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Chapter 2
Digital Electronic Signals and
Switches
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Digital Signals
• Made up of a series of 1’s and 0’s
• Timing Diagram
– voltage versus time
– shows logic state
• If not exactly 0V or 5V
– Still interpreted as a 0 or a 1
• Use Oscilloscope to view
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Clock Waveform Timing
• Precise timing
• Periodic clock waveform
– repetitive form
– specific time interval
– successive pulses identical
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
• Period
– Length of time between rising or falling edges
– Basic unit is seconds
• Frequency
– The reciprocal of the period
– Basic unit is hertz
• f = 1/tp and tp = 1/f
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Serial Representation
• Single electrical conductor
• Slow (relative)
– one bit for each clock period
– telephone lines, intra-computer
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–5
Serial communication between computers.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Serial Representation
• Several standards
– V.90, ISDN, T1, T2, T3, USB, Ethernet,
10baseT, 100baseT, cable, DSL, RS-232
• COM - 115 kbps
• USB
– USB 1.1 12 Mbps
– USB 2.0 480 Mbps
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Parallel Representation
•
•
•
•
•
Separate electrical conductor for each bit
Expensive
Very fast
Inside a computer
External Devices
– Centronics printer interface (LPT1)
– SCSI (Small Computer Systems Interface)
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–7
Parallel communication between a computer and a printer.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Parallel Representation
• LPT1
– 8-bit parallel
– 115 kBps = 115 kBps x 8 bits/byte = 920 Kbps
• SCSI
– 16-bit parallel
– 160 MBps
• Bps - BYTES per second
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–8
Parallel representation of the binary number 01101100.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Basic Electricity
• Current
– Charged particles which flow from the voltage source
through conductive material to a ground
– Basic unit - Amp
• Voltage
– The difference in electrical potential between 2 points of a
circuit
– Basic unit - Volt
• Resistance
– The opposition that a material body offers to the passage
of an electric current
– Basic unit - Ohm
Ohm’s Law
• Defines the relationship between voltage,
current, and resistance
• Voltage = Current x Resistance
• V = I x R, I = V / R
Analogy to Water
• Voltage is analogous to pressure
• Current is analogous to water flow
• Resistance is analogous to flow restrictor
Switches in Electronic Circuits
• Make and break a connection
• Manual Switches - Ideal resistances
– ON - 0 ohms
– OFF - infinite
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
A Relay as a Switch
• Electromechanical Relay
– contacts
– external voltage to operate
– magnetic coil energizes
• NC - normally closed
• NO - normally open
• Total isolation
– triggering source
– output
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–14
Physical representation of an electromechanical relay: (a) normally closed (NC) relay; (b) normally
open (NO) relay; (c) photograph of actual relays.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–14 (continued)
Physical representation of an electromechanical relay: (a) normally closed (NC) relay; (b)
normally open (NO) relay; (c) photograph of actual relays.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
A Relay as a Switch
• Disadvantages
– several milliamperes of current to operate
– slower - several milliseconds vs. micro or nano
– Mechanical movement
• Less reliable than an electronic component
• Advantage
– Switch higher currents
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
A Diode as a Switch
• Semiconductor
• Current flow in one direction only
• Forward-biased
– anode more positive than cathode
– current flow
• Reverse-biased
– anode equal or more negative than cathode
– no current flow
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–22
Diode in a series circuit: (a) forward biased; (b) reverse biased.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
A Diode as a Switch
• Analogous to a water check valve
• Not a perfect short
– Silicon Diode – 0.7 volts
• 0.7 V across its terminals
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–25
Forward-biased diode in an electric circuit: (a) original circuit; (b) equivalent circuit showing the diode
voltage drop and Vout = 5 – 0.7 = 4.3 V.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
A Transistor as a Switch
• Bipolar transistor
– input signal at one terminal
– two other terminals become
short or open
• Types
– NPN
– PNP
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
A Transistor as a Switch
• NPN
– ON
• positive voltage from base to emitter
• collector-to-emitter junction short
– OFF
• negative voltage or 0 V from base to emitter
• collector-to-emitter junction open
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
A Transistor as a Switch
• PNP
– ON
• negative voltage base to emitter
• collector-to-emitter junction short
– OFF
• positive voltage or 0 V from base to emitter
• collector-to-emitter junction open
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–28
NPN transistor switch: (a) transistor ON; (b) transistor OFF.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–31
Equivalent circuits: (a) transistor OFF; (b) transistor ON.
The TTL Integrated Circuit
• Transistor-transistor logic
– Most widely used of IC technology
• Transistor Saturation (Vin = 5V)
• Transistor Cutoff (Vin = 0V)
• Inverter
– takes digital level input
– complements it to the output
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The TTL Integrated Circuit
• 7404
–
–
–
–
–
hex inverter
six complete logic circuits
single silicon chip
14 pin DIP (Dual Inline Package)
7 on a side
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2-39
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–37
Schematic of a TTL inverter circuit.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–40
Photograph of three commonly used ICs: the 74HC00, 74ACT244, and 74150.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The CMOS Integrated Circuit
• Complementary Metal Oxide Semiconductor
–
–
–
–
low power consumption
battery-powered devices
slower switching speed than TTL
sensitive to electrostatic charges
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Surface-Mounted Devices
• SMD
–
–
–
–
–
–
reduced size and weight
lowered cost of manufacturing circuit boards
soldered directly to metalized footprint
special desoldering tools and techniques
chip densities increased
higher frequencies
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Surface-Mounted Devices
• SO (small outline)
–
–
–
–
dual-in-line package
gull-wing format
lower-complexity logic
30% to 50% less space then DIP
• PLCC (plastic leaded chip carrier)
–
–
–
–
square with leads on all four sides
J-bend configuration
more complex logic
requires less space than SO package
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–42
Typical surface-mount devices (SMDs) and their footprints: (a) small outline (SO); (b) plastic leaded
chip carrier (PLCC); (c) photograph of actual SMDs; (d) photograph of SMDs mounted on a printed-circuit board.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 2–42 (continued)
Typical surface-mount devices (SMDs) and their footprints: (a) small outline (SO); (b)
plastic leaded chip carrier (PLCC); (c) photograph of actual SMDs; (d) photograph of SMDs mounted on a printedcircuit board.
William Kleitz
Digital Electronics with VHDL, Quartus® II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• The digital level for 1 is commonly
represented by a voltage of 5 V in digital
systems. A voltage of 0 V is used for the 0
level.
• An oscilloscope can be used to observe the
rapidly changing voltage-versus-time
waveform in digital systems.
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• The frequency of a clock waveform is equal
to the reciprocal of the waveform’s time
• The transmission of binary data in the serial
format requires only a single conductor with
a ground reference. The parallel format
requires several conductors but is much
faster than serial.
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• Electromechanical relays are capable of
forming shorts and opens in circuits
requiring high current values but not high
speed.
• Diodes are used in digital circuitry
whenever there is a requirement for current
to flow in one direction but not the other.
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• The transistor is the basic building block of the
modern digital integrated circuit. It can be
switched on or off by applying the appropriate
voltage at its base connection.
• TTL and CMOS integrated circuits are formed by
integrating thousands of transistors in a single
package. They are the most popular ICs used in
digital circuitry today.
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Summary
• SMD-style ICs are gaining popularity over
the through-hole style DIP ICs because of
their smaller size and reduced
manufacturing costs.
William Kleitz
Digital Electronics with VHDL, Quartus®
II Version
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.