Embedded Systems

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Embedded Systems

• Dinesh Sharma
• Microelectronics group, EE department.
• IIT Bombay
• Mumbai 400 076

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• What are embedded systems?
• Differences from other micro-controller systems
• How are embedded systems designed?
  (Hardware and Software)
• Critical Design challenges
• Tools of the trade
• Future directions

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What are embedded Systems?

• An embedded system is closely integrated with the
  main system
• It may not interact directly with the environment
• For example A micro-computer in a car ignition
  control

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Examples of Embedded Systems

• MP3 music players
• Mobile phone units
• Domestic appliances
• Data switches
• Automotive controls
• Embedded memories to keep configuration
  information

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A typical Embedded System

• A typical embedded system would have
• A micro-controller to provide the intelligence
• Interfacing circuits to connect with the main
  application
• Real time software
• Dedicated hardware for functions whose
  implementation in software might be too slow
• Test and maintenance hardware

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So is embedded systems just another name for
micro-controller systems?

• Not at all!

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How are embedded micro-controller systems
different?

• An embedded micro-controller runs a single
  program which never terminates!
• There is no (separate) operating system or
  monitor program the operating system has to be
  merged with the application program.
• Most embedded micro-controllers have to respond
  in real time.
• In practice, all embedded systems are resource
  constrained. For example, an 8052 based system
  has only 256 bytes of RAM.

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Design of Embedded Systems

• Task partitioning between hardware and software
• Hardware design and integration
• Software development
• System integration
• Test strategies

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Partitioning

• Many tasks can be performed in hardware or in
  software for example timing.
• The choice between hardware and software is
  driven by considerations of speed, cost, need for
  flexibility in modification of underlying
  algorithms.
• Hardware adds a per unit cost where as
  software adds a fixed cost.
• Typically, only those functions are implemented
  in hardware whose speed specifications cannot be
  met by software solution.

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Hardware Design

• Since a micro-controller in an embedded system
  will run just one program all the time, hardware
  resources must be matched to the needs of this
  particular application.
• Modern technology has made it possible to put the
  entire electronics inclusive of sensors, analog
  circuits, digital circuits etc. on a single chip.
  (System On a Chip or SoC).
• The degree of integration used will depend on
  cost, need for small size, availability of
  components etc.

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Hardware design challanges

• Compatibility of system components with each
  other
• Interface design (linear to digital, asynchronous
  to synchronous etc.)
• Low power design
• Modularity and ability to upgrade the system in
  the field.
• Designing for easy testability is difficult.
• Time to market

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Low Power DesignEspecially important in 3 cases

• Battery operated and mobile systems e.g. a
  portable MP3 music player
• Systems with a limited power source such as a
  smart public call booth which derives power from
  phone lines.
• High complexity and speed systems where heat
  dissipation is a problem

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Low Power Design Techniques

• Power smart blocks which go to a low power or off
  mode when not in use.
• Reduced voltage swing on loaded buses
• Coding schemes which minimise the number of
  transitions on signal lines

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Mechatronics

• Mechanics controlled by Electronic Systems.

Mechanical System
Sensors Transducers
Actuators
Electrical System
User Interface
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An embedded system may contain Data Conversion
Devices A ? D D ? A Control Elements
MicroControllers Driver Circuits Motor drivers,
etc. User Interface Key Boards,
Display Transfer of control functions to
electronics can result in significant reduction
in cost, size, weight and response time.
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An example system using MEMS
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Sensing Position and Velocity

• Potentiometric measurements for position
• Linear Variable Differential Transformer

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Distance Measurement

• Slotted disk acts as light interrupter
• There are two channels which are in phase
  quadrature
• Edges are counted to give the position
• Alternate edges imply travel in the same dir.
• Two edges in the same channel imply reversal of
  direction.

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Accelerometers Principle of operation
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Measurement of Displacement

• Dual Capacitance differential method

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Putting it all together

• Hardware/Software partitioning
• Hardware Development
• Software Development
• Interfacing Electrical and Mechanical parts
• Test strategies

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Software Design

• Software must provide the algorithms etc. needed
  for implementing the applications.
• These algorithms often have an impact on the
  choice of hardware as well. For example, whether
  a DSP processor should be used or not.
• Most embedded system software needs to be real
  time software.
• Watchdog timers may be needed.

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Real Time Systems

• Real time systems have to guarantee that they
  will respond to an external event within a
  specified amount of time.
• Real Time systems dont have to be real fast.
  They do have to be reliably on time.

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Types of Real Time Systems

• Based on the type of timing guarantee they
  provide, real time systems are classified as
  soft real time or hard real time.

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Soft Real Time systems

• Soft real time systems provide a time guarantee,
  but missing an event is not catastrophic. For
  example, image decoding used during satellite TV
  reception must be completed within a frame
  time. If this guarantee is missed, there will be
  a visible glitch. Annoying but not catastrophic!

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Hard Real Time Systems

• Hard real time systems are used when missing a
  timing deadline will lead to catastrophic
  results. For example, a missile guidance system
  should not miss any events!

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So why use soft real time systems at all?

• Both soft and hard real time systems provide a
  real time guarantee. But if we can afford to miss
  a few events, this guaranteed response time can
  be much shorter.
• Soft real time systems would be used in
  non-critical applications, which require high
  speed.

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Design strategies forsoft and hard real time
systems

• The time guarantee provided by soft real time
  systems is statistical in nature whereas that
  provided by hard real time systems is absolute.
• Design of soft real time systems optimises
  average case response whereas hard real time
  systems must be designed for worst case
  situations.

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Is real time software any different?

• Interrupt handling has to be specially careful.
• Since interrupt handling and task scheduling is
  done at the operating system level, special real
  time compliant operating systems should be used.

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What are watchdog timers?

• Most embedded system software is in the form of
  an endless loop, which waits for events and
  processes them when they occur.
• A watchdog timer helps with system recovery if
  there is a hang up while servicing some event.

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Tools of the trade .

• Open source systems like real time Linux are
  becoming extremely popular.
• Real time Linux provides the luxury of
  development on an advanced system and then
  migration to the target hardware.
• Because there is no royalty to be paid, a large
  number of developers are now choosing open source
  systems. Example Simputer the palm computer
  designed in India. Even the Sony play station
  runs Linux.

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Future Directions

• Many more embedded systems will be full systems
  on a single chip.
• Already, there are development efforts for
  appliances like a disposable blood tester with on
  chip chemical and bio-sensors and processing
  electronics. (A water safety tester is being
  developed at IIT Bombay).

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Tools of the Trade

• Excellent public domain tools are available for
  software development / debugging and emulation.
  For example, SDCC is a powerful C compiler /
  assembler/ debugger which supports a large
  variety of processors (Z80, 8051, ARM, AVR, PIC
  etc.)
• Hardwarde description languages ease the job of
  designing with IP cores.

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Future directions .

• Embedded systems will be developed in unusual
  applications. For example stress detectors built
  into walls, powered and accessed by RF beams.
• Existing applications will become far more
  sophisticated with standardised user interfaces
  such as web interfaces with XML.

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Future directions for developers

• More and more multi-disciplinary expertise will
  be required. For example biology-chemistry-elect
  ronics and VLSI for bio-sensors
• Fields of micro-processors, VLSI, communications
  and information technology will merge for
  developing embedded systems