1' A First Look at Embedded Systems

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1. A First Look at Embedded Systems
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• As microprocessors have become smaller and
  cheaper, more and more products have
  microprocessors "embedded" in them to make them
  "smart."
• embedded system to mean any computer system
  hidden in any of these products.
• Embedded systems often
• - respond to external events
• - without human intervention
• - subject to deadlines

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1.1 Examples of Embedded Systems

• Telegraph
• connect a printer that has only a high-speed
  serial port to a network.
• Telegraph must receive data from the network and
  copy it onto the serial port.

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Telegraph must do

• On the network, data sometimes arrive out of
  order, data sometimes get lost along the way, and
  some of the data sometimes arrive twice.
  Telegraph must provide a clean data stream to the
  printer.
• Telegraph must feed the printer one print job at
  a time and somehow hold off all the other
  computers.
• Network printers must provide status information
  to any computer on the network

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Telegraph must do

• Telegraph has to work with a number of different
  types of printers without customer configuration.
  
• Telegraph must respond quite rapidly to certain
  events.
• Telegraph must keep track of time. Otherwise, one
  computer crash would make the printer unavailable
  to everybody.

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Telegraph Development Challenges

• To satisfy the list of requirements given above,
  Telegraph has a microprocessor embedded in it.
• What problems arise in developing such software?
• software for Telegraph must be logically correct
  It can't lose track of which computer is printing
  or drop data or report incorrect status.

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• Throughput
• Telegraph must not become a bottleneck between
  the computers on the network and the printer.
• Response
• When a critical network frame arrives,
  Telegraph must respond within 200 microseconds,
  even if it is doing something else when the frame
  arrives.
• embedded-system designers must deal with two
  separate problemsthroughput and response

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• Testability
• It is not at all easy to determine whether
  Telegraph really works.
• It must be able to deal with anything without
  human intervention.
• It must deal with events that are almost
  simultaneous.
• Telegraph contains code to handle this situation,
  but how do you make it happen in order to test
  that code?

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• Debugability
• What do you think typically happens when testing
  uncovers a bug in the Telegraph software?
• Telegraph has no screen no keyboard no speaker,
  not even any little lights.
• When a bug crops up, Telegraph typically just
  stops working.
• You must find other ways to figure out what has
  happened.
• techniques for debugging embedded-systems
  software

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• Reliability
• Telegraph is not allowed to crash.
• whatever happens, the software must function
  without human intervention
• Memory Space
• a very finite amount of memory
• Memory gets nothing but cheaper, but it still
  isn't free.
• Making software fit into the available space is a
  necessary skill for many embedded-system software
  engineers
• Program Installation
• special tools that are needed to install the
  software into embedded systems

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Cordless Bar-Code Scanner

• the cordless bar-code scanner activates its laser
  to read the bar code and then sends the bar code
  across a radio link to the cash register.

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• the problems of developing the software for the
  cordless bar-code scanner? they're mostly the
  same.
• cordless bar-code scanner does not have is the
  problem of throughput
• Power Consumption
• the weight of the battery is limited by what an
  average user can comfortably hold up
• How long does the customer want the battery to
  last?
• it also isn't feasible to run a laser, a
  microprocessor, a memory, and a radio for 8 hours
  on battery power.

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Laser Printer

• that microprocessor is responsible for getting
  data from the various communication ports on the
  printer, for sensing when the user presses a
  button on the control panel, for presenting
  messages to the user on the control panel
  display, for sensing paper jams and recovering
  appropriately, for noticing when the printer has
  run out of paper, and so on.
• the largest work is to deal with the laser engine

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Underground Tank Monitor

• watches the levels of gasoline in the underground
  tanks at a gas station.
• Its principal purpose is to detect leaks before
  the gas station turns into a toxic waste dump by
  mistake
• The system also has a panel of 16 buttons, a
  20-character liquid crystal display, and a
  thermal printer.
• to be as inexpensive as possible, the system will
  be built with an inexpensive microcontroller

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1.2 Typical Hardware

• First, all of the systems need a microprocessor.
• memory for two purposes to store its program and
  to store its data.
• typical embedded system does not have a hard disk
  drive from which to load its program, the program
  must be stored in the memory

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• A keyboard. Some systems may have a few push
  buttons for user input
• A screen. Many systems have a liquid crystal
  display
• A disk drive.
• Compact discs, speakers, microphones, diskettes,
  modems. Most embedded systems have no need for
  any of these items.
• often have a standard serial port, a network
  interface, and hardware to interact with sensors
  and activators

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Chapter Summary

• An embedded system is any computer system hidden
  inside a product other than a computer.
• Embedded systems have a microprocessor and a
  memory. Some have a serial port or a network
  connection.
• They usually do not have keyboards, screens, or
  disk drives.

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difficulties when you write embedded-system
software

• Throughputhandle a lot of data in a short period
  of time.
• Responsereact to events quickly.
• TestabilitySetting up equipment to test can be
  difficult.
• DebugabilityWithout a screen or a keyboard,
  finding out what the software is doing wrong

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difficulties when you write embedded-system
software

• Reliabilitymust be able to handle any situation
  without human intervention.
• Memory Spacemust make the software and the data
  fit into limited memory.
• Program Installationneed special tools to get
  your software into embedded systems.
• Power ConsumptionPortable systems must run on
  battery power,
• Processor HogsComputing that requires large
  amounts of CPU time can complicate the response
  problem.
• CostReducing the cost of the hardware

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