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Chapter 8

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

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Chapter 8

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  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 (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 • A = B • A > B • A < B • 7485 TTL 4-bit comparator 6

  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 • 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 DecoderImplemented with Vectors

  25. VHDL Octal DecoderWith Enable Input

  26. VHDL Octal DecoderIF-THEN-ELSE and CASE Statements

  27. VHDL Octal DecoderSimulation 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 table 23

  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 table 25

  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 converters 33

  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

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