EET 1131 Unit 2 Digital Signals and Switches

Read Kleitz, Sections 2-1 to 2-5, and Section 2-12. Skim the rest of Chapter 2.

Homework #2 and Lab #2 due next week.

Quiz next week.

Digital Signal

In digital circuits, the logic level at any point usually changes as time passes. This changing level is called a digital signal (or digital waveform).

Graph of a Digital Signal

We can describe a digital signal by graphing the voltage (or logic level) versus time.

Oscilloscope

For slow-changing signals, you could graph a digital signal by hand, using a logic probe and a wristwatch.

But usually we use an oscilloscope to do this work for us.

Periodic Digital Signal

A periodic digital signal is one that repeats itself at regular intervals.

Which one of these signals is periodic?

Cycle

In a periodic signal, each repetition is called a cycle.

How many cycles are shown in the diagram below?

Period

The time required for one cycle is called the signal’s period.

The symbol for period is tp. (Some books use T.)

Period is measured in seconds, abbreviated s.

Example: If a signal repeats itself every 3 seconds, we’d write

tp = 3 s

Frequency

A signal’s frequency is the number of cycles that occur in one second.

The symbol for frequency is f. Frequency is measured in hertz,

abbreviated Hz. Some old-timers say “cycles per

second” instead of “hertz.” Example: If a signal repeats itself 20

times every second, we’d writef = 20 Hz

Period and Frequency

Period and frequency are the reciprocal of each other:

f = 1 / tp

tp = 1 / f What are the period and frequency

of this signal?

Large and Small Numbers

Engineers and technicians often deal with very large or very small numbers.

Example: a system’s clock signal might have a frequency of 750,000 Hz and a period of 0.00000133 s.

It’s not convenient to write or discuss numbers using so many zeroes. Instead we use engineering prefixes, which are abbreviations for certain powers of 10.

Copyright ©2012 by Pearson Education, Inc.All rights reserved.

Digital Electronics: A Practical Approach with VHDL, 9th EditionWilliam Kleitz

Engineering Prefix Game

You must memorize these prefixes. To practice, play the Metric Prefix

matching game on my Games page.

Using Engineering Prefixes

Whenever you have a number that’s greater than 1000 or less than 1, you should use these prefixes.

Examples: Instead of writing 750,000 Hz,

write 750 kHz (pronounced “750 kilohertz”).

Instead of writing 0.00000133 s,write 1.33 s (pronounced “1.33 microseconds”).

Calculator’s Exponent Key

Scientific calculators have an exponent key (usually labeled EE, EXP, or E) that lets you easily enter numbers with engineering prefixes.

Examples: To enter 750 k, press 750 EE 3. To enter 1.33 , press 1.33 EE −6.

Calculator Modes

Most scientific calculators also have an engineering mode, which forces the answer always to be displayed with one of the engineering powers of 10.

Learn how to use this feature of your calculator. It will save you from making mistakes.

Function Generator

To produce a periodic digital signal, you could use a switch that you flip up and down by hand at regular intervals.

But usually we use a function generator to do this work for us.

Trainer Function Generator

TTL Mode Output, controlled by the FREQUENCY and RANGE knobs. In this course we’ll always use this one.

Regular Output, controlled by all four knobs. You’ll use this in other courses.

No matter which oneof these you use, youmust also use the GROUND connection.

When we build circuits on the breadboard, we must power each DIP by providing +5 V to the DIP’s power pin.

The Function Generator Does Not Replace the Power Supply

We never connect a DIP’s power pin to the function generator. We connect every DIP’s power pin to the trainer’s power supply instead.

© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved

Digital waveforms change between the LOW and HIGH levels. A positive going pulse is one that goes from a normally LOW logic level to a HIGH level and then back again. Digital waveforms are made up of a series of pulses.

Digital Waveforms

Falling orleading edge

(b) Negative–going pulse

HIGH

Rising ortrailing edge

LOW

(a) Positive–going pulse

HIGH

Rising orleading edge

Falling ortrailing edge

LOWt0 t1 t0 t1

© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved

A timing diagram (or waveform diagram) is used to show the relationship between two or more digital waveforms.

Timing Diagrams

Clock

A

B

C

© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved

Data can be transmitted by either serial transfer or parallel transfer.

Serial and Parallel Data

Computer Modem

1 0 1 1 0 0 1 0

t0 t1 t2 t3 t4 t5 t6 t7

Computer Printer

0

t0 t1

1

0

0

1

1

0

1

Serial Transmission

Uses a single electrical conductor for data, so it’s inexpensive.

Slow, since only one bit for each clock period. Used for telephone lines, computer-to-

computer networks. Computer COM ports and USB ports

are used for serial communications. Ethernet plug-in cards are for serial

networking.

Serial Representation of a Binary Number

A serial representation of the binary string 01101100 is illustrated below. Note that only one bit is transmitted

per clock period.

Parallel Transmission

Separate electrical conductor for each bit, so more expensive than serial.

Very fast Inside a computer

Data bus External Devices

Centronics printer interface (LPT1) SCSI (Small Computer Systems

Interface)

Serial versus Parallel Transmission

Question 1: How long will it take to transmit an 8-bit binary string using serial transmission if the clock frequency is 25 MHz?

Question 2: How long will it take to transmit an 8-bit binary string using 8-bit parallel transmission if the clock frequency is 25 MHz?

Ports on a Typical Laptop Computer

USB(Serial)

IEEE 1394(Serial)

RS-232(Serial)

Printer (Parallel)

PS/2 Mouse (Serial)

VGA (Analog video)

Ports on a Fluke 45 DMM

RS-232(Serial)

IEEE 488 (“GPIB”) option not installed (Parallel)

Ports on a Tektronix TDS2014

RS-232(Serial)

IEEE 488 (“GPIB”) (Parallel)

Printer (Parallel)

• A digital circuit is basically a combination of a huge number of switches.

• Each switch is either on or off at any given time.

• Unlike manual switches that require a person to switch them, we need automatic switches that can be turned on or off by the voltage level (HIGH or LOW) present at the switch’s control input.

Switches

• Before the invention of semiconductor devices in the mid-1900s, the electromechnical relay was the main type of automatic switch.

• Still used in industrial applications, where large currents are involved.

Electromechanical Relays

• Relays are slow, large, and expensive compared to modern electronic switches (diodes and transistors).

• In past decades, some digital electronics used diodes as switches, but today almost all digital electronics is based on using transistors as switches.

Electronic Switches

• In Electronic Devices & Circuits (EET 2201) you’ll study two major classes of transistors:

• Bipolar Junction Transistors (BJTs)

• Metal-Oxide Semiconductor Field Effect Transistor (MOSFETs)

Two Kinds of Transistors

• Two major logic families:

• TTL (Transistor-Transistor Logic) based on bipolar junction transistors

• CMOS (Complementary Metal Oxide Semiconductor) based on MOSFETs

• Within each family are several subfamilies; we’ll study these in Chapter 9.

• Originally, TTL chips were fast but used lots of power, and CMOS chips used little power but were slow.

• CMOS chips are sensitive to static discharge, and must be handled carefully.

Logic Families

• A popular series of TTL chips is the 7400 series that you’ll use in Sinclair’s digital courses: Wikipedia's list

• A popular series of CMOS chips is the 4000 series: Wikipedia's list

7400 Series and 4000 Series