Introduction to Realtime Systems

1
Introduction to Realtime Systems

• Chapter 1
• R. Williams text
• B. Ramamurthy

2
Overview

• Realtime processing requires both parallel
  activities and quick response (See figure in p.2
  of a juggler)
• Design for complexity management. (See figure in
  p.3)
• Microprocessors and realtime applications
• Definition of realtime systems
• Programming structures
• Response latency
• Relative speeds
• Timing issues
• Debugging real-time systems
• Input/output handling
• Deadlines (hard, firm and soft)
• Software quality assurance

3
Design for complexity management

• Choices
• Structured analysis/structured design (SA/SD)
• Concurrent design approach for real-time systems
  (CODARTS)
• Finite state machines (FSM)
• Object-oriented design (OOD)

4
Lets Design a RT Game

• Use two dies
• If the shooter's come-out roll is a 2, 3 or 12,
  it is called "craps" and the round ends with
  player losing.
• A come-out roll of 7 or 11 is called a "natural,"
  resulting in a win.
• If the numbers 4, 5, 6, 8, 9, or 10 are rolled on
  the come-out, this number becomes the "point" and
  the come out roll is now over.
• The shooter will now continue rolling until
  either the point is rolled or a seven.
• If the shooter is successful in rolling the
  point, the result is a win.
• If the shooter rolls a seven (called a
  "seven-out"), the pass line loses.

5
Microprocessor

• Examples vending machines, mobiles phones, alarm
  systems, washing machines, motor car engine
  controllers, heart monitors, microwave ovens all
  operate using embedded microcontrollers running
  dedicated software.
• Microprocessors are the enabling hardware for
  realtime systems.

6
Lets define realtime (RT) systems

• Timing RT systems (RTS) are required to compute
  and deliver correct results within a specified
  period of time. Ex traffic light controller
• Interrupt driven event-driven preemption RTS
  are often involved with handling events.
• Events manifest themselves in terms of interrupt
  signals arising from the arrival data at an input
  port or ticking of a hardware clock, or an error
  status alarm.

7
RTS Definition (contd.)

• Low-level programming RTS often deal with
  devices C language is still a favorite for
  writing device drivers for new hardware.
• Specialized hardware Most RTS work within, or at
  least close beside, specialized electronic and
  mechanical devices. Often closed loop systems.
  (See fig on p.6)
• Volatile data IO Variables that change their
  value from moment to moment. RTS software must be
  structured to check for changes at the correct
  rate, so as not to miss a data update.

8
RTS Definition (contd.)

• Multi-tasking RTS are often multitasking.
  Several processes cooperate to carry out the
  overall job. Divide RTS problem into tasks as a
  design strategy.
• Run-time scheduling Separation of activities
  into tasks leads to question of task sequencing
  or scheduling. Moreover the external events and
  required response to these lead to run-time
  scheduling or dynamic scheduling.
• Unpredictability Being event-driven, RTS are at
  the mercy of unpredictable changes in their
  environment.
• Life-critical code failure to run correctly may
  result in death or at least injury to the user
  and/or others. Life-critical systems requires
  extra testing, documentation and acceptance
  trials.

9
Programming Structures

• Also known as control structures
• Sequence (SEQ)
• Iteration (IT)
• Branches guarded by selection statements (SEL)
• Parallel or concurrent instructions (PAR)
• Critical group of exclusive instructions (CRIT)

10
Relative Speeds

• Polling is a common method used in RTS for
  sensing events/inputs.
• When designing RTS
• Avoid polling input too slowly, you may miss
  events
• Avoid sampling to frequently (called aliasing),
  since it will make the process inefficient.
• Nyquist limit The maximum frequency threshold is
  half the sampling rate referred to as Nyquist
  limit.

11
V2F Voltage to Frequency

• A commonly used environmental monitoring
  arrangement involves a transducer being
  interfaced to a voltage-frequency converter.
• V2F unit converts voltage to frequency larger
  voltage, higher frequency lower voltage, lower
  frequency.
• Computer has to dedicate a single bit input port
  to accept the information in serial mode.
• Problem converting this bit information into
  integer.
• Timing of the two branches in the following
  sequences are not balanced run these two
  alternatingly.

12
Software Timing

• loop for 100 msec
• if (input_bit)
• hcount
• else
• lcount

• loop for 100 msec
• if (!input_bit)
• lcount
• else
• hcount

13
High Speed Timing

• Consider a laser range finder used in civil
  surveying, and battle field targeting.
• Distance is calculated by the timing the duration
  of flight.
• The Speed of light s 3 X 108 m/sec
• Target is 20km away, the pulse of light will
  travel 40km (to and from) d 4 X 104 m
• Time taken d/s 4 X 104 / 3 X 108 130µs
• If the item surveys is only at 150m distance, the
  flight time will be reduced to 325ns.
• Regular 500MHz computer will not do for this.
• Solution Clear a high speed counter when the
  pulse leaves and capture the counter value on
  return of the pulse.
• Assignment Research and explain how this is done
  in a golf distance finder.

14
Output timing

• Timing problems when controlling output devices
  such as motors.
• Cyclic, periodic output data.
• Several types of motors stepper, DC servo,
  universal AC, induction AC, synchronous AC etc.
• For example sometimes we need to dispatch every
  20ms with an accuracy of 50µs.
• Solution need special hardware support
  controller boards.

15
Machine IO

• Programmed IO
• Interrupt driven
• Direct memory access

16
Types of RTS

• Hard RTS tight limits on response time, so that
  a delayed result is a wrong result.
• Ex jet fuel controller and camera shutter unit
• Soft RTS need to meet only time-average
  performance target. As long as most results are
  available before deadline the system will run
  successfully.
• Ex audio and video transmission, single frame
  skip is fie, but repeated loss is unacceptable
• Firm RTS somewhere between the two.
• Ex Space station solar panel unit

17
Software Quality Assurance

• QA is especially important to RTS since many of
  these are deployed in life critical environments
  / situations.
• Patriot missile failure
• Therac-25 accident

18
Summary

• We studies key issues which make development of
  realtime software more challenging than desktop
  or traditional data processing applications.
• Timing is very critical for RTS input, output,
  computing and response.
• See the timing units of measurement on page 26.
• Read the reference papers