Chapter 2

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# Chapter 2 - PowerPoint PPT Presentation

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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Presentation Transcript

### 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:
• 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

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

5

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

Draw timing diagrams for the following circuits:

7

Engineering Notation

See Table 2-1 in your text

8

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

10

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

13

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
• 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
• 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 Switch
• 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
• 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 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
• Bipolar transistor
• Input signal at one terminal
• Two other terminals become short or open
• Types
• NPN
• PNP

22

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 Switch
• PNP
• Negative voltage base to emitter
• ON
• Positive voltage or 0 V from base to emitter
• OFF

24

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
• 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 Circuit
• 7404
• Hex inverter
• Six complete logic circuits
• Single silicon chip
• 14 pins
• 7 on a side

27

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
• Simulation software
• Overview of operation
• Demonstration
• Example circuits

30

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
• 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
• 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 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
• 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

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

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

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

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

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