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Chapter 2

Chapter 2. Digital Electronic Signals and Switches. 1. 2-1 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. 2-2 Clock Waveform Timing. Periodic waveform are Repetitive

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Chapter 2

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  1. Chapter 2 Digital Electronic Signals and Switches 1

  2. 2-1 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

  3. 2-2 Clock Waveform Timing • Periodic waveform are • Repetitive • Have a 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

  4. Engineering Notation 8

  5. Solving for Period and Frequency • Example 2-1 If a clock waveform has a period of 2 µs, its frequency is f = 1/tp= 1/2 µs = 500 kHz • Example 2-2 A 4.17 MHz waveform has a period of tp = 1/f = 1/4.17 MHz = 240 ns

  6. 2-3 Serial Representation • Uses a single electrical conductor -> inexpensive • Slow since only one bit for each clock period • Used for telephone lines, computer-to-computer networks • Computer COM ports are most often used for serial communications • Ethernet plug-in cards are for serial networking 10

  7. Figure 2.5 Serial communication between computers.

  8. Figure 2-6 Serial Representation of a Binary Number • A serial representation of the binary number 01101100 is illustrated below. • Note that there is one bit per clock period and the bit-to-bit change occurs on the falling edge. 11

  9. 2- 4 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

  10. Figure 2.7 Original parallel communication between a computer and a printer.

  11. Figure 2.8 Parallel representation of the binary number 01101100.

  12. 2-5 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

  13. Switches and Logic Levels • A double-throw switch can be used to produce a high (1) or a low (0) at an output. • This circuit produces a high (1) at the output. • This circuit produces a low (0) at the output. 15

  14. Figure 2.14 Physical representation of an electromechanical relay (a) normally closed (NC) relay; (b) normally open (NO) relay; (c) photograph of actual relays.

  15. Figure 2.15 Symbolic representation of an electromechanical relay

  16. 2.7 A Diode as a Switch • A diode is a semiconductor device that allows current to flow in one direction but not the other. Figure 2.22 Diode in a series circuit: (a) forward biased and (b) reverse biased.Forward biased: a diode whose Anode voltage is more positive than its Cathode.Reversed biased: its anode voltage is equal to or more negative than its cathode. Thus not allow current flow.

  17. Figure 2.23 Water system check valve. • A diode is analogous to a check valve in a water system.

  18. Figure 2.24 Diode voltage versus current characteristic curve. • A diode is not a perfect short in the forward-biased condition. • No current flows until Vforw reaches a 0.7v cut-in voltage. • As Vrev becomes more negative, current flow is still practically zero.

  19. Figure 2.25 Forward-biased diode in an electric circuit • original circuit • equivalent circuit showing the diode voltage drop and Vout = 5 – 0.7 = 4.3 V.

  20. V1 = 5-0.7=4.3v V2 = 0v V3 = 0.7v V4 = 5v

  21. V5 = 5-0.7=4.3v V6 = 0.7v

  22. 2.8 A Transistor as a Switch • Bipolar transistor is a very commonly used switch in digital electronic circuits. • A three-terminal semiconductor component • An input signal at one terminal generates a short or open between the other two terminals • Transistor is made by bombarding pure silicon into N-type and P-type materials • N-type: having one more electron than silicon • P-type: having one less electron than silicon 22

  23. 2.8 A Transistor as a Switch • Three distinct regions: base, emitter, and collector • Can be a combination of N-P-N-type or P-N-P-type material bonded together 22

  24. NPN transistor • 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 • Transistor is said to be OFF Vbe Vbe > 0v <= 0v C C E E 23

  25. Figure 2.28 NPN transistor switch: (a) transistor ON and (b) transistor OFF.

  26. Example 2.11

  27. Example 2.11 Continued

  28. Example 2.12

  29. Example 2.12 Continued

  30. PNP transistor • Positive voltage (or 0 V) base-to-emitter opens the transistor output (collector-to-emitter) • Transistor is said to be OFF • Negative base-to-emitter voltage shorts the transistor output • Transistor is said to be ON Vbe Vbe > 0v <= 0v C C E E 24

  31. 2.9 The TTL Integrated Circuit • Transistor-Transistor Logic (TTL) is one of the most widely used integrated circuit technology • A combination of transistors, diodes, and resistors integrated in a single package. • One basic function of a TTL IC is as a complementing switch or inverter • The output is the complement of the input. (Converts 1s to 0s and vice-versa) 26

  32. Figure 2.35 Common-emitter transistor circuit operating as an inverter. • Vin equals 1 (+5v), the transistor is turned on (saturation) and Vout equals 0 (0v), • Vin equals 0 (0v), the transistor is turned off (cutoff) and Vout equals 1 (≈ 5v) • Assume Rload >> Rc

  33. Example 2-13 Let’s assume that Rc=1kΩ, RL=10kΩ, Vin=0v. Vout = But if RL decreases to 1kΩ by adding more loads in parallel with it, Vout will drop to 2.5v Vout =

  34. Figure 2.36 Common-emitter calculations. When the transistor is cut off, Rc is expected to be small to ensure Voutclose to 5v. When the transistor is saturated, Rc is expected to be large to avoid excessive collector current. Need a variable Rc resistance

  35. Figure 2.37 Schematic of a TTL inverter circuit. • Q4 replaces Rc, acts in opposite to Q3. • Such combination is called the totem-pole arrangement. • Q1 (input transistor) drives Q2. • D1 protects Q1from negative voltages at input. • D2 ensures that Q4 cuts off totally when Q3 is saturated. 26

  36. A 7404 TTL IC chip is called 14 pin DIP( Dual-in-line package)

  37. Figure 2.40 Photograph of three commonly used ICs: the 74HC00, 74ACT244, and 74150.

  38. 2-11 The CMOS IC • Another common IC technology is the CMOS (Complementary metal oxide semiconductor) • Use a complementary pair of metal oxide semiconductor field-effect transistors (MOSFETs) • NOT bipolar transistors, which relies on the contact of two types of semiconductor for its operation, in TTL chips • VS. TTL: Lower power consumption (commonly used in battery-powered devices); slower switching speed.

  39. Figure 2.42 A 4049 CMOS hex inverter pin configuration.

  40. 2-12 Surface-Mounted Devices (SMD) • Modern electronics depends on the ability to design smaller, more dense components and systems. • SMDs reduce the size of DIP-style logic by 70% • SMDs reduce the weight by 90% • 7400 IC in the DIP style:19.23mm X 6.48mm • 7400 SMD: 8.75mm X 6.20mm • SMDs significantly lower the manufacture cost

  41. Popular SMD package styles c) ball grid array (BGA) (b) plastic leaded chip carrier (PLCC) (a) small outline configuration (e) photograph of SMDs mounted on a printed-circuit board (d) photograph of actual SMDs 33

  42. 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

  43. 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

  44. 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 vs. CMOS circuits (Complete coverage in Chapter 9) 38

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