1. Code Converters, Multiplexers and Demultiplexers
Chapter 8
Code Converters, Multiplexers and Demultiplexers 1

2. Objectives You should be able to:
Use an IC magnitude comparator to perform binary comparisons.
Describe the function of a decoder and an encoder.
Design the internal circuitry for encoding and decoding.
Use manufacturers’ data sheets to determine operation of IC decoder and encoder chips. 2

3. Objectives (Continued)
Explain the procedure involved in binary, BCD, and Gray code conversion.
Explain the operation of code converter circuits built with SSI and MSI ICs.
Describe the function and uses of multiplexers and demultiplexers.
Design circuits that employ multiplexer and demultiplexer ICs. 3

4. Comparators Compare two binary strings or words Digital comparator
Compare bit-by-bit
Outputs a 1 if they are exactly equal
Using exclusive-NOR gates is easiest 4

5. Comparators
Evaluating two 4-bit numbers 5

6. Comparators Magnitude Comparators 7485 TTL 4-bit comparator A = B6

7. Comparators (Continued)
7485 TTL 4-bit comparator logic symbols 6

8. Comparators
Magnitude comparison of two 8-bit strings 7

9. VHDL Comparator using IF-THEN-ELSE

10. PROCESS Flowchart

11. 8-bit Comparator Simulation

12. Decoding
Process of converting some code (binary, BCD, or hex) to a single output
4-bit BCD decoder
Comprised of a combination of logic gates 8

13. Decoding
3-Bit Binary-to-Octal Decoder Truth Tables 9

14. Decoding
Complete Octal Decoder (active LOW outputs) 10

15. Decoding Octal Decoder Decoder ICs Also known as 1-of-8 decoder
Also known as 3-line-to-8-line decoder
Decoder ICs 11

16. Decoding
Octal Decoder IC
74138 pin configuration and logic symbol. 12

17. Octal Decoder IC
Decoding
74138 logic diagram and function table 13

18. Decoding 7442 BCD-to-DEC Decoder IC
1-of-10 decoder pin configuration and logic symbol 14

19. Decoding 7442 BCD-to-DEC Decoder IC
1-of-10 decoder logic diagram and function table 15

20. Decoding 74154 Hexadecimal Decoder IC
1-of-16 Decoder pin configuration and logic symbol 16

21. Decoding 74154 Hexadecimal Decoder IC
1-of-16 Decoder logic diagram and function table 17

22. Decoders in VHDL More flexible than fixed-function 7400-series ICs
Can be customized to meet specific needs.

23. VHDL Octal Decoder Boolean Equations and Block Diagram

24. VHDL Octal Decoder Implemented with Vectors

25. VHDL Octal Decoder With Enable Input

26. VHDL Octal Decoder IF-THEN-ELSE and CASE Statements

27. VHDL Octal Decoder Simulation with Enable Control

28. Encoding Opposite process from decoding
Used to generate a coded output from a singular active numeric input line. 18

29. Encoding
Octal-to-binary and decimal-to-BCD encoders 19

30. Encoding
A truth table can be used to design encoders using combinational logic. 20

31. Encoding
A basic decimal-to-BCD encoder 21

32. Encoding 74147 Decimal-to-BCD Encoder
Inputs and outputs are Active-LOW
Note bubbles on logic symbols
Priority encoder - highest input has priority 22

33. Encoding 74147 Decimal-to-BCD Encoder logic symbol and function table23

34. Encoding 74148 Octal-to-Binary Encoder Eight active-low inputs
Three active-low outputs
Priority encoder 24

35. Encoding 74148 Octal-to-Binary Encoder logic symbol and function table25

36. VHDL Octal Priority Encoder
VHDL listing and block symbol file

37. VHDL Octal Priority Encoder
Octal encoder simulated waveforms

38. Discussion Point
Explain the difference between an encoder and a decoder.
How does a priority encoder determine which input to encode if more than one is active? 26

39. Code Converters Convert a coded input into another form
BCD-to binary is a good example
Computer program (software)
MSI integrated circuits (hardware) 27

40. Code Converters Software BCD-to-Binary conversion
Must develop an algorithm
Weighting factor of 10 28

41. Code Converters 74184 BCD-to-Binary Converter logic symbol
Hardware approach 29

42. Code Converters
Six-bit BCD-to-Binary Converter using 74184 30

43. Code Converters
BCD-to-binary for two BCD decades 31

44. Code Converters
BCD-to-binary for three BCD decades 32

45. Code Converters 6 bit binary-to-BCD and 8 bit binary-to-BCD converters33

46. Code Converters BCD-to-Seven Segment Converters
4-bit BCD into a 7-bit code to drive display segments
Useful in calculators and any application that requires a 7 segment display.
The 7447 is a commonly used IC 34

47. Code Converters
Gray Code is used to indicate the angular position of a rotating machine.
Varies by only 1 bit from one entry to the next 35

48. Code Converters
Gray Code 36

49. Code Converters
Conversion between binary and Gray code using exclusive-OR gates 37

50. Multiplexers
Funneling several data lines into a single one for transmission to another point
Data select control determines which input is transmitted 38

51. Multiplexers
S0 and S1 determine which data is transmitted 39

52. Multiplexers
Logic diagram for a four-line multiplexer
Figure 8-31 40

53. Multiplexers
Some popular TTL and CMOS multiplexers 40

54. Multiplexers 74151 Eight-Line Multiplexer logic symbol
Note 3 data select inputs for 8 inputs (23 = 8) 41

55. Multiplexers
74151 Eight-Line Multiplexer logic diagram 42

56. Multiplexers Provide Combination Logic Functions
Multiplexers can be used to implement combinational logic circuits.
A multiplexer can replace several SSI logic gates 43

57. VHDL 4-Line Multiplexer
Program listing and block symbol file 43

58. VHDL 4-Line Multiplexer
Simulated waveforms 43

59. Demultiplexers Opposite procedure from multiplexing Data distributor
Single data input routed to one of several outputs 45

60. Demultiplexers
In this simulation file, the D waveform is routed to the Y outputs as determined by the data select inputs (S0 and S1). 45

61. Demultiplexers
74139 Dual 4-line Demultiplexer logic symbol and logic diagram 46

62. Demultiplexers
74139 connected to route an input signal to the 2a output 47

63. Demultiplexers 74154 4-line-to16-line hexadecimal decoder
Used as a 16 line demultiplexer
Connected to route a signal to the 5 output 48

64. Demultiplexers Analog Multiplexer/Demultiplexer
4051, 4052, 4053 CMOS devices
High-speed CMOS also available (74HCT4051)
Function as either device since inputs and outputs are bidirectional
Analog since outputs can be any value between positive and negative supplies (not just 0 and 1) 49

65. Demultiplexers
4051 CMOS analog multiplexer /demultiplexer 50

66. System Design Application
The as a memory address decoder 51

67. System Design Application
The used to encode an active alarm 52

68. Serial Data Multiplexing for a Microcontroller
One serial receive line
One serial transmit line
74HCT151 is the multiplexer
74HCT238 is the demultiplexer
HCT versions used to match the microcontroller`s speed 53

69. 54

70. 74HCT4051 Staircase Generator55

71. 74HCT4051 Staircase Generator
The voltage divider at the Y-inputs creates outputs at 8 different voltage levels.

72. Multiplexed Display Application
Allows sharing of common ICs, components, and conductors
Four-digit registered are multiplexed to a single-digit bus
Display bus is multiplexed into the four-digit displays 56

73. Multiplexed Display Application57

74. FPGA Design Using LPMs
LPM module for an 8-bit comparator 58

75. FPGA Design Using LPMs
Simulation waveforms for the LPM comparator 58

76. FPGA Design Using LPMs
LPM module for an octal decoder 58

77. FPGA Design Using LPMs
Simulation waveforms for the LPM decoder 58

78. FPGA Design Using LPMs
LPM module for an 8-line multiplexer 58

79. FPGA Design Using LPMs
Simulation waveforms for the LPM multiplexer 58

80. Summary
Comparators can be used to determine equality or which of two binary strings is larger.
Decoders can be used to convert a binary code into a singular active output representing its numeric value.
Encoders can be used to generate a coded output from a singular active numeric input line. 59

81. Summary ICs are available to convert BCD to binary and binary to BCD.
The Gray code is useful for indicating the angular position of a shaft on a rotating device, such as a motor.
Multiplexers are capable of funneling several data lines into a single line for transmission to another point. 60

82. Summary
Demultiplexers are used to take a single data value or waveform and route it to one of several outputs. 61