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Chapter 2. Digital Electronic Signals and Switches. 1. Objectives. You should be able to: Describe the parameters of digital vs. time waveforms. Convert a periodic waveform between frequency and period. 2. Objectives. You should be able to:

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chapter 2

Chapter 2

Digital Electronic Signals and Switches

1

objectives
Objectives
  • You should be able to:
    • Describe the parameters of digital vs. time waveforms.
    • Convert a periodic waveform between frequency and period.

2

objectives1
Objectives
  • You should be able to:
    • Sketch the timing waveform for a binary string in parallel and serial forms.
    • Discuss switch and relay applications
    • Explain the characteristics of diodes and transistors when forward and reverse biased.

3

objectives2
Objectives
  • You should be able to:
    • Calculate output voltage in circuits containing diodes or transistors used as digital switches.
    • Perform I/O timing analysis in circuits containing relays or transistors.
    • Explain the operation of a common-emitter transistor circuit used as a digital inverter.

4

digital signals
Digital Signals
  • Timing diagram
    • Voltage versus time
    • Shows logic state
  • If not exactly 0V and 5V
  • Use an oscilloscope to view

5

clock waveform timing
Clock Waveform Timing
  • Periodic clock waveform
    • Repetitive form
    • Specific time interval
    • Successive pulses identical
  • Period
  • Frequency
  • F = 1/tp and tp = 1/f

6

practice problem
Practice Problem

Draw timing diagrams for the following circuits:

7

engineering notation
Engineering Notation

See Table 2-1 in your text

8

discussion points
Discussion Points
  • What does the vertical scale of an oscilloscope represent?
  • What does the horizontal scale of an oscilloscope represent?
  • Describe frequency and period.
  • What is the period of a 75 MHz waveform?
  • What is the frequency of a waveform with a period of 20 ns?

9

serial representation
Serial Representation
  • Single electrical conductor
  • Slow
    • One bit for each clock period
    • Telephone lines, intra-computer
  • COM ports
  • Plug-in cards

10

serial representation1
Serial Representation
  • Several standards
    • V.90, ISDN, T1, T2, T3, USB, Ethernet, 10baseT, 100baseT, cable, DSL
  • COM - 115 kbps
  • USB – Different speeds, depending on version

11

parallel representation
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)

12

parallel representation1
Parallel Representation
  • LPT1
    • 8-bit parallel
    • 115 kBps
  • SCSI
    • 16-bit parallel
    • 160 MBps
  • Bps - BYTES per second

13

discussion points1
Discussion Points
  • Describe the difference between parallel and serial transmission.
  • What advantage does parallel transmission have over serial transmission?
  • Are there any disadvantages to parallel transmission?
  • How long will it take to transmit two 8 bit binary strings using both serial and parallel if the clock frequency is 25 MHz?

14

switches in electronic circuits
Switches in Electronic Circuits
  • Make and break a connection
  • Manual switch vs. electromechanical relay
  • Semiconductor devices
    • Diodes
    • Transistors
  • Manual Switches - ideal resistances:
    • ON - 0 ohms
    • OFF - infinite

15

the relay as a switch
The Relay as a Switch
  • Electromechanical relay
    • Contacts
    • External voltage to operate
    • Magnetic coil energizes
  • NC - normally closed
  • NO - normally open
  • Provides isolation
    • Triggering source
    • Output

16

the relay as a switch1
The Relay as a Switch
  • Disadvantages
    • Relatively high current is required
    • Slow - milliseconds vs. micro or nanoseconds
  • Energized relay coil
  • Replace source with clock oscillator
  • Timing diagrams
  • See Figure 2-17

17

a diode as a switch
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

19

a diode as a switch1
A Diode as a Switch
  • Analogous to a water check valve
  • Not a perfect short
    • See Figure 2-24
  • 0.7 V across its terminals

20

a transistor as a switch
A Transistor as a Switch
  • Bipolar transistor
    • Input signal at one terminal
    • Two other terminals become short or open
  • Types
    • NPN
    • PNP

22

a transistor as a switch1
A Transistor as a Switch
  • NPN
    • Positive voltage from base to emitter
    • Collector-to-emitter junction short
    • ON
    • Negative voltage or 0 V from base to emitter
    • Collector-to-emitter junction open
    • OFF

23

a transistor as a switch2
A Transistor as a Switch
  • PNP
    • Negative voltage base to emitter
    • ON
    • Positive voltage or 0 V from base to emitter
    • OFF

24

discussion points2
Discussion Points
  • Name the three pins (leads) of a transistor.
  • Describe how to turn an NPN transistor ON.
  • Describe how to turn a PNP transistor ON.

25

the ttl integrated circuit
The TTL Integrated Circuit
  • Transistor-transistor logic
  • Inverter
    • Provides the complement (inversion) of an input at the output.
  • Transistor saturation
  • Transistor cutoff
  • TTL Integrated Circuit
    • Totem-pole output

26

the ttl integrated circuit1
The TTL Integrated Circuit
  • 7404
    • Hex inverter
    • Six complete logic circuits
    • Single silicon chip
    • 14 pins
    • 7 on a side

27

the ttl integrated circuit2
The TTL Integrated Circuit
  • DIP - dual-in-line package
    • NC - not physically or electrically connected
  • Pin configuration
    • See Figure 2-39

28

multisim simulation of switching circuits
MultiSIM Simulation of Switching Circuits
  • Simulation software
  • Overview of operation
  • Demonstration
  • Example circuits

30

the cmos integrated circuit
The CMOS Integrated Circuit
  • Complementary Metal Oxide Semiconductor
    • Low power consumption
    • Useful in battery-powered devices
    • Slower switching speed than TTL
    • Sensitive to electrostatic discharge

31

surface mount devices
Surface-Mount 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

32

surface mounted devices
Surface-Mounted Devices
  • SO (small outline)
    • Dual-in-line package
    • Gull-wing format
    • Lower-complexity logic
  • PLCC (plastic leaded chip carrier)
    • Square with leads on all four sides
    • J-bend configuration
    • More complex logic

33

discussion points3
Discussion Points
  • What are some key characteristics of:
    • TTL devices
    • CMOS devices
    • Surface mount devices
  • From a technician’s standpoint, is there a problem troubleshooting and repairing SMD based equipment?

34

summary
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.

35

summary1
Summary
  • The frequency of a clock waveform is equal to the reciprocal of the waveform’s speed
  • 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.

36

summary2
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.

37

summary3
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.

38

summary4
Summary
  • SMD-style ICs are gaining popularity over the through-hole style DIP ICs because of their smaller size and reduced manufacturing costs.

39