EIE 322 Interface and Embedded Systems

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EIE 322 Interface and Embedded Systems
Embedded-software architectures
Chapter 5 of D. Simon E. An Embedded
Software Primer QA76.6.S5726 1999
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Embedded Software Architectures

• Which kind of architecture to choose largely
  depends on the control you need to have over
  system response.
• How hard is it to achieve good response?
• Depends on
• responses needed specified in absolute time
• speed of your microprocessor
• other processing requirements

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Round-Robin

• This is effectively a large main program loop
  that checks conditions successively and perform
  needed actions. (Figure 5.1)
• No interrupts, no shared data, no latency
  concerns.
• Main advantage over other architectures
  Simplicity.
• It is an attractive potential architecture, if it
  can satisfy the systems performance
  requirements. Example Digital Multimeter p.116

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Round-Robin (cont)

• Disadvantages
• Worst case response time is the time it takes the
  microprocessor to get once around the main loop
  in the worst-case.
• Even when response times are not critical, the
  system may not perform well if there is any
  lengthy processing to do.
• This architecture is fragile. Addition of a
  device or requirement may break everything.
• It is chosen only for very simple devices, such
  as toys, calculators, alarm clocks and digital
  watches.

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Round-Robin with Interrupts

• In this architecture, interrupt routines deal
  with the very urgent needs of the hardware and
  then set flags the main loop polls the flags and
  does any follow-up processing required by the
  interrupts. (Figure 5.4)
• Compare to the Round-Robin architecture this one
  has more control over priorities.
• As all of the task code executes at the same
  priority, device B will need to wait after the
  execution of device A before it can start to
  execute.

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Shared-data Problem

• Shared-data problems can occur.
• You need to use the various techniques that
  discussed in last lecture for dealing with shared
  data.

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Example 1 A simple bridge

• Consider a two-ported communications bridge that
  forwards data traffic received on the first port
  to the second and vice versa. (Figure 5.6 in
  book)
• The data on one of the ports is encrypted and
  that it is the job of the bridge to encrypt and
  decrypt the data as it passes it through.
• Code for a very simple bridge is shown in Figure
  5.7 of book.

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Example 2 Cordless Bar-Code Scanner

• Although more complicated, a bar-code scanner is
  essentially a device that gets the data from the
  laser that reads the bar codes and sends that
  data out to the radio.
• As in the bridge, the only real response
  requirement is to service the hardware quickly
  enough. The task can be done quickly enough by a
  round-robin loop.

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Disadvantages

• What is the worst-case response for the task code
  for any given device ?
• Worst-case response occurs when the target
  devices interrupt is asserted just after the
  round-robin loop have skipped the devices task
  code, and every other device needs service before
  the target device will be checked again.
• The worst-case response is at least the sum of
  the execution times of the task code for every
  other device

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Disadvantages (cont)

• Examples of systems for which the
  round-robin-with interrupts architecture does not
  work well include the following ones
• A laser printer
• Why? ANS.
• Hint calculating the locations where the black
  dots go is very time-consuming.
• The underground tank-monitoring system
• Why? ANS.

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Function-Queue-Scheduling Architecture

• Figure 5.8 shows another, yet more sophisticated
  architecture.
• Upon occurrence of an event, the interrupt
  routines add function pointers to a queue of
  function pointers .
• The main loop repeatedly fetch pointers from the
  queue to call the functions.

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Function-Queue-Scheduling Architecture (cont)

• Advantages
• The main loop can call functions based on any
  priority scheme that suits your purposes. Any
  task code functions that need quicker response
  can be executed earlier.
• Draw backs
• If one of the lower-priority task code functions
  is quite long, it will affect the response for
  the higher-priority functions.

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Real-Time Operating System Architecture

• A very simple sketch of how it works is shown in
  Figure 5.9.
• The differences between this architecture and the
  previous ones are that
• Signaling between the interrupt routines and the
  task code is handled by the real-time operating
  system. Share variables can also be omitted.

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Real-Time Operating System Architecture (cont)

• What needs to be done next is not decided by a
  main loop.
• The real-time operating system decides which of
  the task code functions should run next.
• The real-time operating system knows which task
  is more urgent.
• The real-time operating system can suspend on
  task code subroutine in the middle of its
  processing in order to run a higher priority one.
• Sophisticated uses of this architecture will be
  discuss more in later lectures.

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Advantages

• Advantages
• System using the real-time-operating-system
  architecture can control task code response as
  well as interrupt routine response. The possible
  priority levels for a real-time operating system
  architecture is shown in Figure 5.10.
• Changes to lower-priority functions do not
  generally affect the response of higher-priority
  functions.

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Disadvantages

• Disadvantages
• The primary disadvantage is that the real-time
  operating system is a big piece of code and
  itself uses a certain amount of processing time.
  You are getting better response at the expense of
  code memory and some processing time.

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Characteristics of Various Software Architectures

• Priorities
• Response
• Stability
• Simplicity

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Selecting an Architecture

• Suggestions about selecting an architecture for
  your system.
• Select the simplest architecture that will meet
  your response requirement.
• If your system has many tasks of wide response
  requirements, you should then consider using a
  real-time operating system.

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Selecting an Architecture

• Hybrid architectures can make sense for some
  systems. Example even if you are using a
  real-time operating system, you can let a
  low-priority task to poll those parts of the
  hardware(e. g. keypad) that do not need fast
  response.

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Capture event A Enqueue pointer_to_function_A
Capture event B Enqueue pointer_to_function_B
Main
Call up functions in the queue
Actions for A
Actions for B
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Answer A laser printer

• Since calculating the locations where the black
  dots go is very time-consuming the only code
  that will get good response is code in interrupt
  routines.
• Any task code may potentially be stuck while the
  system calculates more locations for black dots.
• Back to the question

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Answer the underground tank-monitoring system

• Similar to the laser printer, the tank-monitoring
  system has a processor hog the code that
  calculates how much gasoline is in the tanks.
• To avoid putting all the rest of the code into
  interrupt routines, a more sophisticated
  architecture is required for this system as well.
• Back to the question