also on the time at which the

results are produced.


Definition of Real Time System

Hard real-time system and soft real-time systems:

(1) Hard real-time systems are those where it is

absolutely imperative the response occur within

the specified deadline.

(2) Soft real-time systems are those where

response time are important but the system will

still function correctly if deadlines are

occasionally missed.

Examples of Real-time System:

Process Control

Process control

Computer

Valve Temperature

transducer

Stirrer

Chemicals

and

materials

Finished

Product

Operator

console

Plant

A Process Control System


Examples of Real-time Systems:

Manufacturing

Production control

computer

. . . .

Machine tools Manipulators Conveyor belts

A Production Control System

PartsFinished

Product


Examples of real-time Systems:

Communication,command and control

Command Post

Command and

control computer

Temperature,pressure,power and so on

Terminals Sensors/actuators

A Command and Control System


Examples of Real-time Systems-

Generalized Embedded Computer Systems

Algorithms for

digital control

Data logging

Data retrieval

and display

Operator

interface

Real-time computer

Real-time

clock

Database

Operators

console

InterfaceEngineering

system

Remote

monitoring

system

Display

devices

A Typical

Embedded System


Characteristics of Real-time Systems

A real-time system has many special characteristics (either inherent or imposed). Clearly, not all real-time systems will exhibit the same characteristics.

(1) Large and complex.

-Largeness of the system is related to variety which is caused by the continuous changing requirements of the real world. Traditional indicators, such as number of instructions and development effort, are just symptoms of variety. Therefore, real-time systems undergo constant maintenance and enhancements during their lifetimes. They must be extensible.

-Although real-time software is often complex, it cab be broken down into smaller components which can be managed effectively.



(2) Manipulation of real numbers:

-Many real-time systems involve the control of some

engineering activity and most controllers are nowadays

implemented by computers.

-The implementation of control algorithms using

computers is often mathematically complex and require high

degree of a real time programming language, therefore, is the

ability to manipulate real, fixed- or floating-point numbers.



(3) Extremely reliable and safe:

-The more society passes the control of its vital functions to computers, the more important it becomes that those computers do not fail.

*failure of a system involved in automatic fund transfer between banks may cause the loss of millions of dollars.

*a faulty component in electricity generation could result in the failure of the vital life-support system in a intensive care unit.

-The large size and complexity of real-time systems make the reliability become worse; not only must one expect difficulties inherent in the application to be taken into account, but also those introduced by faulty software or hardware design.



(4) Concurrent control of separate system

components:

-In a typical embedded computer example, the

program has to interact with an engineering

system(which will consist of many parallel activities

such as robots, conveyor belts, sensors and so on) and

the computers display devices, the operators console,

the database and the real-time clock.



-A major problem is how to express that concurrency in the structure of the program.

Older real-time programming languages, for example RTL/2,Coral 66 and C, relied on operating system support for concurrency.

The more modern languages, such as Ada, Pearl and Occam, have direct support for concurrent programming.



(5) Real-time facilities:

-Response time is crucial in any real-time

system.Unfortunately, it is very difficult to design and

implement systems which will guarantee that the

appropriate output will be generated at the appropriate

times under all possible conditions. For this, reason

real-time systems are usually constructed using

processors with considerable spare capacity.



-Given adequate processing power, language and run-time support, the programmer is to give:

#specify times at which actions are to be performed.

# specify times at which actions are to be completed.

#responses to situations where all or any of the timing requirements cannot be met.

# responses to situations where all or any of the timing requirements are changed dynamically.



(6) Interaction with hardware interfaces:

-Most of the real-time systems requires the computer components to interact with the external devices (e.g. sensors, actuators). These devices interface to the computer via input and output registers.Devices may also generate interrupts to signal to the processor.

-The methods used in the past is:

~under the control of operating system, or

~the application programmer is required to resort to assembly language control and manipulate the registers and

interrupts.

-The methods used nowadays are:

~the facilities provided by modern high level real-time languages allows the control to be direct, and not through a layer of operating system.



(7) Efficient implementation and execution environment:

-In real-time systems, what is actually important is the meeting of deadlines of adequate response times in a particular execution environment.

The term execution environment is used to mean those components which are used together with the applications code to make the complete system: the processors, networks, operating system and so on. Clearly, the more efficient use of the execution environment, the more likely the requirements will be met.

End


Inroduction to Real time systems

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