Introduction to

Digital Design

Text Book• Digital Design by - John F. Wakerly

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

• www.xilinx.com - Xlinix Web site

Digital Design• Digital Design is also know as “Logical Design”.

• The purpose is to build Digital Systems.

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.)

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

Benefits of Digital over Analog

• Reproducibility

• Not effected by noise means quality

• Ease of design

• Data protection

• Programmable

• Speed

• Economy

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 4.5v

Output Output

Logic levels• Undefined region

is inherent– digital, not analog– amplification,

weak => strong• Switching threshold varies with voltage, temp, process,

phase of the moon– need “noise margin”

• The more you push the technology, the more “analog” it becomes.

• 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

Alternate transistor symbols

CMOS Gate Characteristics • No DC current flow into MOS gate terminal

– However gate has capacitance ==> current required for switching (CV2f power)• No current in output structure,

except during switching– Both transistors partially on– Power consumption related

to frequency– Slow input-signal rise times

==> more power• Symmetric output structure

==> equally strong drive in LOW and HIGH states

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 digital

design.• 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

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.

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 designusing Truth tables

• Logic designusing boolean algebra

Equations: Z = S A+ S B

• Logic diagrams

• Prepackaged building blocks, e.g. multiplexer

• Various hardware description languages– 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

Structural VHDL program for the multiplexer

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.