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Implementing Digital Systems
Chapter 27
 Introduction
 Semiconductor Memory
 Array Logic
 Microprocessors
 Programmable Logic Controllers
 Selecting an Implementation Method
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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)
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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
Ultra large scale integration (ULSI)
107 – 109
Giga-scale integration (GSI)
109 – 1011
Tera-scale integration (TSI)
1011 – 1013
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 Integration densities of Intel microprocessors
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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
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 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
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 Memory organisation
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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
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 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
X  A B C D+A B CD
Y  A B CD  ABCD
Z  A B C D  A B CD  ABCD
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 The structure
of a simple PLA
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 The PLA programmed
to give the required
output functions
– the device is
programmed by
blowing fusible
links at the various
interconnection
points
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 Field programmable gate arrays
– a programmable device using more complex cells
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Microprocessors
27.4
 A microcomputer system
– the CPU take the form of a microprocessor
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 Communication within the microcomputer
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 Registers
– fundamental building blocks within computers
– can be constructed using D flip-flops
– some are used for storage, others for input/output
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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
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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
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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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