lec 11 introduction to digital electronics

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Introduction

to

Digital

Electronics



Suplementary Reading

• Digital Design

by - John F. Wakerly

– www.ddpp.com - you will find some solutions at this site.

– www.xilinx.com - Xlinix Web site

• Logic and Computer Design Fundamentals

by - M. Morris Mano & Charles R. Kime

• Digital Design

by - M. Morris Mano

• Digital Logic Circuit Analysis and Design by - Victor P. Nelson, H. Troy Nagle, J. David Irwin & Bill D. Carrol



Digital Electronics

• Digital Electronics represents information (0, 1) withonly two discrete values.

• Ideally

“no voltage” (e.g., 0v) represents a 0 and

“full source voltage” (e.g., 5v) represents a 1• Realistically

“low voltage” (e.g., <1v) represents a 0 and

“high voltage” (e.g., >4v) represents a 1

• We achieve these discrete values by using switches.

• We use transistor switches, which operates at high speed,

electronically, a small in size.



Analog versus Digital

• Analog systems process time-varying signals that

can take on any value across a continuous range

of voltages (in electrical/electronics systems).

• Digital systems process time-varying signals that

can take on only one of two discrete values of

voltages (in electrical/electronics systems).

– Discrete values are called 1 and 0 (ON and OFF,HIGH and LOW, TRUE and FALSE, etc.)



Representing Information Electronically

• A light bulb has to represent 2 different information:

Bulb off - no student inside

Bulb Full lit - 1 student inside

• A light bulb has to represent 4 different information: – How? With one bulb?

– Use two bulbs

• A light bulb has to represent 10 different information:

– Use four bulbs



Representing Information Electronically

• “Analog electronics” deals with non-discrete values

• “Digital electronics” deals with discrete values



Benefits of Digital over Analog

• Reproducibility

• Not effected by noise means quality

• Ease of design

• Data protection• Programmable

• Speed

• Economy



Digital Revolution

• Digital systems started back in 1940s.

• Digital systems cover all areas of life:

– still pictures

– digital video

– digital audio

– telephone

– traffic lights

– Animation



Digital Devices

•Gates•Flip-Flops

•PLDs

•FPGAs



Gates

• The most basic digital devices are called gates.

• Gates got their name from their function of

allowing or blocking (gating) the flow of digital

information.

• A gate has one or more inputs and produces an

output depending on the input(s).

• A gate is called a combinational circuit.

• Three most important gates are: AND, OR , NOT



Digital Logic

• Binary system -- 0 & 1, LOW & HIGH,

negated and asserted.• Basic building blocks -- AND, OR, NOT



AND, OR , NOT Gates



Electronic Aspects of Digital Design

• How we represent digital information in electronic devices?

• By discrete voltages.



What is the

Basic Digital Element

in Electronics

?

a Switch



Using Switch to represent digital information



Digital Abstraction

• It is difficult to make ideal switches means a

switch is completely ON or completely OFF.

• So, we impose some rules that allow analog

behavior to be ignored in most cases, so circuits

can be modeled as if they really did process 0s

and 1s, known as digital abstraction.

• Digital abstraction allows us to associate a noise

margin with each logic values (0 and 1).



Real Switches to represent digital information

5v 5v

1k

10k

5v 4.5vOutput Output



Logic levels• Undefined region

is inherentdigital, not analog

• Switching threshold varies with voltage, temp

need “noise margin”

• Logic voltage levels decreasing with new processors.

5 , 3.3 , 2.5 , 1.8 V



MOS Transistors

NMOS

PMOS

Voltage-controlled resistance



CMOS Inverter



Switch model



Flip-flops

• A device that stores either a 0 or 1.

• Stored value can be changed only at certain times

determined by a clock input.

• New value depend on the current state and it’s

control inputs

• A digital circuit that contains filp-flops is called a

sequential circuit



Flip-flops

S-R latch symbols D flip-flop

J-K flip-flops



Integrated Circuits

• A collection of one or more gates fabricated on a

single silicon chip is called an integrated circuit

(IC).

• ICs were classified by size:

– SSI - small scale integration - 1~20 gates

– MSI - medium scale integration - 20~200 gates

– LSI - large scale integration - 200~200,000 gates

– VLSI - very large scale integration - over 1M

transistors

• Pentium-III - 40 million transistors



DIP Packages



Gates in ICs



Programmable Logic Devices

• PLDs allow the function to be programmed into

them after they are manufactured.

• Complex PLDs (CPLD) are a collection of PLDs

on the same chip.

• Another programmable logic chip is FPGA -

field-programmable gate arrays.



CPLDs and FPGAs

FPGACPLD



Application Specific ICs (ASICs)

• Chips designed for a particular application are

called semicustom ICs or application-specific ICs

(ASICs).

• ASICs generally reduce the total component and

manufacturing cost of a product by reducing chip

count, physical size, and power consumption, and

they often provide higher performance.

• But costly if not produced in bulk.



Printed-Circuit Boards

• An IC is normally mounted on a printed-circuit

board (PCB) that connects it to other ICs in a

system.

• Individual wire connection or traces can be as

narrow as 4 mils with 4 mils spacing (one-

thousandth of an inch)

• Now a days, most of the components use surface

mount technology.

• They are normally layered.



Software Aspects of Digital Design

• Today software tools are an essential part of digitaldesign.

• Software tools improve productivity, correctness and

quality of designs

• Software tools are:

– Schematic entry

– HDL (Hardware Description Language) Editors

– Simulators - to verify the behaviour of the design

– Synthesis tools - circuit design

– Timing analyzers and verifiers

Di i l D i L l



Digital Design Levels

• the lowest level of design is device physics and

IC manufacturing processes.

• design at the transistor level

• level of functional building blocks

• level of logic design using HDLs

• computer design and overall system design.

Diff D i L l



Different Design Levels

Consider a simple design example:

Build a multiplexer with two data inputs A and B, a

control input S, and an output Z.

Switch model for the example multiplexer



Designing at the transistor level

• Transistor-levelcircuit diagrams

• Gate symbols (for simple elements)



• Logic design

using Truth tables



• Logic design

using boolean algebra

Equations: Z = S A + S B

• Logic diagrams



• Prepackaged building blocks, e.g. multiplexer



• Various

hardware

descriptionlanguages

– ABEL

– VHDL

• We’ll start with

gates and work

our way up



• Name of the program

module

• the type of PLD

• pin numbers

• ABEL statement to

achieve the

multiplexer

• Standard library

• and a set of definitions

• Inputs and outputs

• functions behaviour

St t l VHDL f th lti l



Structural VHDL program for the multiplexer

S



Summery

• Design to minimize cost.

• Rule of thumb is to minimize the number of ICs.

• Though PLDs costs more but uses less PCB area.

• Unless mass production avoid ASIC design.• Design to solve real life problems.

lec 11 introduction to digital electronics

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