Real-Time Computing
For Human Computer Interfacing

November 4, 2002

Copyright* 1997-2002
Perry R. Cook, Princeton University

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I. What's Real-Time? Some Definitions

Encyclopedia of Computer Science (VNR 1993) says:

    Computer Processing Modes and Times:

    Card Oriented Batch


    Keyboard Oriented Batch


    Interactive Computing


    Online Inquiry and Transactions


    Message Switching


    Data Acquisition and Control


Ib. What's Real-Time? Continued

What do the Computer Companies Say?
  • SGI: Media Playback is Synchronizable
    to 1 Audio Sample

  • Windows: Graphics Display Looks Smooth,
    Audio Doesn't Click too Much
    Prerecorded Game Sounds Play Quickly

  • Apple, NeXT, Others: Still different definitions.

  • Computer Company Definitions Typically Say Little
    About Latency Guarantees!

Ic. What's Real-Time? Continued

What do the Controls/Robotics People Say?
  • Systems Function Under Time Constraints

  • Imposed by External Processes

  • Scheduling and Resource Allocation Guarantees

  • Synchronous, or Guaranteed Timings (Stability)

II. Controls Systems vs. Media Delivery

    Media Delivery:
    • Make it look/sound smooth, degrade gracefully
    • No clicks or pops in audio, no jerks in video.

    Control Systems:

    • Close a feedback loop with sensors and motors.
    • Guarantee STABILITY.

    HCI is Both:

    We're really interested in combining two areas. We want to get user input,
    process it in a timely fashion, and display something smoothly and convincingly
    (with small or controllable delay) in response. The human and computer make
    a complete feedback system. Many modern HCI applications are active
    feedback control systems, and thus stability can be a consideration.

III. Control Systems

  • A Classic Feedback Control System

    • Feedback allows system to control itself

    • Human sets desired state, but is not part of feedback

    • Must worry about stability of feedback systems

  • What are some systems where human is part of feedback?

IIIb. Control Systems

    Humans in the Feedback Loop:
    • Cars and things you steer

    • Musical Instruments (some, but not all)

      • Theremin
      • Brass
      • Bowed

IIIc. Control Systems


    • Actuator: Steering system
    • Mechanical System: Car, Wheels, Tires


    • Sensor: Visual Sensors (maybe more!)
    • Controller: Brain and Arms

A More General Picture of a Feedback Control System:

IIId. Some Feedback Control Systems with Humans

    Broom Balancing

    • Actuator: Hand
    • System: Physics of Broomstick
    • Sensor: Visual/Tactile/Kinesthetic
    • Controller: Human

    Video Game

    • Actuator: Hand/Joystick
    • System: Computer running game software
    • Sensor: Visual (auditory) System(s)
    • Controller: Human


    • Actuator: Hands
    • System: Theremin electronics
    • Sensor: Auditory
    • Controller: Human

IIIe. Diagram of Control System with Human

    • Latency

    • Overcompensation, stability (gain)

      • Feedback means possibility for instability
      • System Delay (VR Sickness)
      • Sampling and measurement errors

      • Human overcompensation
      • Variable Reaction time (attention, ...)
      • Resolution
      • Training

IV. Synchronous vs. Asynchronous Models
Polled vs. Interrupt Operation

  • Synchronous Real-Time
      Based on time-ordered external events.
      Processor responds instantaneously to events.
      (or time between events >> response time)

  • Asynchronous Real-Time
      Events at times indexed on the real numbers.
      System responds within specified bounds.
To accomplish: We can Poll, use Interrupts, or both.

V. Data and Processing Models

  • Queues and Scheduling:
    • FIFOs (fixed priority)
    • Priority-based

  • Interrupts:
    • Single Priority
    • Multi-Level
    • Nestable
    • Hardware vs. Software

Vb. Data and Processing Models Continued

  • Process/Device Communication:
    • Shared Memory
    • Messages
    • DMA
    • Multi-Bus/Ring/Star/Common Bus

  • Input/Output Buffers
    • Ping Pong
    • Single with Hi/Low Water

VI. Complexity!!!

Online: Scheduling, Interrupt Handling, Load Monitoring, ..
Development: Debugging, Optimization, Profiling, ..

=> Need for an Operating System

Some Real-Time Operating Systems











RT Mach

80960, 80486, MIPS R3000

DSP/Media Processor Real-Time OS:






TICX0, Motorola 56x, ADI 21x

VII. Multiple Processes

One Common Software Model:
  • Main Loop
    • Polls Non-Critical Inputs
    • (Maybe) Services Queue
    • Waits Around a Lot
  • Interrupt Service Routines (ISRs)
    • Respond Instantaneously
    • Typically Very Short

Queue Servicing Can Also Be Done on a Clocked ISR

VIIb. Multiple Processors

  • Segments and Isolates the Problems
  • Improves Response Times
  • Saves Cost
Why Not?
  • Can Increase Complexity and Indeterminacy
  • Can Degrade Response Times

Symmetric Processors (all the same type) or Not
May Share Memory or Not
May Run Synchronously or Not


What's a DSP?

    • Processor good for DSP algorithms

      • Filters
      • FFT's
      • Matrix and Vector Math

    • Fast (Single Cycle) Multiplies +

    • Optimized Data Paths (2+ operands)

    • Address Generation Hardware:

      • Pointer Arithmetic
      • Modulo incrementing/decrementing
      • Bit Reversing
      • Increment/Decrement by N

    • I/O Support (A/D and D/A, Host Communication)

VIIIb. What are DSPs used for?


    • Data Compression/Decompression
    • Telephony (MODEMS)
    • Audio Processing
    • Audio Synthesis
    • Speech Synthesis/Recognition
    • 3D Audio
    • 3D Graphics
    • More suitable
    • Frees up less suitable processors
    • $$$$$
    • Interactivity (latency, time)
Why Not?
    • Hard to develop for
    • Communications with other processors
    • Data flow
    • Synchronization

VIIIc. DSP -> Media Processors -> Media Extensions

Media Processors:
    • Combine Video, Audio, and more
    • Share memory between these types
    • Split ALU and data paths into smaller sub-units

      Media Data Types:
      • Video: 8 bit data, lots of it
      • Audio: 16+ bits, less of it
      • Compressed media: Varies

This leads also to "Media Extensions" on Hosts

  • Single cycle multiplies
  • Fixed Pt. Extensions
      Use existing floating point regs,
      but do parallel fixed point ops.
  • Examples: MMX, VIS, HP Media Extensions

VIIId. DSP Trends

Trends for General DSPs:

  • DMA, Local Memory Interfaces
    • Supports I/O
    • Supports Host Communication
    • Supports Data Transfer
  • Speeds go up some, but not as much as hosts
  • Multi Processor support (SHARC, C40)

Trends for Media Processors:

  • Multi function
  • Don't know yet

Trends for Host Based DSP:

    Media Extensions
Overall Need: Operating Systems?

IX. Microcontrollers

  • Microcontrollers Historically:
    • Small Words and Integer Math
    • Low Level Language
    • Hardware Interrupt Capabilities
    • Peripheral Devices for Inputs
    • Low Cost

  • Modern Microcontrollers:
    • Small Words (relatively) Integer Math (or not)
    • Higher Level Language Support
    • Peripheral Inputs Integrated
    • Low Cost

IXb. Microcontrollers Continued

  • Old: H8 (Minuteman missles), 68xx, 65xx, Zx, 80x

      • 8 bit data, 8 bit instruction, 2+K address
      • Clock Rate: 1-2 MHz
      • Language and Interface: Assembler via TTY
      • System cost: $100.00

  • New:

    • PIC Chip ($4-10.00)

    • BASIC Stamp = PIC + More
      • 16 bit data, high level instructions, 2K memory
      • Clock Rate: 4-20 MHz
      • Language and Interface: BASIC via PC Serial Port
      • System cost: $10-35.00

Simple Basic Stamp II Program Example

The KIM-1 microcomputer, 1977

Basic Stamp II SX, 1998

X. Micro/DSP/Host Real-time Processing Examples

  • Basic Stamp Programming Env.

  • Micro -> Micro+DSP via MIDI

  • Micro -> Host via MIDI

Some Web References:

Educational Research Groups and Projects:

U. Michigan Real-Time Computing Lab

Carnegie Melon Real-Time Groups


From Artesyn: Choosing an OS and Choosing a Processor

Resource List Compiled by E. Douglas Jensen

Some Non-Web References Made from Dead Trees:

"Real-time programming : neglected topics"
Caxton C. Foster.
Reading, Mass. : Addison-Wesley Pub. Co., c1981.

"Real-time software for control : program examples in C"
David M. Auslander, Cheng H. Tham.
Englewood Cliffs, N.J. : Prentice Hall, c1990.

"Real-time systems, Specification, Verification, and Analysis,"
Mathai Joseph, ed.
Englewood Cliffs, N.J. : Prentice Hall, 1996

"Introduction to Real-time Software Design,"
S.T. Allworth and R.N. Zobel
New York, Springer Verlag, 1989

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