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Chapter 2

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

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Chapter 2

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  1. Chapter 2 Digital Electronic Signals and Switches 1

  2. Objectives You should be able to: • Describe the parameters of digital-versus-time waveforms. • Convert between frequency and period for a periodic waveform. • Sketch the timing waveform for a binary string in either serial or parallel form. • Discuss the applications of mechanical switches and electromechanical relays in electric circuits. 2

  3. Objectives (Continued) • Explain the characteristics of diodes and transistors when forward and reverse biased. • 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

  4. Digital Signals • Timing diagram • Voltage is measured on the y-axis; time on the x-axis • 0 and 1 correspond to 0 V and 5 V • Can be viewed using an oscilloscope. 5

  5. Clock Waveform Timing • Periodic waveform • Repetitive • Specific time interval • Successive pulses are identical • Period – the time from one edge to the corresponding edge on the next cycle • tp = 1/f • Frequency – reciprocal of waveform period • f = 1/tp 6

  6. Figure 2–3 Solution to Example 2–3.

  7. 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

  8. Table 2–1 Common engineering prefixes.

  9. Serial Representation • Single electrical conductor • Slow • One bit for each clock period • Telephone lines, computer-to-comcomputer • COM ports are most often used for serial communications • Plug-in cards 10

  10. Figure 2–5 Serial communication between computers.

  11. Serial Representation • Standards • V.90, ISDN, T1, T2, T3, USB, Ethernet, 10baseT, 100baseT, cable, DSL • Standard COM port transmission rate: 115 kbps • USB – Different speeds, depending on version – typically in hundreds of Mbps. 11

  12. Parallel Representation • Separate electrical conductor for each bit • Expensive • Very fast • Inside a computer • Data bus • External Devices • Centronics printer interface (LPT1) • SCSI (Small Computer Systems Interface) 12

  13. Parallel Representation • LPT1 • 8-bit parallel • 115 kBps • SCSI • 16-bit parallel • Up to 160 MBps • Bps - bytes per second 13

  14. Figure 2–7 Parallel communication between a computer and a printer.

  15. Figure 2–6 Serial representation of the binary number 0110110.

  16. Switches in Electronic Circuits • Make or break connections between conductors • Manual switches • Electromechanical relays • Semiconductor devices • Diodes • Transistors • Manual Switches - ideal resistances: • ON - 0 W • OFF -   15

  17. Figure 2–11 Manual switch: (a) switch open, R ohms; (b) switch closed, R 0 ohms.

  18. Figure 2–12 1-Level output.

  19. Figure 2–13 0-Level output.

  20. 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

  21. Figure 2–14 Physical representation of an electromechanical relay: (a) normally closed (NC) relay; (b) normally open (NO) relay; (c) photograph of actual relays.

  22. 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.

  23. 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

  24. Figure 2–15 Symbolic representation of an electromechanical relay: (a) NC relay used in a circuit and (b) NO relay used in a circuit.

  25. Timing Diagram (Figure 2-17) 18

  26. A Diode as a Switch • Semiconductor • Current in one direction only • Forward-biased • Anode more positive than cathode • Current • Reverse-biased • Anode equal or more negative than cathode • No current 19

  27. A Diode as a Switch • Analogous to a water check valve • Not a perfect short • 0.7 V across its terminals 20

  28. Figure 2–25 Forward-biased diode in an electric circuit: (a) original circuit and (b) equivalent circuit showing the diode voltage drop and Vout  5 – 0.7  4.3 V.

  29. Figure 2–22 Diode in a series circuit: (a) forward biased and (b) reverse biased.

  30. Figure P2–6

  31. A Transistor as a Switch • Bipolar junction transistor (BJT) • Three-terminal component • An input signal at one terminal generates a short or open between the other two terminals • Terminals: base, emitter, and collector • Two types • NPN • PNP

  32. Figure 2–27 The NPN bipolar transistor: (a) physical layout; (b) symbol; (c) photograph.

  33. A Transistor as a Switch • NPN • Positive base-to-emitter voltage shorts the transistor output (collector-to-emitter) • Transistor is said to be ON • Negative voltage (or 0 V) base-to-emitter opens the transistor output (collector-to-emitter) • Transistor is said to be OFF 23

  34. A Transistor as a Switch • PNP • Negative base-to-emitter voltage shorts the transistor output (collector-to-emitter) • Transistor is said to be ON • Positive voltage (or 0 V) base-to-emitter opens the transistor output (collector-to-emitter) • Transistor is said to be OFF 24

  35. Figure 2–31 Equivalent circuits: (a) transistor OFF and (b) transistor ON.

  36. Figure 2–28NPN transistor switch: (a) transistor ON and (b) transistor OFF.

  37. Figure 2–29

  38. Figure 2–30

  39. Figure 2–32

  40. Figure 2–33

  41. Figure 2–34 Equivalent circuits: (a) transistor OFF and (b) transistor ON.

  42. Figure 2–35 Common-emitter transistor circuit operating as an inverter.

  43. Figure 2–36 Common-emitter calculations.

  44. Figure P2–10

  45. The TTL Integrated Circuit • Transistor-Transistor Logic (TTL) • Inverter • The output is the complement of the input. (Converts 1 to 0 and vice-versa) • TTL Integrated Circuit • Totem-pole output 26

  46. The TTL Integrated Circuit • 7404 Hex Inverter • Six complete logic circuits on a single silicon chip • 14 pins, 7 on a side • DIP (dual-in-line package) • Pin configuration 27

  47. 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 • NC = Not Connected 31

  48. Figure 2–38 A 7404 TTL IC chip.

  49. Figure 2–40 Photograph of three commonly used ICs: the 74HC00, 74ACT244, and 74150.

  50. Surface-Mount Devices • SMD • Reduced size and weight • Lowered cost of manufacturing circuit boards • Soldered directly to metal footprint • Special desoldering tools and techniques • Chip densities increased • Higher frequencies 32

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