Implementing digital systems
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Chapter 27. Implementing Digital Systems. Introduction Semiconductor Memory Array Logic Microprocessors Programmable Logic Controllers Selecting an Implementation Method. 27.1. Introduction. In this lecture we will look at the techniques used to implement complex digital systems

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Implementing Digital Systems

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Implementing digital systems

Chapter 27

Implementing Digital Systems

  • Introduction

  • Semiconductor Memory

  • Array Logic

  • Microprocessors

  • Programmable Logic Controllers

  • Selecting an Implementation Method




  • In this lecture we will look at the techniques used to implement complex digital systems

  • We will begin by looking at the evolution of complex integrated circuits, and then progress to look at implementation strategies

  • Many terms are used to describe integration level

  • Available integration level increases exponentially with time (Moore’s Law)

Implementing digital systems

  • Integration densities of Intel microprocessors

Semiconductor memory


Semiconductor Memory

  • Random access memory (RAM)

    • this is read-write memory

    • write describes the process of storing information

    • read described the process of retrieval

    • RAM is volatile in nature

    • several forms:

      • static RAM - uses circuitry similar to a bistable

      • dynamic RAM – uses charge on capacitors, needs refreshing

    • battery backup can be used to provide non-volatility

Implementing digital systems

  • Read-only memory (ROM)

    • this can be read from, but not written to

    • is inherently non-volatile (useful for programs, etc)

    • many forms available

      • some are programmed by the manufacturer (such asmasked programmed devices)

      • others are user programmable (such as EPROM, and EEPROM)

    • memory such as EEPROM can be written to (programmed) as well as read, but it is not RAM

      • it can only be programmed relatively slowly

Implementing digital systems

  • Memory organisation

Array logic


Array Logic

  • Array logic has two major forms:

    • programmable logic devices (PLDs)

    • field programmable gate arrays (FPGAs)

  • Programmable logic devices (PLDs)

    • these are examples of uncommitted logic

    • forms include:

      • PLA – programmable logic array

      • PAL – programmable array logic

      • GAL – generic array logic

      • EPLD – erasable programmable logic device

      • CPLD – complex programmable logic device

Implementing digital systems

  • Programmable logic array (PLA)

    • has an array of inverters, AND gates and OR gates

    • can implement any logic function (given limits on numbers of inputs and outputs)

      Example: consider a system with four inputsA, B, C and D and three output X, Y and Z, where

Implementing digital systems

  • The structureof a simple PLA

Implementing digital systems

  • The PLA programmedto give the requiredoutput functions

    • the device isprogrammed byblowing fusiblelinks at the variousinterconnectionpoints

Implementing digital systems

  • Field programmable gate arrays

    • a programmable device using more complex cells




  • A microcomputer system

    • the CPU take the form of a microprocessor

Implementing digital systems

  • Communication within the microcomputer

Implementing digital systems

  • Registers

    • fundamental building blocks within computers

    • can be constructed using D flip-flops

    • some are used for storage, others for input/output

Programmable logic controllers


Programmable Logic Controllers

  • Programmable logic controllers (PLCs) are self-contained microcomputers that are optimised for industrial control

  • They consist of one or more processors together with power supply and interface circuitry

  • A range of input and output modules are available to allow the units to be used in a range of situations

  • Facilities are also provided for programming and for system development

Selecting an implementation method


Selecting an Implementation Method

  • The implementation method will depend on the complexity of the required functionality

    • applications requiring just a handful of gates might use CMOS or TTL devices

    • slightly more complex applications will often make use of array logic

    • complex digital applications will probably use either complex programmable devices (such as CPLDs or FPGAs) or a microprocessor

Key points

Key Points

  • Technologies can be categorised into a number of levels of integration from ‘zero-scale’ to ‘tera-scale’ integration

  • The available complexity doubles every couple of years

  • Semiconductor memory can be divided into RAM and ROM

  • Array logic integrates large numbers of gates within a single package that is then configured for a particular application

  • Complex digital systems can also be implemented using a microcomputer

  • A programmable logic controller is a self-contained microcomputer that is optimised for industrial control

  • The implementation method used will depend on the complexity of the required system

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