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Introduction to VLSI - VHDL

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    INTRODUCTION TO VLSI

    Introduction

    Integrated circuits were made possible by experimental discoveries which showed that

    semiconductor devices could perform the functions of vacuum tubes, and by mid-20th-century

    technology advancements in semiconductor device fabrication. The integration of large numbers

    of tiny transistors into a small chip was an enormous improvement over the manual assembly of

    circuits using discrete electronic components. The integrated circuit's mass production capability,

    reliability, and building-bloc approach to circuit design ensured the rapid adoption of

    standardi!ed I"s in place of designs using discrete transistors. There are two main advantages of

    I"s over discrete circuits - cost and performance. "ost is low because the chips, with all their

    components, are printed as a unit by photolithography and not constructed a transistor at a time.#erformance is high since the components switch $uicly and consume little power, because the

    components are small and close together. %s of 200&, chip areas range from a few s$uare mm to

    around 20 mm2, with up to ( million transistors per mm2.

    Advances in Integrated circuits

    %mong the most advanced integrated circuits are the microprocessors, which control

    everything from computers to cellular phones to digital microwave ovens. )igital memory chips

    are another family of integrated circuit that is crucially important to the modern information

    society. *hile the cost of designing and developing a complex integrated circuit is $uite high,

    when spread across typically millions of production units the individual I" cost is minimi!ed.

    The performance of I"s is high because the small si!e allows short traces, which in turn allows

    low power logic +such as "/ to be used at fast switching speeds.

    I"s have consistently migrated to smaller feature si!es over the years, allowing morecircuitry to be paced on each chip. %s the feature si!e shrins, almost everything improves - the

    cost per unit and the switching power consumption go down, and the speed goes up. owever,

    I"'s with nanometer-scale devices are not without their problems, principal among which is

    leaage current, although these problems are not insurmountable and will liely be solved or at

    least ameliorated by the introduction of high- dielectrics. ince these speed and power

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    chips produced in (66@ contained more than three million transistors.This step was largely made

    possible by the codification of design rules for the " technology used in >

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    nown as B. These devices are used in a variety of commercial and defense applications,

    including pro3ectors, in 3et printers, and accelerometers used to deploy the airbag in car

    accidents. In the past, radios could not be fabricated in the same low-cost processes as

    microprocessors. 4ut since (66=, a large number of radio chips have been developed using

    " processes. Bxamples include Intel's )B"T cordless phone, or %theros's =02.(( card.

    Moore&s La!

    The growth of complexity of integrated circuits follows a trend called oore's

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    the structure of modern societies. That is, modern computing, communications, manufacturing

    and transport systems, including the Internet, all depend on the existence of integrated circuits.

    Indeed, many scholars believe that the digital revolution brought about by integrated circuits was

    one of the most significant occurrences in the history of manind.

    $# VLSI(

    Integration improves the designC

    lower parasitics D higher speed.

    lower power.

    physically smaller.

    Integration reduces manufacturing cost-no manual assembly.

    C$allenges in VLSI Design

    ultiple levels of abstractionC transistors to "#?s.

    ultiple and conflicting constraintsC low cost and high performances are often at odds.

    hort design timeC

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    "reate lower levels of abstraction from upper levels.

    4ottom-up design creates abstractions from low-level behavior.

    Aood design needs both top-down and bottom-up efforts.

    Design Strategies

    I" design productivity depends on the efficiency with which the design may be converted

    from concept to architecture, to logic and memory, to circuit and hence to a physical layout.

    % good design strategy with a good design system should provide for consistent descriptions invarious abstraction levels.

    The role of good design strategies is to reduce complexity, increase productivity, and assureworing product.

    )esign is a continuous trade-off to achieve ade$uate results forC

    #erformance - speed, power, function, flexibility

    i!e of die +hence cost of die/

    Time to design

    Base of test generation and testability

    ard!are Descri%tion Langauges DLs.

    IBBB standardi!ed

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    S'2CI1ICATION

    This is the stage at which we define what are the important parameters of the

    system9design that you are planning to design. % simple example would beC I want to design a

    counter5 it should be @ bit wide, should have synchronous reset, with active high enable5 when

    reset is active, counter output should go to 0.

    I3 L2V2L D2SI3N

    This is the stage at which you define various blocs in the design and how they

    communicate.

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    erilog9>)< code, using

    synthesi!able constructs of the language. Formally we lie to lint the code, before starting

    verification or synthesis.

    SIMULATION

    imulation is the process of verifying the functional characteristics of models at any level

    of abstraction. *e use simulators to simulate the ardware models. To test if the 1T< code meets

    the functional re$uirements of the specification, we must see if all the 1T< blocs are

    functionally correct. To achieve this we need to write a test4enc$, which generates cl, reset andthe re$uired test vectors. *e use the waveform output from the simulator to see if the )?T

    +)evice ?nder Test/ is functionally correct.

    S5NT2SIS

    ynthesis is the process in which synthesis tools lie design compiler or ynplify tae

    1T< in >erilog or >)

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    The purpose of >)< descriptions is to provide a model for digital circuits and systems.

    This abstract view of the real physical circuit is referred to as entity. %n entity normally consists

    of five basic elements, or design units.

    In >)< one generally distinguishes between the external view of a module and its

    internal description. The external view is reflected in the entity declaration, which represents an

    interface description of a 'blac box'. The important part of this interface description consists of

    signals over which the individual modules communicate with each other.

    The internal view of a module and, therefore, its functionality is described in the

    architecture body. This can be achieved in various ways. ne possibility is given by coding a

    behavioral description with a set of concurrent or se$uential statements. %nother possibility is a

    structural description, which serves as a base for the hierarchically designed circuit architectures.

    Faturally, these two inds of architectures can also be combined. The lowest hierarchy level,

    however, must consist of behavioral descriptions. ne of the ma3or >)< features is the

    capability to deal with multiple different architectural bodies belonging to the same entity

    declaration.

    4eing able to investigate different architectural alternatives permits the development of

    systems to be done in an efficient top-down manner. The ease of switching between different

    architectures has another advantage, namely, $uic testing. In this case, it is necessary to bind

    one architecture to the entity in order to have a uni$ue hierarchy for simulation or synthesis.

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    *hich architecture should be used for simulation or synthesis in con3unction with a given entity

    is specified in the configuration section. If the architecture body consists of a structural

    description, then the binding of architectures and entities of the instantiated submodules, the so-

    called components, can also be fixed by the configuration statement.

    The pacage is the last element mentioned here. It contains declarations of fre$uently

    used data types, components, functions, and so on. The pacage consists of a pacage declaration

    and a pacage body. The declaration is used, lie the name implies, for declaring the above-

    mentioned ob3ects. This means, they become visible to other design units. In the pacage body,

    the definition of these ob3ects can be carried out, for example, the definition of functions or the

    assignment of a value to a constant. The partitioning of a pacage into its declaration and body

    provides advantages in compiling the model descriptions.

    2ntit# Declaration

    %n entity declaration specifies the name of an entity and its interface. This corresponds to

    the information given by the symbols in traditional design methods based on drawing

    schematics. ignals that are used for communication with the surrounding modules are called

    ports.

    Inter"ace o" a "ull-adder module

    2)am%le /

    entity :?

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    The module :?)

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    The second important component of a >)< description is the architecture. This is

    where the functionality and the internal implementation of a module are described. In general, a

    complex hierarchically structured system may have the topology.

    ierarc$ical circuit design

    C structural description4C behavioral description49C mixed description

    In order to describe such a system both behavioral and structural descriptions are

    re$uired. % behavioral description may be of either concurrent or se$uential type. verall,

    >)< architectures can be classified into the three main typesC

    )ata flow modeling.

    4ehavioral modeling.

    tructural modeling.


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