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Chapter 13 Components in Real-Time Systems. Overview. Introduction Characteristics and challenges of real-time systems Real-time component models Designing component-based real-time systems Composition of components Example: RT components in Rubus OS. Introduction. Embedded computers

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Chapter 13

Components in Real-Time Systems

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Overview
Overview

  • Introduction

  • Characteristics and challenges of real-time systems

  • Real-time component models

  • Designing component-based real-time systems

  • Composition of components

  • Example: RT components in Rubus OS

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Introduction
Introduction

  • Embedded computers

    • Medical control equipment, mobile phones, and vehicle control systems.

    • Most of such embedded systems can also be characterized as real-time systems.

    • They must usually meet stringent specifications for safety, reliability, limited hardware capacity etc

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Characteristics and challenges of rts
Characteristics and Challenges of RTS

  • Real-time systems are computing systems in which the meeting of timing constraints is essential to correctness.

  • If the system delivers the correct answer, but after a certain deadline, it could be regarded as having failed.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Tasks
Tasks

  • Real-time systems can be constructed of sequential programs, but typically they are built of concurrent programs, called tasks.

  • Tasks are usually divided into:

    • Periodic tasks: consist of an infinite sequence of identical activities, called instances, which are invoked within regular time periods.

    • Non-periodic : are invoked by the occurrence of an event.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Scheduling
Scheduling

  • Offline scheduling:

    • The scheduler has complete knowledge of the task set and its constraints.

  • Online scheduling:

    • Make their scheduling decisions during run-time.

  • Deadline:

    • Is the maximum time within which the task must complete its execution with respect to an event.

    • Real-time systems are divided into two classes, hard and soft real-time systems

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Limited resources
Limited Resources

  • Common CBSE technologies (JavaBeans, CORBA and COM) are seldom used as they:

    • Require excessive processing requirements

    • Require excessive memory requirements

    • Provide unpredictable timing characteristics

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


System level analysis
System Level Analysis

  • At system level we analyze to determine if the system composed fulfils the timing requirements.

    • Several different mature analysis methods exist, for example, analysis for priority-based systems and pre-run-time scheduling techniques

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Real time component models
Real-time Component Models

  • Using a standard operating system in a real-time application, such as windows NT must be done carefully, as it was designed to be used so.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Application specific component models
Application-specific Component Models

  • Maintain a component library which the application engineer can use when developing an application.

  • In addition to infrastructure components, domain specific component models, which in fact have been used for many years for certain domains must be considered.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Iec 61131 3 application structure
IEC 61131-3 Application Structure

Configuration

Variable

Resource

Resource

access

path

Task

Task

Task

Task

FB

Function

Block

Program

Program

Program

Program

Variable

FB

FB

FB

FB

Global and

direct

variables

Execution

control

path

Access

path

Communication

Function

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


A configuration in iec 61131 3
A Configuration in IEC 61131-3

  • Encapsulates all software for an application and consists of one or several resources which provide the computational mechanisms.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


A program in iec 61131 3
A Program in IEC 61131-3

  • A program is written in any of the languages proposed in the standard, for example:

    • Instruction lists

    • Assembly languages

    • Structured text

    • A high level language similar to Pascal

    • Ladder diagrams

    • Function block diagrams (FBD)

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Function block diagram
Function Block Diagram

A simple function block diagram representing a feedback control loop.

FB1

FB2

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


A port based object approach
A Port-based Object Approach

  • The model is based upon the development of domain-specific components which maximize usability, flexibility and predictable temporal behavior.

    • Independent tasks are the bases for the PBO model.

    • Whenever a PBO needs data for its computation, it reads the most recent information from its in-ports, irrespective of its producer.

    • The PBOs are in their nature periodic and the system can be analyzed using traditional schedulability analysis.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


A port based object
A Port-based Object

Configuration parameters

Variable input ports

Variable output ports

Port-based

object

Resource ports for communication with sensors and actuators

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Designing component based rts
Designing Component-based RTS

System specification

Component library

Top-level design

Detailed design

Architecture analysis

Create specifications for the new components

Add new components to library

Implement and verify new components using classical development methods

Scheduling / interface check

Obtain components timing behavior on target platform

System verification

Final product

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Top level design
Top-level Design

  • The first stage of the development process involves de-composition of the system into manageable components

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Detailed design
Detailed Design

  • At this stage a detailed component design is performed, by selecting components to be used from the candidate set.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Architecture analysis
Architecture Analysis

  • At this stage it is time to check that the system under development satisfies extra-functional requirements such as:

    • Maintainability

    • Reusability

    • Modifiability

    • Testability

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Scheduling1
Scheduling

  • At this point we must check that the temporal requirements of the system can be satisfied, assuming time budgets assigned in the detailed design stage.

  • In other words, we need to make a schedulability analysis of the system based on the temporal requirements of each component

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Wcet verification
WCET Verification

  • Performing a worst-case analysis can either be based on measurements or on a static analysis of the source code.

  • What is more interesting in the test cases is the execution time behavior shown as a function of input parameters as shown in the following slide.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


An execution time graph

Execution time

Input

domain 2

domain 1

domain 3

An Execution Time Graph

  • The execution time shows different values for the different

    input sub-domains.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Maximum execution time per sub domain

Execution time

Input

domain 2

domain 1

domain 3

Maximum execution time per sub-domain

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Implementation of new components
Implementation of New Components

  • New components; Those not already in the library must be implemented. The designer of the component has two requirements:

    • The functional requirements

    • The assigned time budget

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


System build and test
System Build and Test

  • Finally, we build the system using old and new components.

  • We must now verify the functional and temporal properties of the system obtained.

  • If the verification test fails, we must return to the relevant stage of the development process and correct the error.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Component library
Component Library

  • Is the most central part of any CBSE system as it contains binaries of components and their descriptions.

  • A component library containing real-time components should provide the following:

    • Memory requirements

    • WCET test cases

    • Dependencies

    • Environment assumptions

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Composition of components
Composition of Components

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


End to end deadlines
End-To-End Deadlines

  • End-to-end deadlines

    • Are set such that the system requirements are fulfilled in the same way as the time budgets are set

    • Should be specified for the input to and output from the component since the WCET cannot be computed since its parts may be executing with different periods.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Specification of timing attributes
Specification Of Timing Attributes

  • We specify virtual timing attributes of the composed component, which are used to compute the timing attributes of sub-components, ie:

    IF virtual period is set to P,

    THEN the period of a sub-component A should be fA * P

    AND the period of B is fB * P,

    WHERE fA and fB are constants for the composed component, which are stored in the component library

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Rt components in rubus os
RT Components in Rubus OS

  • Rubus:

    • Is one of a few real-time operating systems currently available which have some concept of components.

    • Is a hybrid operating system, in the sense that it supports both pre-emptive static scheduling and fixed priority scheduling.

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


A task and its interfaces
A Task and Its Interfaces

  • The timing requirements are specified by release-time, deadline, WCET and period

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


A composed system in the red model of rubus
A Composed System in the Red Model of Rubus

  • The task depicted below is required to execute before the outputBrakeValues task, (i.E. Task BrakeLeftRight precedes task outputBrakeValues).

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


Composition of components in rubus
Composition of Components in Rubus

Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems


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