2. Chapter 20
Digital Circuits

3. Objectives Explain the difference between analog and digital systems.
Convert decimal numbers to their binary equivalents and binary numbers to their decimal equivalents.
Name seven types of logic gates.
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4. Objectives Explain the operation of various types of logic gates.
Use truth tables to determine the output of a logic gate.
Discuss two types of logic families.
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5. Objectives Explain the digital encoders and decoders.
Explain analog-to-digital and digital-to-analog devices.
List three types of flip-flops and explain their truth tables.
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6. ICs Include transistors, diodes, resistors, and capacitors
Major advantage is size
Linear circuits
Used as amplifiers and have variable outputs
Digital circuits
Used as switches and work in the on or off state
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7. Digital Fundamentals Binary numbering system Voltage logic levels
Bits, nibbles, and bytes
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8. Digital Advantages Inexpensive, compared to analog systems
Easier information storage
Speed is greater than in analog systems
Compatibility with computers
Less effect from temperature changes
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9. Digital Characteristics
Capable of replacing analog systems
Cannot be distinguished from analog systems in most cases
Information is handled using switching circuits
Combination of logic gate and flip-flop circuits
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10. Binary Numbering System
Digital circuits act in on or off states
Comparable to a single-pole single-throw (SPST) switch
Switch in “on” position represents a 1
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11. Basic Counting Rules
Digits must be recorded one after the other for each counting unit
When count exceeds total number of available digits, a second column begins
Decimal system’s first column ends at 9, and second column begins at 10
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12. Binary System 2 is comparable to decimal system’s 10
2 moves to second column, becomes a 0 or 1
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13. Decimal to Binary Conversion Table
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14. Binary Numbers Count binary numbers on your fingers
Number the fingers on your left hand as shown
Fingers pointing up are 1s, fingers folded down are 0s
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15. Large Binary Numbers and Addition
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16. Voltage Logic Levels in Digital Circuits
Type of logic circuitry or family affects operating voltages needed in a circuit
Valid logic high range
Valid logic low range
Invalid value range, or intermediate range
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17. Bits, Nibbles, and Bytes Bit comes from joining binary and digit
Bit is smallest unit of information
Computer memory
Kilobytes
Megabytes
Gigabytes
Computer storage abilities are growing
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18. Review
What is the primary difference between the decimal numbering system and the binary numbering system?
The decimal system has numbers 0–9, and the binary system has 0 and 1
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19. Review What is the smallest unit of information in binary code? A bit
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20. Logic Gates AND gates OR gates NOT gates NAND gates NOR gates
XOR gates
XNOR gates
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21. AND Gates Accepts high and low inputs (1 and 0) High and low outputs
Output of 1 if all inputs are 1
If only one gate is on, circuit will not work
Truth tables
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22. AND Gates (Cont.) © Goodheart-Willcox Co., Inc.
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23. AND Gates (Cont.) Valid logic highs and lows
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24. OR Gates Output signal of 1 if either or both inputs is 1
When all inputs are 0, output is 0
Acts similar to two switches in parallel
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25. OR Gates (Cont.) Valid logic highs and lows
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26. NOT Gates Used to invert polarity of input signal
Also called inverters
If input is 1, output is 0
If input is 0, output is 1
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27. NOT Gates (Cont.) Valid logic highs and lows
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28. NAND Gates Made of an AND gate and a NOT gate
Reverse of AND application
Also called NOT AND gates
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29. NAND Gates (Cont.) Valid logic highs and lows
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30. NOR Gates Opposite of OR gate Made of OR and NOT gates
Used to test for any kind of input
No input will have output of 1
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31. NOR Gates (Cont.) Valid logic highs and lows
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32. XOR Gates Also called exclusive OR gates
Provide high output if any, but not all, inputs are logic high
Provide low output if all inputs are logic high
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33. XOR Gates (Cont.) Valid logic highs and lows
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34. XNOR Gates Also called exclusive NOR gates
XOR gate with inverted output
High output only when all inputs are logic high or logic low
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35. Review
Which gate has an output signal of 1 if either or both inputs is 1?
OR gate
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36. Review
Which gate provides high output if any, but not all, inputs are logic high, but provides a low output if all inputs are logic high?
XOR gate
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37. Review Which gate is made of an AND and a NOT gate? NAND gate
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38. Review
Which gate has an inverted output so it has a high output only when all inputs are logic high or logic low?
XNOR
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39. Logic Families
Traits of one logic family must match traits of another family when many digital ICs are used in one device
Complementary metal-oxide semiconductor logic (CMOS)
Transistor-transistor logic (TTL)
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40. CMOS Logic Circuits have good resistance to noise Use FETs
Require small amounts of power
Can be damaged by static electricity
Worker and work surface must be grounded through high-resistance resistor
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41. TTL Circuits work quickly Function from 20 megahertz to 60 megahertz
Faster than CMOS ICs
Require high power dissipation and high current
Many gates can be placed in one TTL IC
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42. Review
If many digital circuits are used in one device, how should logic families be treated?
Traits of one logic family must match traits of another family
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43. Review
Which of the two types of logic families can be easily damaged by static electricity?
CMOS
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44. Digital Applications Logic probes Digital encoders and decoders
Digitized analog signals
Flip-flops
Counters
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45. Logic Probes Indicate a high or low signal using LEDs
Connect to power supply of circuit being tested
Equipped with a slide switch to select the logic family
If both LEDs light, there is an invalid logic level voltage
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46. Digital Encoders and Decoders
Electronic systems translate binary system into decimal system
Analog-to-digital converters can change analog numbers to a digital number equivalent
Digital multimeter as example
Can convert electrical pulses into binary numbers
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47. Digitized Analog Signals
Digital signals representing sound waves or linear voltage
Analog-to-digital encoders are used
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48. Flip-Flops Set-reset (R-S) flip-flops J-K flip-flops D flip-flops
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49. R-S Flip-Flops When S is high, Q is high
Output represents last input setting if both inputs are low
Outputs are complementary
Clocked R-S flip-flops
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50. J-K Flip-Flops Clock driven
Retain output status when two inputs are low
When both inputs are high, outputs toggle on and off
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51. D Flip-Flops Do not require two inputs
Q outputs toggle when input signal is received if clock signal is applied
Output state of Q will not change without clock signal
Output Qs are always complementary
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52. Counters Can carry and borrow Fabricated from individual flip-flops
Available as integrated chips
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53. Decade Counters Can count based on ten Can be used to divide by ten
Used as frequency dividers for oscilloscopes or in digital clocks
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54. Computerized Tomography (CT)
Uses photomultiplier tubes, computer scans, and X-rays to produce digitized images of the body
Photocathodes
Dynodes
Anode grid
Gantry
Contrast agent
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55. Review How can binary numbers be translated into the decimal system?
Digital decoders
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56. Review What is a flip-flop?
A semiconductor device that can assume one of two stable states
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57. Review What kind of flip-flop does not require two inputs? D flip-flop
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58. Review What does it mean if two outputs are complementary?
When one output is high, the other is low
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59. For Discussion
Give examples of advantages integrated circuits have over transistors.
What are the differences among the logic gates?
Give examples of possible advances that will be made in the computer field in the next 25 years, based on what you have learned in this chapter.
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60. Glossary AND gate Binary Bit
A logic gate that is used to determine the presence of yes signals or 1s.
Binary
A number system having a base of 2, using only the symbols 0 and 1.
Bit
One binary digit (0 or 1).
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61. Glossary Byte Clocked R-S flip-flop
Two binary nibbles, or eight binary bits.
Clocked R-S flip-flop
A flip-flop in which the output changes when there is a change in the R or S input and a pulse appears at the clock input.
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62. Glossary Complementary
A principle behind the operation of a flip-flop that results in one high output when the other is low.
Complementary metal-oxide semiconductor logic (CMOS)
A digital circuit arrangement that uses field-effect transistors for its logic circuits.
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63. Glossary Counter D flip-flop
Series of events that can be triggered by a pulse train or a lever.
D flip-flop
Semiconductor device similar to the J-K flip-flop, except it does not require two inputs.
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64. Glossary Decade counter Decimal system
A common digital counter that counts in base ten.
Decimal system
A numbering system in which ten digits are available.
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65. Glossary Decoder Encoder
Part of a communications system that changes a coded message into an uncoded message.
Encoder
Part of a communications system that changes the information source into coded form.
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66. Glossary Exclusive NOR gate Exclusive OR gate
A logic gate that provides a logic high output (1) only if all inputs are logic high or logic low.
Exclusive OR gate
A logic gate that provides a high output (1) whenever any, but not all, inputs are logic high.
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67. Glossary Flip-flop Gigabyte
A digital device based on the operation of combined logic gates.
Gigabyte
A measure of computer memory, equal to 1,000,000,000 bytes.
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68. Glossary Intermediate range Invalid value range
In digital circuits, the area between value logic high and value logic low that acts as a buffer range. Also called invalid value range.
Invalid value range
In digital circuits, the area between value logic high and value logic low that acts as a buffer range.
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69. Glossary Inverter J-K flip-flop
A gate that changes the polarity of an incoming signal in the output.
J-K flip-flop
A clock-driven flip-flop, similar to the R-S flip-flop, except that it retains its output status when two lows are present at its inputs.
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70. Glossary Kilobyte Logic gate
A measure of computer memory equal to 1000 bytes.
Logic gate
An integrated circuit used mostly as a digital switching device.
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71. Glossary Megabyte NAND gate Nibble NOR gate
A measure of computer memory equal to 1,000,000 bytes.
NAND gate
A negative AND logic gate.
Nibble
One-half of a byte, or four bits.
NOR gate
A negative OR logic gate.
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72. Glossary NOT gate OR gate
An inverter that changes the polarity of an incoming signal in the output.
OR gate
A logic gate that will provide an output signal if there is a signal on either of its inputs.
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73. Glossary Set-reset (R-S) flip-flop Transistor-transistor logic (TTL)
A flip-flop in which outputs are complementary, but in which both inputs cannot be high.
Transistor-transistor logic (TTL)
An arrangement of digital circuits using transistors to perform logic functions.
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74. Glossary Truth table Valid logic high
A binary table that explains the operation of digital logic circuits.
Valid logic high
Operating voltage required for a digital circuit to be in the 1, or on, position. Voltage range is usually 2 to 5 volts.
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75. Glossary Valid logic low XNOR gate
Operating voltage required for a digital circuit to be in the 0, or off, position. Voltage range is usually 0 to 1.5 volts.
XNOR gate
A logic gate that provides a logic high output (1) only if all inputs are logic high or logic low.
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76. Glossary
XOR gate
A logic gate that provides a high output (1) whenever any, but not all, inputs are logic high.
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