LECTURE 19 DIGITAL ELECTRONICS

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LECTURE 19 DIGITAL ELECTRONICS
Dr Richard ReillyDept. of Electronic
Electrical EngineeringRoom 153, Engineering
Building
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Why use a HDL (Hardware Descriptive Language) ?

• Power and Flexibility
• Has language constructs to write succinct code
  descriptions of complex control logic
• Supports design libraries and creation of
  reusable components.
•  
• Device Independent Design
• Can create a design without having to first
  choose a device for implementation. Thus can be
  FPGA or ASIC.
• Portability
• VHDL is a standard
• Can use design description on many types of
  platforms, from one simulator to another
• Can use VHDL design in many different projects

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Hardware Descriptive Languages

• Benchmarking Capabilities
• Being device independent and portable allows the
  designer to benchmark a design using different
  device architectures and different synthesis tools

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Shortcomings of VHDL

• 1. No longer have control over defining gate
  level implementation of circuits when design
  described using high level abstract constructs
•  most synthesis tools give the designer some
  level of control
• Area-efficiency or Speed efficiency
• Technology specific implementations
• VHDL will not always produce optimal
  implementations
• Needs creativity of designer to code design, to
  shape the implementation.
• Inefficiently written code, give inefficient VHDL
• 3. Benchmarking is not always produces accurate
  timings
• need accurate models of capacitance in MOS devices

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Behavioural, Structural and Physical Domains

• An Integrated Circuit may be described in terms
  of three domains
• Behavioural Domain
• Structural Domain
• Physical Domain
•  
• In each of these domains there are a number of
  design options that may be selected to solve a
  particular problem.
•  

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Behavioural, Structural and Physical Domains

• At the Behavioural Level
• can choose sequential or parallel algorithm
  (Boolean or VHDL)
•  
• At the Structural Level
• can choose logic family, clocking strategy and
  circuit style
•  
• At the Physical Level
• can choose how circuit is implemented in terms of
  chips, boards, packages

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Behavioural, Structural and Physical Domains

• These Domains can be hierarchically divided into
  levels of design abstraction.
•  
• Architectural or Functional Level
• Register Transfer Level (RTL)
• Logic Level
• Circuit Level
•  
• The relational between descriptions domains and
  levels of design abstraction are shown in the
  Y-chart.
• 3 radial lines represent the three description
  domains
• along each line are described types of objects in
  the domain

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Behavioural, Structural and Physical Domains
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Design Strategies

• Economic viability of an IC dependent largely by
  the productivity that can be brought to bear on
  design.
•  
• Depends on efficiency with which design may be
  converted
• From concept to architecture, to logic and
  memory, to circuit
• and hence physical layout.
•  
• A good VLSI design system should provide for
  consistent descriptions in all 3 descriptions
  domains
• Behavioural, Structural and Physical Domains
• and at all relevant levels of abstraction (Arch.,
  RTL, Logic, Circuit)

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Design Strategies

• Need to see importance based on the application.
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• In summary
• Performance speed, power, function
• Size of die hence cost of die
• Time of Design hence cost and schedule
• Ease of test generation and testability

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Structured Design Strategies

• A number of structured design techniques have
  been developed to deal with complex hardware.
• Borrowed from design techniques for software.
• Can be used for a small IC designed by a single
  designer
• or
• a large system designed by a team of designers.

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Hierarchy

• "Divide and conquer
• divide module into submodules,
• repeating until complexity of submodule is at a
  appropriately comprehensible level of detail.
• Allocate submodules to different designers
• At a system level, use of hierarchy allows one to
  specify single-designer projects thus project
  schedule proportional to number of available
  personnel.

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Regularity

• Hierarchy alone doesnt necessarily solve
  complexity problem
• With regularity as a guide, designer attempts to
  divide the hierarchy into a set of similar
  building blocks.

At Circuit level uniform sized transistors
(instead of optimising each device).   At
Logic Level identical Gate structure   At Higher
Level architecture that use a number of
identical processor structures.   Regularity
allows an improvement in productivity by reusing
specific designs in a number of applications
(within a number of designs).
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Modularity

• Modularity adds to hierarchy and regularity that
  submodules have "well-defined" functions and
  interfaces.
• If "well-defined" thus then well characterised.
• RS-232, RS-485, USB 1.0 or USB 2.0 serial
  interfaces
•  
• In the case of IC design corresponds to a
  well-defined behavioural, structural and physical
  interfaces that indicates the
• position, name, layer type, size and signal type
  of external interconnections.
• along with
• logic functions and electrical characteristics.
• Dividing tasks into a set of well defined modules
  aids in a team design approach.

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Locality

• By defining well-characterised interfaces for a
  module.
• stating all the other internals of a module are
  unimportant to exterior interface.
• some form, of "information hiding" is being
  carried out
• reduces apparent complexity of the module.

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Behavioural, Structural and Physical Domains
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Summary

• Strong parallels exist between methods of design
  for software systems and hardware systems.
• Hence the new term of co-design
• Use of a HDL to describe hardware systems in
  essence merges these two disciplines.