Chapter 8

1. 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 • You should be able to: • 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 • Digital comparator • Compare bit-by-bit • Outputs a 1 if they are exactly equal • Use exclusive-NOR gates • Evaluating two 4-bit numbers - see Figure 8-1 4

5. Comparators • Evaluating two 4-bit numbers – Figure 8-1 5

6. Comparators • Magnitude Comparators • A = B • A > B • A < B • 7485 4 bit comparator • Figure 8-2 6

7. Comparators • Magnitude comparison of two 8-bit strings • Figure 8-3 7

8. Decoding • Converting some code (binary, BCD, or hex) to a single output • BCD decoder • Figure 8-4 8

9. Decoding • 3-Bit Binary-to-Octal Decoding • Truth Table for active HIGH and active LOW 9

10. Decoding • Complete Octal Decoder (active LOW out) • Figure 8-6 10

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

12. Decoding • Octal Decoder IC • 74138 pin configuration and logic symbol. Figure 8-7 12

13. Decoding • Octal Decoder IC • 74138 logic diagram and function table • Don’t- Care level • Figure 8-7 (continued) 13

14. Decoding • BCD Decoder IC • 7442 1-of-10 decoder pin configuration and logic symbol – Figure 8-10 14

15. Decoding • BCD Decoder IC • 7442 1-of-10 decoder logic diagram and function table – Figure 8-10 (continued) 15

16. Decoding • Hexadecimal Decoder IC • 74154 1-of16 Decoder pin configuration and logic symbol – Figure 8-11 16

17. Decoding • Hexadecimal Decoder IC • 74154 1-of16 Decoder logic diagram and function table – Figure 8-11 (continued) 17

18. Encoding • Opposite process from decoding • Used to generate a coded output • Decimal-to-BCD encoder block diagram: Figure 8-12 18

19. Encoding • Octal to binary encoder – Figure 8-12 (continued) 19

20. Encoding • The truth table can be used to design encoders using combinational logic. • See Table 8-3 in your text 20

21. Encoding • Combinational logic for decimal to BCD encoder based on truth table – Figure 8-13 21

22. Encoding • Decimal-to-BCD Encoder • 74147 • Inputs and outputs are Active-LOW • Priority encoder - highest input has priority 22

23. Encoding • Decimal-to-BCD Encoder • 74147 logic symbol and function table – Figure 8-14 23

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

25. Encoding • Octal-to-Binary Encoder • 74148 logic symbol and function table – Figure 8-17 25

26. 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

27. Code Converters • Convert a coded input into another form • Computer program (software) • MSI integrated circuits (hardware) 27

28. Code Converters • BCD-to-Binary conversion • weighting factor of 10 • Figure 8-20 28

29. Code Converters • 74184 BCD-to-Binary Converter logic symbol – Figure 8-21 29

30. Code Converters • Six-bit BCD-to-Binary Converter using 74184 – Figure 8-22 30

31. Code Converters • BCD to binary for two BCD decades – Figure 8-23(a) 31

32. Code Converters • BCD to binary for three BCD decades – Figure 8-23(b) 32

33. Code Converters • 6 bit binary to BCD and 8 bit binary to BCD converters – Figure 8-23 (c) and (d) 33

34. 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. 34

35. Code Converters • Gray Code • used to indicate angular position of rotating shafts • varies by only 1 bit from one entry to the next Figure 8-25 35

36. Code Converters • Gray Code • Comparison between regular binary and Gray code: 36

37. Code Converters • Conversion between binary and Gray code using XOR gates • Figure 8-26 and 8-27 37

38. Multiplexers • Funneling several data lines into a single one for transmission to another point • Data selector • Figure 8-30 38

39. Multiplexers 39

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

41. Multiplexers • 74151 Eight-Line Multiplexer logic symbol – Figure 8-32 41

42. Multiplexers • 74151 Eight-Line Multiplexer logic diagram – Figure 8-32(continued) 42

43. Multiplexers • Providing Combination Logic Functions • Multiplexers can be used to implement combinational logic circuits. • A multiplexer can replace several SSI logic gates • Example 8-12 43

44. 44

45. Demultiplexers • Opposite procedure from multiplexing • Data distributor • Single data input routed to one of several outputs • Figure 8-37 45

46. Demultiplexers • 74139 Dual 4-line Demultiplexer logic symbol and logic diagram- Figure 8-38 46

47. Demultiplexers • 74139 connected to route an input signal to the 2a output – Figure 8-39 47

48. Demultiplexers • 74154 4-line-to16-line hexadecimal decoder • Used as a 16 line demultiplexer • Connected to route a signal to the 5 output – Figure 8-40 48

49. Demultiplexers • Analog Multiplexer/Demultiplexer • 4051, 4052, 4053 CMOS devices • Both functions • Bidirectional • Analog and digital 49

50. 4051 CMOS analog multiplexer/demultiplexer – Figure 8-41 50