Lecture-1 Time: 9:00-9:50 am

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Lecture-1 Time 900-950 am
Welcome to IS ZC 424 Software for Embedded
Systems Instructor in charge Virendra S Shekhawat

• Course Objective and Introduction
• Introduction to Real Time Embedded Systems
• Timeliness Constraints
• Questions???

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Course Objective

• Primary focus on
• Introduction to embedded and real-time system
  software process
• Issues and challenges in developing software for
  embedded systems
• Methodologies, tools and techniques for
  developing such kind of software
• Why we need a software development process for
  embedded and real time systems???

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The course will enable you to

• Identify the unique characteristics of real-time
  and embedded system
• Explain the general structure of a real-time and
  embedded system
• Define the unique design problems and challenges
  of real-time and embedded systems using UML
• Apply real-time systems design techniques to
  various software programs

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What is an embedded system? -1

• Definition(s)
• A embedded system contains a computer as a part
  of a large system and does not exist primarily to
  provide standard computing services to the user
• An embedded system is a special-purpose computer
  system designed to perform one or a few dedicated
  functions.
• A specialized computer system that is part of a
  larger system or machine. Typically, an embedded
  system is housed on a single microprocessor board
  with the programs stored in ROM

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What is an embedded system? -2

• Is a special purpose system designed to perform a
  few dedicated functions.
• Small foot prints (in memory)
• Highly optimized code
• Examples
• Robot controller, Microwave Oven, VCR, Cell
  phones, mp3 players
• The components in an mp3 player are highly
  optimized for storage operations. (For example,
  no need to have a floating point operation on an
  mp3 player!)

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What are Real Time Systems?

• Definition(s)
• Real time systems are concurrent systems with
  timing constraints.
• Real time systems are computer systems that
  monitor, respond to, or control external
  environment. This environment is connected to the
  computer system through sensors, actuators and
  other input-output devices
• Examples
• Medical systems, Manufacturing systems with
  robots, chemical plants, firing weapons, traffic
  control systems, process control for power
  plants, aircraft monitoring system etc

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Example Real-time systems
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Realtime System Characteristics

• RTS have to respond to events in a certain
  pre-determined amount of time.
• The time constraints have to be considered during
  planning, design, implementation and testing
  phases.
• Internal failures due to software and hardware
  fault have be handled satisfactorily.
• You cannot simply pop-up a dialog box that says
  send report or dont send report.
• Also external failures due to outside sources
  need to be handled.

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Real-time system concepts

• A system is a mapping of a set of input into a
  set of outputs.
• Example A digital camera
• Set of input including sensors and imaging
  devices producing control signals and display
  information
• Real time system can be viewed as a sequence of
  job to be scheduled.
• Response time
• It is the time between presentation of a set of
  inputs to a system and the realization of the
  required behavior, including availability of all
  associated outputs

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Computing in Real Time Systems

• By Computers we often mean
• General-purpose, digital computers that interact
  with humans and possibly with each other.
• Change in Trend
• More special purpose-computers
• More scenarios without humans in the loop
• More environments where computing is not the
  explicit / main goal.

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Contrast with traditional system

• Computers
• Interactive
• Discrete Time not counted
• Stand-alone / Networked
• Isolated Environment

• Embedded RT Systems
• Reactive (Event-driven)
• Timeliness is important to critical
• Computing Component may be controlled by external
  system
• Alien Environment (may affect function)

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Implications (of being embedded) 1

• Low cost hardware
• Software Development cost (one-time) vs.
  Per-shipped-item cost (recurring)
• Very Limited Storage Space
• Convoluted Algorithms and Aggressive Optimization
• Development Environment is different from
  execution environment
• Constrained process innovative techniques?

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Implications (of being embedded) 2

• Long and un-interrupted running times
• Cant reset flight control system or cardiac
  pacemaker at whim
• Alien (Uncontrolled) Environments
• Increase reliability constraints
• Timeliness expectations
• Specific approaches for design, development and
  testing

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Basic Model of A Real-Time System1
Input Interface
Input Conditioning Unit
Sensor
Human Computer Interface
Real Time Computer
Output Interface
Output Conditioning Unit
Operators
Actuator
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Basic Model of A Real-Time System 2

• Sensor
• Converts some physical characteristic of its
  environment into electrical signals
• Ex. Photo-voltaic cell, thermocouple
• Actuator
• Takes inputs from the output interface of the
  computer and converts electrical signals into
  physical actions
• Ex. Motion, change of thermal, electrical
  characteristic of some objects
• Heaters and motors are common examples of
  Actuators

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Basic Model of A Real-Time System 3

• Signal Conditioning Unit
• Signals have to be improved before sending to
  computer/actuator
• Ex. Voltage amplification, voltage level
  shifting, frequency range shifting and filtering
• Interface Unit
• A/D or D/A conversion by using a register

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Notions of (Real) Time

• Real-time system is the one in which logical
  correctness is based on both the correctness of
  the output as well as their timeliness.
• Time Constraints 3 common types
• Hard, Soft and Firm
• A soft real-time system is one in which
  performance is degraded by failure to meet
  response-time constraints.
• A hard real-time system is one in which failure
  to meet a single deadline may lead to complete
  and catastrophic failure.

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Hard Time

• Correctness of a response (to a stimulus)
  includes a description of timeliness
• Delay is unacceptable (i.e. treated as failure)
• Deadlines specified as points in time fixed
  relative to an event
• Cardiac pacemakers pacing time
• Robot
• Tasks are communication with the host system,
  logging all completed activities, sensing the
  environment to detect any obstacles present,
  tracking the objects of interest, path planning,
  effecting next move, etc.
• Hard Real time tasks Detecting obstacles and
  reacting to it

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Soft Time

• Timeliness constraints that do not strictly
  affect correctness
• Delays in individual computations acceptable
• Accumulated delays may lead to failure
• Typical scenarios Transactions, User Interfaces
• Delay in booking a ticket is tolerable
  repeated/accumulated delays may close the booking
  counter
• Delay in refreshing the screen is annoying but
  acceptable. Repeated/Accumulated delays can make
  the system unusable.
• Web browsing

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Firm Time

• A combination of hard and soft constraints
• Short term softness and long term hardness
• Cumulative hard deadline
• Monitoring software if a system does not
  respond within a certain deadline it is
  pronounced dead (which is a form of failure)
• Example Video conferencing, Satellite based
  tracking of enemy movements

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Events in a Real Time System

• Stimulus Events
• Generated by environment act on the system
• Produced asynchronously (i.e. aperiodically) or
  periodically
• Response Events
• Usually produced by the system in response to
  some stimulus events
• Ex temperature exceeds 1000 degree in a chemical
  plant ?system responds by switching off the
  heater

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Source of time constraints

• Time bound between a (stimulating) event and its
  reaction
• Reaction includes one or more tasks (sequential
  or concurrent)
• Bound determined by external often physical
  requirements
• Time bound is distributed (i.e. budgeted for each
  task that must be performed)

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Expectations of Timeliness

• Timeliness of behavior
• Expressed as actual execution time
• An associated budget time constraint
• Extreme case deadline (single time value)
• Schedulability of a system
• Hard real time systems all deadlines can be
  guaranteed (to be met) under all scenarios
• Often hard real-time is easier to specify and
  analyze (used as extreme case for others as well)

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Classification of timing Constraints

• Delay Constraint
• Notion of the minimum time that must elapse
  between the occurrence of two arbitrary events.
• t(e2) - t(e1) gt d
• Deadline Constraint
• Notion of permissible maximum separation between
  any two arbitrary events
• t(e2) - t(e1) lt d
• Duration Constraint
• Period of time over which event acts

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Source of time constraints

• Time bound between a (stimulating) event and its
  reaction
• Reaction includes one or more tasks (sequential
  or concurrent)
• Bound determined by external often physical -
  requirements
• Time bound is distributed (i.e. budgeted for each
  task that must be performed)

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Summary

• Basic characteristics of Real time Embedded
  systems
• Basic model of a RT system
• Various timing constraints

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• Questions???