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EET 1131 Unit 2
Digital Signals and Switches
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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
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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
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We can describe a digital signal by
graphing the voltage (or logic level)
versus time.
Oscilloscope
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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
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A periodic digital signal is one that
repeats itself at regular intervals.
Which one of these signals is
periodic?
Cycle
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In a periodic signal, each repetition
is called a cycle.
How many cycles are shown in the
diagram below?
Period
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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
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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.
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Some old-timers say “cycles per
second” instead of “hertz.”
Example: If a signal repeats itself
20 times every second, we’d write
f = 20 Hz
Period and Frequency
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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
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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.
Digital Electronics: A Practical Approach with VHDL, 9th Edition
William Kleitz
Copyright ©2012 by Pearson Education, Inc.
All rights reserved.
Engineering Prefix Game
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You must memorize these prefixes.
To practice, play the Metric Prefix
matching game on my Games page.
Using Engineering Prefixes
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Whenever you have a number that’s
greater than 1000 or less than 1,
you should use these prefixes.
Examples:
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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
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Scientific calculators have an
exponent key (usually labeled EE,
EXP, or E) that lets you easily enter
numbers with engineering prefixes.
Examples:
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To enter 750 k, press 750 EE 3.
To enter 1.33 , press 1.33 EE −6.
Calculator Modes
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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
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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
Regular Output,
controlled by all four
knobs. You’ll use
this in other courses.
No matter which one
of these you use, you
must also use the
GROUND connection.
TTL Mode Output, controlled
by the FREQUENCY and
RANGE knobs. In this
course we’ll always use
this one.
The Function Generator Does Not
Replace the Power Supply
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When we build circuits on the
breadboard, we must power each
DIP by providing +5 V to the DIP’s
power pin.
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.
Digital Waveforms
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.
HIGH
HIGH
Rising or
leading edge
LOW
Falling or
trailing edge
t0
(a) Positive–going pulse
t1
Falling or
leading edge
LOW
Rising or
trailing edge
t0
t1
(b) Negative–going pulse
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Timing Diagrams
A timing diagram (or waveform diagram) is used to show
the relationship between two or more digital waveforms.
Clock
A
B
C
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Serial and Parallel Data
Data can be transmitted by either serial transfer or parallel
transfer.
1
t0
0
t1
1
t2
1
t3
0
0
t 4 t 5 t6
1
0
t7
Computer
Modem
1
Computer
Printer
0
1
1
0
0
1
0
t0
t1
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Serial Transmission
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Uses a single electrical conductor
for data, so it’s inexpensive.
Slow, since only one bit for each
clock period.
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Used for telephone lines, computer-tocomputer 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
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A serial representation of the binary
string 01101100 is illustrated
below.

Note that only one bit is transmitted
per clock period.
Parallel Transmission
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Separate electrical conductor for
each bit, so more expensive than
serial.
Very fast
Inside a computer
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Data bus
External Devices
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Centronics printer interface (LPT1)
SCSI (Small Computer Systems
Interface)
Serial versus Parallel
Transmission
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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
IEEE 488 (“GPIB”) option
not installed (Parallel)
RS-232
(Serial)
Ports on a Tektronix TDS2014
RS-232
(Serial)
Printer
(Parallel)
IEEE 488 (“GPIB”)
(Parallel)
Switches
•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.
Electromechanical Relays
•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.
Electronic Switches
•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.
Two Kinds of Transistors
•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)
Logic Families
•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.
7400 Series and 4000 Series
•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