Introduction to Realtime and Embedded Systems

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Introduction to Real-time and Embedded Systems

• Robert Oshana
• Engineering Manager
• Texas Instruments
• Roshana_at_ti.com

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Lecture 2

• Introduction to Real-Time Systems

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Agenda

• What is a real-time system?
• Characteristics of real-time systems
• Timing constraints
• Example

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What is a real-time system ?
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Introduction

• 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
• Oxford Any system in which the time at which
  output is produced is significant
• Input corresponds to some movement in the
  physical world
• Output has to relate to that same movement

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Introduction

• Lag from input time to output time must be
  sufficiently small for acceptable timeliness.
• Generally, real-time systems are systems that
  maintains a continuous timely interaction with
  its environment
• Correctness of a computation depends not only
  upon its results but also upon the time at which
  its outputs are generated

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Introduction

• A real-time system must satisfy bounded response
  time constraints or suffer severe consequences
• If the consequences consist of a degradation of
  performance, but not failure, the system is
  referred to as a soft real-time system (e.g. time
  adjusting system on computers over the network)
• If the consequences are system failure, the
  system is referred to as a hard real-time system.
  (e.g. emergency patient management system in
  hospitals)

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Reactive and embedded

• There are two types of real-time systems
• Reactive real-time system involves a system that
  has constant interaction with its environment.
  (e.g. a pilot controlling an aircraft)
• Embedded real-time system used to control
  specialized hardware that is installed within a
  larger system. (e.g. a microprocessor that
  controls the fuel-to-air mixture for automobiles)

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Signals from the environment
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Analog to digital and back
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RT event characteristics

• Real-time events fall into one of the three
  categories
• Asynchronous events
• entirely unpredictable
• the event that a user makes a telephone call..as
  far as the telephone company is concerned, the
  action of making a phone call cannot be predicted

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RT event characteristics

• Synchronous event predictable and occur with
  precise regularity if they are to occur (the
  audio and video in a movie take place in
  synchronous fashion)
• Isochronous events occur with regularity within
  a given window of time (audio bytes in a
  distributed multimedia application must appear
  within a window of time when the corresponding
  video stream arrives)
• Isochronous is a sub-class of asynchronous

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Real time vs Shared
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Characteristics of real time systems
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Large and complex

• Most of the problems associated with developing
  software are those related to size and complexity
• This largeness is related mostly to variety
• The real world is continuously changing
• large programs, like all complex systems, must
  continuously evolve
• Software programs tend to exhibit the undesirable
  property of largeness. This is mainly due to
  continuous change 

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Manipulation of real numbers

• Many real-time systems involve the control of
  some engineering activity
• outputs compared to a desired or reference signal
  to produce an error signal 
• A real-time requirement of many systems is to
  move to a new point set within a fixed time
  period
• adds to the complexity of the computations

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Reliable and safe

• The more society relinquishes control of its
  vital functions to computers, the more it becomes
  imperative that those computers do not fail
• In hostile environments, systems must be able to
  fail in a controlled way
• All expected difficulties inherent in the
  application must be taken into account (including
  those introduced by faulty software design!)

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Concurrent control of separate system components

• Typical RT embedded system consists of computers
  and sensors and actuators
• Usually several co-existing external elements
  which the computer must interact with
  simultaneously
• The very nature of these external elements is
  that they exist in parallel
• Actions performed by the computer must be carried
  out in sequence but give the allusion of being
  simultaneous
• data that must be collected and processed at
  various geographical points

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Real time facilities

• Response time is crucial to any embedded system
• Very difficult to design and implement systems
  which will guarantee the appropriate output will
  be generated at the appropriate times under all
  possible conditions
• Doing this and making use of all computing
  resources at all times is often impossible

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Real time facilities

• Real-time systems usually constructed using
  processors with considerable space capacity
• ensures worst case behavior does not produce any
  unwelcome delays during critical periods of the
  systems operation
• The designer must be cognizant of weight and
  power issues!

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Real time facilities

• Given adequate processing power, a good real-time
  programming language, and run-time support is
  required to enable the programmer
• to specify times at which actions are to be
  performed
• to specify times at which actions are to be
  completed
• to respond to situations where all timing
  requirements cannot be met
• respond to situations where the timing
  requirements are changed dynamically (mode
  changes)

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Interaction with hardware devices

• Nature of embedded real-time systems requires
  them to interact with the external world
• Many of the operational requirements for
  real-time systems are device and computer
  dependent
• These devices may generate interrupts in response
  to certain events and errors

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Efficient execution and the execution environment

• Real-time systems are time critical
• efficiency of their implementation is more
  important than in other systems
• Benefit of using a HLL is to allow the
  programmer to abstract away the details and
  concentrate on solving the problem
• not always true in the embedded system world
• Higher level languages can be used in real-time
  systems effectively

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Efficient execution and the execution environment

• A system operates in real time as long as its
  actions which have time constraints are performed
  with acceptable timeliness
• Acceptable timeliness is defined as part of the
  behavioral requirements for the system
• Resource management may be performed statically
  off-line or dynamically on-line
• Real-time resource management comes at a cost

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Efficient execution and the execution environment

• The degree to which a system is required to
  operate in real time cannot necessarily be
  attained solely by hardware over-capacity (e.g.,
  high processor performance)
• There must exist some form of real-time resource
  management
• Systems which must operate in real time need an
  balance of real-time resource management and
  hardware resource capacity

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Efficient execution and the execution environment

• Embedded systems have interactions with physical
  devices and processes whose dynamics follow their
  own laws in a system-wide physical time frame
• Physical (as opposed to logical) time is
  necessary in real-time resource management for
• relating events to their moments of occurrences
• state variables to their values -- e.g. for
  action time constraints
• measuring rewards or costs incurred as actions
  progress to completion
• history logging etc

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Timing Constraints
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Timing Constraints

• Define response time requirements for software
  and/or the environment
• Many simple cases
• key press response time lt 10 msec
• Four basic types of timing constraints
• stimulus-response
• response-response
• stimulus-stimulus
• response-stimulus

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Timing Constraints

• Stimulus is an action performed by the user or
  environment on the system
• Response is an action by the system on the user
  or environment

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Stimulus-Response

• Stimulus-response constraint that the system
  must produce a response in accordance with a
  specified timing relationship to an earlier user
  stimulus to the system

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Stimulus-Response

• Examples
• The system shall generate a dial tone within 15
  seconds of a user taking the phone off the hook
  (maximum constraint)
• The system shall arm the door no sooner than 1
  minute after the alarm on button is pressed
  (minimum constraint)
• Stimulus and response do not need to be adjacent
  in time

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Response-Response

• Response-response used to specify a temporal
  relationship that must exist between two
  arbitrary system responses in the environment
• Examples
• The system shall initiate the door closing
  operation within 20 seconds of locking the
  landing gear in the retracted state (maximum)

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Response-Response

• Examples
• The system shall generate a launch missile
  command no sooner than 5 seconds after generating
  a start battery warm up command (minimum)

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Stimulus-Stimulus

• Stimulus-stimulus enables us to specify
  expected behavior of a user or environment of a
  system in terms of maximum or minimum timing
  constraints between two stimuli
• Examples
• Users must type their password within 15 seconds
  of typing their identification or they will be
  denied access to the database (maximum)

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Stimulus-Stimulus

• Examples
• Pilots must not press the launch weapon button
  sooner than 10 seconds after pressing the fire
  ready button (minimum)

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Response-Stimulus

• Response-stimulus enables us to specify a
  temporal relationship that must exist between a
  system response and a subsequent user stimulus
• Examples
• The user must dial the complete phone number
  within 1 minute of hearing the dial tone
  (maximum)

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Response-Stimulus

• Examples
• The user may not make a menu selection sooner
  than 5 seconds after completion of the menu
  display (minimum)

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System vs user ??

• S-R and R-R define timing requirements on the
  system being specified
• Function must be implemented in such a way as to
  be fast enough (or slow enough) to meet the
  timing requirement

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Timing Constraints

• S-S and R-S constraints imply the system must be
  able to detect a violation of timing constraints
  by the user or environment
• Do not imply the software must be rapid or slow
  but there must be additional software
• detect inappropriate timed user stimuli
• generate alternative response to the user
• warning
• error message

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Example
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Monitoring and control systems

• Important class of real-time systems
• Continuously check sensors and take actions
  depending on sensor values
• Monitoring systems examine sensors and report
  their results
• Control systems take sensor values and control
  hardware actuators

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Security system

• System is required to monitor sensors on doors
  and windows to detect the presence of intruders
  in a building
• When a sensor indicates a break-in, system
  switches on lights around the area and calls
  police automatically
• Includes provision for operation without a mains
  power supply

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Security system

• Sensors
• Movement detectors, window sensors, door sensors.
• 50 window sensors, 30 door sensors and 200
  movement detectors.

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Security system

• Actions
• When an intruder is detected, police are called
  automatically.
• Lights are switched on in rooms with active
  sensors.
• An audible alarm is switched on.
• The system switches automatically to backup power
  when a voltage drop is detected.

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The R-T system design process

• Identify stimuli and associated responses
• Define the timing constraints associated with
  each stimulus and response
• Allocate system functions to concurrent processes

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The R-T system design process

• Design algorithms for stimulus processing and
  response generation
• Design a scheduling system which ensures that
  processes will always be scheduled to meet their
  deadlines

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Stimuli to be processed

• Power failure
• Generated aperiodically by a circuit monitor.
  When received, the system must switch to backup
  power within 50 ms
• Intruder alarm
• Stimulus generated by system sensors. Response is
  to call the police, switch on building lights and
  the audible alarm

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Timing requirements
Stimulus/Response
Timing requirements
Power fail interrupt
The switch to backup power must
be completed
within a deadline of 50 ms.
Door alarm
Each door alarm should be polled twice
per
second.
Window alarm
Each window alarm should be polled twice
per
second.
Movement detector
Each movement
detector should be polled twice
per second.
Audible alarm
The audible alarm should be switched
on within
1/2 second of an alarm being raised by a sensor.
Lights switch
The lights should be switched on
within 1/2
second of an alarm being raised by a sensor.
Communications
The
call to the police should be started within 2
seconds of an alarm being raised by a sensor.
Voice synthesiser
A synthesised message should be available
within
4 seconds of an alarm being raised by a sensor
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Summary

• Embedded systems and real-time systems are unique
• Must understand the constraints of these systems
• Failure to specify and define these constraints
  early can lead to trouble down the road
• Complexity and size is growing!