Chapter 2 Digital Electronic Signals and Switches 1.

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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: –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/t p and t p = 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  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

Figure 2-17 18

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

Figure 2-24 21

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

Figure 2-39 29

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. 39