CPE-746-JUST-Fall 2006

1
Real Time Embedded Systems

• Done by Lara Quadan Lina Sawalha
• Supervised by Dr. Loai Tawalbeh
• Computer Engineering Department
• Jordan University of Science and Technology

2
Introduction

• Computer is entering all facets of life from home
  electronics to production of different products
  and material.
• Many of the computers are embedded and thus
  hidden for the user.
• So it is necessary to know about the control and
  characteristics of embedded real-time systems to
  implement reliable ones.

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Real-time Systems

• A real-time system is one in which the
  correctness of the computations not only depends
  on their logical correctness, but also on the
  time at which the result is produced. In other
  words, a late answer is a wrong answer.
• A real-time system consists of tasks under
  deadline constrains.
• Real-time systems maybe distributed systems,
  consisting of components that are physically
  distributed. These systems consists of multiple
  processing units and communication links.

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Functional Requirements

• Data Collection
• Sensors
• Signals
• Alarm
• Direct Digital Control
• Actuators
• Man-machine interaction

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Temporal Requirements

• Tasks may have deadline
• Minimal latency jitter
• Minimal error detection latency
• Timing requirements due to tight software control
  loops.
• Human interface timing requirements

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Application Areas

• Customer electronics
• Cameras
• Camcorders
• Customer products
• Dish washers
• Microwave ovens
• Cars
• Anti-lock braking
• Engine control
• Drive-by-wire
• Planes
• Stability
• Jet engine
• Fly-by-wire
• Military
• Weapons
• Satellites
• Industrial process controllers

• Computer/Communication products
• Peripherals
• Fax machines
• Protection security systems
• Intruder Alarm
• Smoke/Gas detection
• Robotics

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Control Theory

• A control system is an interconnection of
  components forming a system configuration that
  will provide a desired system response.
• Types of control
• Open-loop control a system that utilizes a
  device to control the process without feedback.
  Thus the output has no effect upon the signal to
  the process.
• Closed-loop control A system that uses a
  measurement of the system output through sensors
  and compares it with the desired output (called
  feedback system).

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Embedded digital controlled systems

• When a digital controller (microprocessor/Microcon
  troller) is used in the embedded system to
  control the plant or process an analog/digital
  converter (A/D) is required as well as
  digital/analog converter (D/A) to interface the
  input/output controller digital signals with the
  analog signals from the other component in the
  system.
• it is very important to consider the timing for
  the real-time digital controlled system.

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Cruise Control

• Regulate the speed of a car by adjusting the
  throttle
• Input by the driver ? sets a speed and car
  maintains it.
• Measures the speed through device connected to
  the driver shaft.
• Hard real-time driver shaft revolution event.
• Soft real-time driver inputs, throttle
  adjustments.

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Cont.

• Disturbances road surface and grade, wind and
  obstacles.
• Input by the driver ? sets a speed and car
  maintains it.
• If the driver give a brake command to the car the
  cruise control system will be interrupted and the
  car speed will slow down or be stopped.

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Other car embedded systems
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Autopilot

• Objective function to control the direction
• and speed of the plane.
• Outputs actual direction and speed of the plane
• Control inputs path markings and speed.
• Disturbances wind, obstacles.
• subsystems power system,
• engines, steering system,
• braking system, . . .

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Cont.
14
Robot Vacuum cleaner
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Cont.

• Operating environment
• closed indoor environments
• such as rooms.
• Controlling System
• Human turn on and off the robot
• Computer Sensors stairs and wall detection
  (IR sensors).
• Controls robot location through controlling the
  robot speed and steering to follow the walls.
• Actuators motors and wheels.

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Hard Disk Drive Control

• The disk rotates at a speed between 1800 and 7200
  rpm.
• The head flies above the disk at a distance of
  less than 100 nm.
• Position accuracy is 1 micro meter.
• Move the head from track a to track b within 50
  ms.
• Motor to actuate the arm to the desired location
  on the disk.

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Cont.

• For DVD
• system?

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Wireless Sensor Network Embedded System
Sensor nodes scattered in a sensor field
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Cont.

• Collection of sensor nodes in a field.
• Each node collects data and send it to a sink.
• The sink aggregates the data and send it to a
  main processing unit.
• Each sensor node can manipulate data using its
  own embedded microcontroller (microprocessor),
  transceiver and communication model.

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Keck Astronomical Telescope

• World largest telescope
• The main objective is to collect and focus
  starlight using a large concave mirror.
• The shape of the mirror determines the quality of
  the observed image.
• The diameter of the mirror is 10 m.
• The mirror is a mosaic of 36 hexagonal small
  mirrors.
• The 36 segments must be aligned so that the
  resulting mirror has the desired shape

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• Behind each segment are three actuators applying
  forces at three points on the segment affecting
  its orientation.
• In the gap between every two adjacent segments a
  capacitor type sensors measuring the local
  displacement between segments.
• These displacements should be controlled to give
  the desired shape and direction for the mirror.

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Auto-washing machines

• The user will select the required washing
  program.
• A set of sensor will monitor the washing process
  variables such as water level, temperature, and
  the rotation speed. Where the embedded controller
  will keep these values agree with the selected
  washing program. through out the washing time.

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Classifications of RT systems

• Hard vs. Soft systems
• Fail-safe vs. Fail operational
• Guaranteed-response vs. Best-effort
• Resource-adequate vs. Resource-inadequate
• Event-triggered vs. Time-triggered

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Hard vs. Soft Real Time Systems

• Hard RTS
• A RTS where the tasks have to be
• performed not only correctly but also
• on time. i.e. a catastrophic system
  failure can
• occur if the system is not responding
  on time.
• E.g., the aircraft turbulence (instability in the
  atmosphere) controller should respond on time, if
  not there will be a definite disaster.
• Other examples
• control systems for space probes and nuclear
  reactors.
• refresh rates for video, or DRAM.
• collision alert.

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Cont.

• Soft RTS
• A RTS where the tasks have to be
• performed as fast as possible, but the
  tasks
• need not be finished within deadlines.
• I.e. It may go beyond specified deadlines without
  catastrophic failures (For example, if the video
  game is not responding on time, its only a delay
  where the user may not want to wait, but even
  then nothing awful is going to happen).

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Cont.

• Another e.g. Laser printer rated by
  pages-per-minute, but can take differing times to
  print a page (depending on the "complexity" of
  the page) without harming the machine or the
  customer.
• Task execution may also be timed-out display
  updates, connection establishment.
• Other examples of Soft real time systems
• videoconferencing
• stock price quotation systems
• airline reservation systems
• automatic teller

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Characteristics of RTS

• Response time - hard real-time applications,
  milliseconds or less, preclude direct human
  intervention during normal operation and in
  critical situations. - Soft real-time and on-line
  systems are often in the order of seconds.
• Peak-load performance - hard real-time system
  the peak-load scenario must be well-defined,
  guaranteed to meet the specified deadlines in all
  situations.- soft-real time system, the average
  performance is important, and a degraded
  operation in a rarely occurring peak load case is
  tolerated
• Control of Pace A hard real-time system must
  remain synchronous with all the state of the
  environment in all cases. This is in contrast to
  an on-line system, which can exercise some
  control over the environment in case it cannot
  process the offered load.
• Safety Hard real-time systems are often safety
  critical.
• Size of Data files Hard real-time systems have
  small data files and real-time databases.
  Temporal accuracy is often the concern here. Soft
  real time systems has larger databases so
  long-term integrity of large data files is the
  key issues.

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Characteristics of RTS

• Redundancy Type If an error occurs in a soft
  real-time system, the computation is rolled back
  to a previously established checkpoint to
  initiate a recovery action. In hard real-time
  systems, recovery is of limited use.

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Common Misconceptions

• Misconceptions about Real-Time Systems
• Stankovic'88
• faster hardware implies all deadlines will be met
• real-time computing is merely fast computing
• RT systems are low-level coding done using ad-hoc
  methods.
• Fast is relative. More important that system is
  fast enough", deterministic and predictable.
• Worst-case response times of interest rather than
  average-case.
• Scheduling theory, software design, formal
  methods and RTOS are changing things.
• It is important to note that hard versus soft
  real-time does not necessarily relate to the
  length of time available.

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Real Time terminologies

• Synchronous events occur at predictable times in
  the flow-of-control.
• Asynchronous interrupts
• Multitasking The process of scheduling and
  switching the CPU between several tasks, a single
  CPU switches its attention between several
  sequential tasks. Multiple is like
  foreground/background with multiple backgrounds.
  Multitasking maximizes the utilization of the CPU
  and also provides for modular constructions of
  applications.
• Task A task is called a Thread, is a simple
  program which thinks it has the CPU all to
  itself. Each task typically is an infinite loop
  that can be in any one of the three states
  RUNNING, READY (-TO-RUN), BLOCKED.
• Resource A resource is any entity used by a
  task. A resource can thus be an I/O device, such
  as a printer, a keyboard, or a display, or a
  variable, a structure, or an array.

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Real Time terminologies

• Shared Resource A shared resource is a resource
  that can be used by more than one task. Each task
  should gain exclusive access to the shared
  resource to prevent data corruption. This is
  called Mutual Exclusion. For e.g., a global
  variable can be used by many tasks, or printer
  which could be shared by "n" users.
• Critical Section of Code Also called Critical
  Region, is code that needs to be treated
  indivisibly once the section of code starts
  executing, it must not be interrupted. To ensure
  this, interrupts are typically disabled before
  the critical code is executed and enable when the
  critical code is finished.
• Reentrancy A reentrant function can be used by
  more than one task without fear of data
  corruption. a reentrant function can be
  interrupted at any time and resumed at a later
  time without loss of data. Reentrant functions
  either use local variables (i.e. CPU registers or
  variables on the stack) or protect data when
  global variables are used.

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Operating System

• Real-time operating systems must provide support
  for
• Guaranteeing real-time constraints.
• Supporting fault tolerance and distribution.
• Time constraint resource allocations and
• scheduling.
• Time constraint end-to-end communications.

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Real Time Operating System (RTOS)

• Fundamental requirements for an RTOS
• The OS behavior must be predictable
• The OS must be multithreaded and preemptive.
• The OS must support thread priority.
• The OS must support predictable thread
  synchronization mechanisms.
• Additional Requirements
• The maximum time that device drivers use to
  process an interrupt, and specific IRQ
  information relating to those device drivers,
  must be known.
• The interrupt latency (the time from interrupt to
  task run) must be predictable and compatible with
  application requirements

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• Kernel
• Is the core program of the Operating System, and
  determines its behavior
• Is responsible for System Resources and for
  context switch.
• Also responsible for task scheduling.
• In RTOS since the size of the kernel is small,
  its also referred to as Micro Kernel.
• Kernel types
• Microkernel - scheduler
• Kernel - a microkernel with intertask
  synchronization
• Executive - a kernel that includes privatized
  memory blocks, I/O services, and other complex
  issues. Most commercial realtime
• kernels are in this category.
• Operating system - an executive that also
  provides generalized user interface, security,
  file management system, etc.

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RTOS

• Three groups
• Small, fast, proprietary kernels
• Real-time extensions to commercial operating
  systems
• Research operating systems
• Small, fast, proprietary kernels
• homegrown
• commercial offerings
• QNX, PDOS, pSOS, VCOS, VRTX32, VxWorks
• To reduce the run-time overheads incurred by the
  kernel and to make the system fast, the kernel
• has a small size
• responds to external interrupts quickly
• minimizes intervals during which interrupts are
  disabled
• provides fixed or variable sized partitions for
  memory
• management as well as the ability to lock code
  and data in memory
• provides special sequential files that can
  accumulate data at a fast rate

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RTOS

• To deal with timing constraints, the kernel
• provides bounded execution time for most
  primitives
• maintains a real-time clock
• provides primitives to delay processing by a
  fixed amount of time and to suspend/resume
  execution
• Also, the kernel
• performs multitasking and intertask communication
  and synchronization via standard primitives such
  as mailboxes, events, signals, and semaphores.
• For complex embedded systems, these kernels are
  inadequate as they are designed to be fast rather
  than to be predictable in every aspect.

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Examples of RTOS

• Open source
• eCos
• Fiasco (L4 clone) 1
• FreeRTOS
• Linux as of kernel version 2.6.18
• Phoenix-RTOS
• Nut/OS 2
• Prex
• RTAI
• RTEMS
• RTLinux
• SHaRK 3
• TRON Project
• Xenomai 4

• Proprietary
• Ardence RTX - BeOS
• ChorusOS - DNIX
• DSOS - embOS (Segger)
• ITRON
• LynxOS - MicroC/OS-II
• MQX RTOS 5 - Nucleus
• OS-9 - OSE
• OSEK/VDX - OSEKtime
• PDOS - Phar Lap ETS
• PikeOS - Portos
• pSOS - QNX
• RMX - RSX-11
• RT-11 - RTOS-UH
• RTXC - Salvo RTOS 6
• SINTRAN III - Symbian OS
• ThreadX - VRTX
• VxWorks - Windows CE
• µnOS - UNIX-RTR

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RTLinux

• Real-Time Linux (now part of FSMLabs Inc.)
• that is suitable for Real-Time applications.
• Its view is that the application system can be
  split into two parts
• Real-time
• Non real-time
• With this approach, it splits the applications to
  run on either the Linux kernel, or a real-time
  kernel!

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RTLinux

• Between the real and non-real parts communication
  is performed through FIFOs called RT-FIFOs.
• These FIFOs are
• Locked to memory in kernel space.
• FIFOs appear as devices to Linux user processes.
• Reads and writes are non-blocking and atomic.

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RTLinux

• RTLinux eliminates the problem of the kernel
  blocking interrupts by replacing HW interrupts by
  SW emulated interrupts.
• Thus, the RT kernel intersects all interrupts!
• If an interrupt is to let a RT task execute, then
  the RT kernel pre-empts any general application
  and runs the RT task.
• Flags are used to emulate interrupt disabling.

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VxWorks

• Monolithic Kernel
• Reduced run-time overhead, but increased kernel
  size compared to Microkernel designs
• Supports Real-Time POSIX standards
• Common in industry
• Mars missions
• Honda ASIMO robot
• Switches
• MRI scanners
• Car engine control systems

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Software vs. Hardware

• Hardware
• Fast
• Power-efficient
• Used for performance and security
• Software
• Flexible
• Reusable

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Examples on Software Subsystems

• The software components of an embedded system can
  be diverse.
• Software running on a microprocessor core.
• Written in a High level language
• Cooperative multi-tasking software running on one
  or more DSP processors
• Largely written in Assembly code due to a lack of
  high quality compilers
• An RTOS kernel running on the microprocessor core
  and/or DSP core
• Control Processes running on a microcontroller
• User interface modules running on the
  microprocessor
• Devise drivers for interface protocols e.g.
  TCP/IP, ATM

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Software Languages

• Specifying machine code concisely
• Sequential semantics
• Perform this operation
• Change system state
• Raising abstraction symbols, expressions,
  control-flow, functions, objects, templates,
  garbage collection.
• C/C/Java
• Real Time Operating Systems Adds concurrency,
  timing control

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Hardware Subsystems

• Microprocessor cores
• DSP cores
• Field Programmable Gate Arrays (FPGAs)
• Application specific integrated circuits
• (ASICs)
• standard cell/synthesized deigns
• custom ICS
• Memory
• RAM, ROM, PROM, ...
• Conten addressable memory (CAM)
• Specialized DRAMs, multi-bank memories,
• System Bus structures interfaces
• I/O interfaces
• Others

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Hardware languages

• Specifying connected gates concisely
• Originally targeted at simulation
• Discrete event semantics skip idle portions
• Mixture of structural and procedural modeling
• Hardware Languages
• Verilog
• Structural and procedural modeling
• Four-valued vectors
• Gate and transistor primitives
• Less flexible, succinct
• VHDL
• Structural and procedural modeling
• Few built-in types Powerful type system
• Fewer built-in features for hardware modeling
• More flexible, verbose
• SystemC 1.0, Cynlib VHDL/Verilog in C

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Hardware/Software Codesign

• Most systems and mixed between hardware and
  software.
• Traditional Design
• SW and HW partitioning is decided at an early
  stage, and designs proceed separately from then
  onward.
• CAD today addresses synthesis problems at a
  purely hardware level
• efficient techniques for data-path and control
  synthesis down to silicon.
• ECS use diverse (commodity) components
• uP, DSP cores, network and bus interfaces, etc.
• "New fangled" Codesign
• A flexible design strategy, wherein the HW/SW
  designs proceed in parallel, with feedback and
  interaction occurring between the two as the
  design progresses.
• Final HW/SW partition/allocation is made after
  evaluating trade-offs and performance of options.

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Examples of mixed hardware-software embedded
systems

• Embedded controllers for reactive real-time
  applications are implemented as mixed
  software-hardware systems. These controllers
  utilize Micro-processors, Micro-controllers and
  Digital Signal Processors but are neither used
  nor perceived as computers. Generally, software
  is used for features and flexibility, while
  hardware is used for performance. Some examples
  of applications of embedded controllers are
• Consumer Electronics microwave ovens, cameras,
  compact disk players.
• Telecommunications telephone switches, cellular
  phones.
• Automotive engine controllers, anti-lock brake
  controllers.
• Plant Control robots, plant monitors.

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Embedded System Design

• Modelling
• the system to be designed, and experimenting with
  algorithms involved
• Refining (or partitioning)
• the function to be implemented into smaller,
  interacting pieces
• HW-SW partitioning Allocating
• elements in the refined model to either (1) HW
  units, or (2) SW running on custom hardware or a
  general microprocessor.
• Scheduling
• the times at which the functions are executed.
• This is important when several modules in the
  partition share a single hardware unit.
• Mapping (Implementing)
• a functional description into (1) software that
  runs on a processor or (2) a collection of
  custom, semi-custom, or commodity HW.

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References

• Kopetz, H. , Real-Time Systems Design Principles
  for Distributed Embedded Applications.
• Calton Pu, 1B RTE Concepts and Examples
• A Framework for Hardware-Software Co-Design of
  Embedded Systems, University of California,
  Berkeley.
• http//www.ics.uci.edu/rgupta/ics212/w2002/models
  .pdf
• http//www.ics.uci.edu/rgupta/ics212/w2002/intro.
  pdf
• http//www.cs.unb.ca/courses/cs4405/lectures/real-
  time.pdf
• http//www.ece.cmu.edu/koopman/des_s99/real_time/
  introduction
• http//en.wikipedia.org/wiki/
• http//www.ics.uci.edu/rgupta/iec/introduction/