Implementing Digital Systems Chapter 27 Introduction Semiconductor Memory

Implementing Digital Systems Chapter 27 § Introduction § Semiconductor Memory § Array Logic § Microprocessors § Programmable Logic Controllers § Selecting an Implementation Method Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 1

Introduction 27. 1 § 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) Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 2

Integration level Number of transistors Zero scale integration (ZSI) 1 Small scale integration (SSI) 2– 30 Medium scale integration (MSI) 30 - 103 Large scale integration (LSI) 103 - 105 Very large scale integration (VLSI) 105 – 107 Storey: Electrical 7& Electronic 9 Systems © Pearson Education Limited 2004 Ultra large scale 10 – 10 OHT 27. 3

§ Integration densities of Intel microprocessors Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 4

Semiconductor Memory 27. 2 § 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 Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 5

§ 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 as masked 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 Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 6

§ Memory organisation Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 7

Array Logic 27. 3 § 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 Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 8

§ 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 inputs A, B, C and D and three output X, Y and Z, where Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 9

§ The structure of a simple PLA Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 10

§ The PLA programmed to give the required output functions – the device is programmed by blowing fusible links at the various interconnection points Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 11

§ Field programmable gate arrays – a programmable device using more complex cells Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 12

Microprocessors 27. 4 § A microcomputer system – the CPU take the form of a microprocessor Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 13

§ Communication within the microcomputer Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 14

§ Registers – fundamental building blocks within computers – can be constructed using D flip-flops – some are used for storage, others for input/output Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 15

Programmable Logic Controllers 27. 5 § Programmable logic controllers (PLCs) are selfcontained 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 Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 16

Selecting an Implementation Method 27. 6 § 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 Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 17

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 Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 27. 18