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COMP3222/9222 Digital Circuits & Systems

21T3 L01 - Introduction

Introduction
What you will learn in this class
• Introduction to the design of digital logic circuits
– Boolean algebra, logic minimization, combinational logic components, sequential circuits, simple systems
• Principles of creating digital circuit designs
– using VHDL hardware description language
– simulation techniques to verify the correct working of designs
– logic compilers to synthesize hardware
– implementing and testing designs using programmable hardware
Introduction
What is logic design?
• What is design?
– given a specification of a problem, come up with a way of
solving it choosing appropriately from a collection of available
tools, techniques and components
– while meeting certain performance criteria for size, cost, power,
beauty, elegance, etc.
• What is logic design?
– determining the collection of digital logic components and
the interconnections between them to perform a specified
data gathering, processing, storage, control and/or
communication function
– which logic components to choose? – there are many
implementation technologies, with differing costs/benefits
– the design may need to be optimized and/or transformed to meet
design constraints
Introduction
Applications of logic design
• Computer hardware & systems
– design of processors, CPUs, systems, accelerators,
custom chips, buses, interfaces, memories, peripherals
• Communications & networks
– I/O devices, phones, switches, routers, base stations, satellites
• Embedded systems
– consumer electronics, appliances, entertainment devices, IoT,
medical devices, security, transport, robotics, mining, agriculture and
manufacturing equipment
• Scientific instruments & equipment
– simulation, testing, sensing, logging, reporting, analyzing
• Challenge:
– Which human activities (a) don’t yet, and (b) won’t ever involve
digital technology?
Introduction
Why study logic design?
• Obvious reasons
– fundamental abstraction for implementing all digital devices
– to study the digital design process
– this course is part of the CompEng requirements
• Other important reasons
– it’s an important counterpart to software design
– it’s essential to furthering our understanding of the efficient
implementation of computation
– the inherent parallelism in hardware provides exposure to real
parallelism on a large, fine-grained scale
low cost
high performance
COMP3222/9222 details
• Lectures, Weeks 1 – 10
– Mon 16-18 Colombo Th B
– Wed 14-16 Ainsworth G02
• Labs, Weeks 1 – 10
– 2 hr Labs: Tue 12-14, Wed 16-18, Wed 18-20, Thu 10-12, Thu 12-14
in K17 Lyre lab
– Pick up take-home lab kit during first lab (this week)
• Tutes, Weeks 2 – 10
– 1 hr Tutes: Tue 14-15, Tue 15-16 Quad G045,
Thu 14-15, Thu 15-16 Quad G031
5, 7-10
Recordings availa le on website for asynchronous viewing
• I assume you have viewed the relevant recording before the synchronous
meetings – see the Lecture schedule on the course website
– Synchronous meetings to review main points, discuss questions and work
through exercises: Mon 14–16 & Wed 16–18
• Labs, Weeks 1 – 10
– Lab exercises and resources available on website
– 3 hr official online drop-in session where you can obtain help and ask
questions of demostrators: Wed 12–15
– 2 hr optional online drop-in sessions: Tue 14–16, Thu 12–14
– Submissions due roughly each week on Mondays at 23:59
COMP3222/9222 assessment
• Assessment:
– 7 lab exs (due the week after they are scheduled): 40% total
– 4 fortnightly quizzes on theory in Weeks 3, 5, 7 & 9: 15% total
– 1 hr Final Theory Test: 15%
– 2 hr Final Practical Test: 30% (need to score >40% in Prac Exam
to pass course)
• Supplementaries
– Only one eligibility criterion:
1. Need to score >45% overall and >36% in Practical Test ⇒ final score = 50% if
supp passed
2. Certified medical excuse ⇒ final score calculated as if sitting normal exam
– Note: there is no supp for missing the lab submission deadlines or
quizzes
Introduction
COMP3222/9222 text
• Brown & Vranesic, Fundamentals of Digital Logic with
VHDL Design, 3ed, McGraw-Hill
– out of print, so not available at bookshop
– available for short loan from the Library
– exercises drawn from text
– extracts of Ch 1 & 2 provided on course website
– text is designed to be used with the FPGA board you will be
using
• contains tutorials on loading & using the CAD tools
• provides a detailed reference on VHDL
• will be an invaluable aid in the follow-on computer architecture &
design project courses
Introduction
Acknowledgement
• Most of the slides in this course are modified from the
current text (Brown & Vranesic), or from Katz,
Contemporary Logic Design, 2ed, Pearson
• The material provided by these authors is gratefully
acknowledged
Introduction
Digital circuits
• The study of digital logic circuits is primarily motivated
by their use in digital computers
• Logic circuits perform operations on digital signals and
are usually implemented as electronic circuits in which
the signal values are restricted to a few discrete values
– In binary logic circuits, there are only two values, 0 and 1
– Decimal logic circuits would have 10 values
• In contrast, in analog circuits, signals may take on
continuous values between maximum and minimum
levels
• We will only consider binary digital circuits
– These are dominant because of their simplicity
– The simplest binary element is a switch that has two states
Introduction
• We say the switch is controlled by input variable x, and
that the switch is open if x = 0 and closed if x = 1
x 1 = x 0 =
(a) Two states of a switch
S
x
(b) Symbol for a switch
A binary switch
open closed
L01/S11
• The state of the
light L is dependent
on the state of the
switch
• The light is on, L =
1, if the switch is
closed, x = 1,
and vice versa
• Thus L(x) = x
• li t i , 1,
if the switch is closed,
x = 1, and vice versa
• Thus L(x) = x
Introduction
(a) Simple connection to a battery
S
(b) An equivalent circuit using a ground connection as the return path
Battery Light
Power
supply
S
Light
x
x
A light controlled by a switch
Introduction
(a) The logical AND function (series connection)
S
Power
supply
S
S
Power
supply S
(b) The logical OR function (parallel connection)
Light
Light x1 x2
x1
x2
Two basic functions
• In a series connection,
both switches need to
be closed for the light
to be on
• Thus, L(x1, x2) = x1 ∙ x2
• In a parallel connection,
either one of the switches
need to be closed for the
light to be on
• Thus, L(x1, x2) = x1 + x2
Introduction
S
Power
supply S Light
S X1
X2
X3
A series-parallel connection
L(x1, x2, x3) = ?
Which function determines the output of the light?
How many possible states are there for this circuit?
How many of these have the light on?
Introduction
• Since the switch, when it is closed, short-circuits the
potential difference across the light:
L(x) = x, where L = 1 if x = 0 and L = 0 if x = 1
• The complement op, NOT, is expressed in many ways:
x = x’ = !x = ~x = NOT x
S Light
Power
supply
R
x
Inversion
Introduction
Truth tables for the AND, OR and NOT operations
AND OR NOT
x 1
x 2
x n
x 1 x 2 … x n + + +
x 1
x 2
x 1 x 2 +
x 1
x 2
x n
x 1
x 2
x 1 x 2 ∙ x 1 x 2 … x n ∙ ∙ ∙
(a) AND gates
(b) OR gates
x x
(c) NOT gate
Basic gate symbols
• Each logic operation
can be implemented
electronically with
transistors, resulting in a
circuit element called a
logic gate
• Each logic gate has one
or more inputs and one
output that is a function
of its inputs
• A logic circuit is often
described by drawing a
circuit diagram, or
schematic, consisting of
graphical symbols
representing the logic
gates
Introduction
• A larger circuit is implemented by a network of gates
• Such circuits are called logic networks or logic circuits
• The complexity of a given network (in terms of the gate
count & number of gate inputs) has a direct impact on its
cost
• In order to reduce manufactured cost, we seek ways to
implement logic circuits as inexpensively as possible