COMP3221: Microprocessors and Embedded Systems

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COMP3221 Microprocessors and Embedded
Systems Lecture 31 Embedded Systems
http//www.cse.unsw.edu.au/cs3221 Lecturer
Hui Wu Session 2, 2005
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• Overview
• What is an embedded system?
• Characteristics of embedded systems
• Embedded system requirements

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Embedded Systems

• An embedded system
• is a combination of hardware and software to
  perform a specific function
• is part of a larger system
• works in a reactive and time-constrained
  environment.

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Characteristics of Embedded Systems

• Application specific
• An embedded system performs a single or fixed
  set of functions
• All functions are known a priori before the
  system design begins.
• The fixed functionality provides opportunities
  for design optimization.
• Application specific processor design can be a
  significant component of some embedded systems
• Advantages
• Customization yields lower area, power, cost and
  higher performance.
• Disadvantages
• Higher hardware/software development overhead,
  resulting in longer time-to-market.
• Strict design constraints
• performance, timing, power, area, cost,
  reliability etc.

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Characteristics of Embedded Systems (Cont.)

• Multiple heterogeneous processing units
• General processor, ASIC (Application Specific
  Integrated Circuit) , ASIP (Application Specific
  Instruction set Processor), DSP (Digital Signal
  Processing processor) etc.
• Reactive
• Embedded systems constantly interact with their
  environment, taking in data from sensors and/or
  other input devices and making appropriate
  responses.
• Real-time
• Embedded systems interact with their
  environments in a timely manner.
• Parallel and distributed computing
• Many embedded systems use parallel/distributed
  architecture where multiple processing units are
  tightly or loosely coupled.

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Examples of Embedded Systems

• Consumer electronics, e.g., cellular phones,
  personal digital assistants, interactive game
  boxes, cameras, camcorders, ....
• Consumer products, e.g., washers, microwave
  ovens, ...
• Automobiles (anti-lock braking, engine control,
  ...)
• Industrial process controllers
  avionics/defence applications
• Computer/Communication products, e.g., printers,
  FAX machines, ...

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Traditional Embedded Systems Design Major
Procedures

• Modelling
• Specifying the behaviours of the target embedded
  system.
• Hardware-software partitioning
• Partitioning the specifications into either
  hardware components or software components.
• Hardware components are implemented in
  co-processors.
• Software components run on custom hardware or a
  general microprocessor.
• Hardware design and software design
• Hardware design includes co-processor design,
  interfaces etc.
• Software design includes interrupt handlers,
  task scheduler etc.

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Traditional Embedded Systems Design Major
Procedures (Cont.)
Modelling
Hardware-software partitioning
Hardware design
Software design
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Problems with Traditional Embedded Systems Design

• The precise information (execution time etc)
  about each task is not available at the
  partitioning stage.
• Designers have to use estimated values in
  partitioning, leading to bad partitioning and
  therefore bad design.
• How to solve this problem?
• Use hardware-software co-design.

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What Is Hardware-Software Co-design?

• The hardware/software designs proceed in
  parallel, with feedbacks and interactions
  occurring between the two as the design
  progresses.
• An multi-objective function of cost, area,
  power etc is used to find an optimal design.

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Goals of Embedded System Design

• Reduce time-to-market.
• Produce an optimal design which minimize the
  multi-objective function of cost, area, power
  etc.

• New design methodology and CAD tools for
  automating embedded system design are needed.
• CAD today addresses synthesis problems at a
  purely hardware level
• efficient techniques for data-path and control
  synthesis down to silicon.

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Disciplines Involved in Embedded System Design

• Application domain (Signal processing, process
  control, machine control, robot, ...).
• Software engineering
• How to build a correct and reliable embedded
  system?
• Software reuse?
• Programming Languages and Compilers
• How to reduce the execution time of each task?
• How to reduce the power consumption of
  processors and memory?

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Disciplines Involved in Embedded System Design
(Cont.)

• Operating Systems
• How to schedule tasks such that all timing
  constraints are satisfied?
• How to schedule tasks such that the processor
  power consumption is minimized?
• VLSI (computer aided) design
• How to minimize the area and maximize the
  performance for a co-processor?
• How to minimize the power consumption of a
  co-processor?

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Disciplines Involved in Embedded System Design
(Cont.)

• Parallel/Distributed systems
• Many embedded systems use parallel/distributed
  architecture where the multiple processors are
  tightly coupled or loosely coupled. Many issues
  exist.
• Task scheduling
• Resources sharing etc.
• Real-time systems (Hard soft real time
  systems)
• How to specify and satisfy timing requirements?
• How to share resource such that timing
  constraints are still satisfied?

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Embedded System Requirements

• Functional requirements
• Timing requirements
• Dependability requirements

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Functional Requirements

• Data collection
• Sensors
• AD converters
• Signal conditioning etc
• Direct digital control
• Actuators
• Man-machine interface
• Informs the operator of the current state of the
  controlled object
• Assists the operator in controlling the system.

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Timing Requirements

• Tasks Release times and deadlines
• Minimal task distance
• Maximal task distance
• Task Periods
• Minimal error detection latency
• Minimal latency jitter etc.

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Timing Requirements (Cont.)
Timing constraints are often imposed on tasks.
Typical timing constraints include

• Release time A task cannot be executed before
  its release time.
• Deadline A task is required to finish by its
  deadline.
• Minimal distance The distance between two tasks
  is required to be greater than a specified value.
  
• The distance is defined to be the difference of
  the start time of the other task completed later
  and the completion time of the task completed
  earlier.
• Maximal distance The distance between two tasks
  is required to be less than a specified value.

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Timing Requirements (Cont.)

• Period A periodic task must be executed
  periodically. For example, if the period of a
  task is 5, it must be executed and completed
  every 5 time units.

T1
T1
T1
0 2 5 7 10
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Figure 1 A periodic task T1 has a period of 5
and a worst-case execution time of 2.
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Timing Requirements (Cont.)

• Hard Timing constraints Miss of any hard timing
  constraints may cause catastrophes e.g., control
  systems for aircraft/space probes/nuclear
  reactors.
• Soft timing constraints The violation of soft
  timing constraints only causes performance
  degradation. e.g., game box.
• Embedded systems may contain both hard and soft
  timing constraints.
• Task scheduler is responsible for satisfying all
  timing constraints.

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Timing Requirements (Cont.)

• Consider an embedded system with a single
  processor and a set of 3 tasks T1, T2 and T3 with
  the following attributes
• T1 is a periodic task with a period of 4 and a
  worst-case execution time of 2
• T2 is a periodic task with a period of 5 and a
  worst-case execution time of 2
• T3 is a non-periodic task with a release time of
  0, a deadline of 20 and a worst-case execution
  of 2.

T1
T2
T1
T1
T1
T1
T2
T2
T3
T2
0 2 4 6 8 10
12 14 16 18 20
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Dependability Requirements

• Reliability
• Number of failures per hour or
  Mean-Time-To-Failure (MTTF) in hours.
• Safety
• critical failure modes
• certification
• Maintainability
• Mean-Time-To-Repair (MTTR).
• Availability
• A MTTF / (MTTF MTTR)
• Security

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Major Components in Embedded Systems

• Microprocessors/microcontrollers, co-processors,
  DSP cores, ASICs, ASIPs, FPGAs (Field
  Programmable Gate Arrays), memory (RAM, ROM,
  FLASH, EEPROM) and buses.
• Data acquisition and processing
• Communication
• System logic and control
• Interfaces
• Auxiliary units
• display
• storage
• monitoring and protection
• test and diagnosis.

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Example Embedded System (I) DVD
From LSI Logic web Page Courtesy R.
Gupta, UC Irvine
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Example Embedded System (II) Dryer
From Siemens web page Courtesy R. Gupta,
UC Irvine
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Reading Material

• Chapter 1 in Embedded Systems Design An
  Introduction to Processes, Tools, and Techniques
  by Arnold S. Berger.
• S. Edwards, L. Lavagno, E. Lee, A.
  Sangiovanni-VincentelliDesign of Embedded
  Systems Formal Methods, Validation and Synthsis.
  Proceedings of the IEEE, vol. 85 (no.3) , March
  1997, p366-290.