Code Converters, Multiplexers and Demultiplexers 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. Use manufacturers’ data sheets to determine operation of IC decoder and encoder chips. 2

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Multiplexers 39

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

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

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

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

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

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

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

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

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

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

System Design Applications The 74138 as a memory address decoder – Figure 8-42 51

System Design Applications The 74148 used to encode an active alarm – Figure 8-43 52

System Design Applications Serial Data Multiplexing for a Microcontroller One serial receive line One serial transmit line See Figure 8-44 Analog Multiplexer superimposed 4051 See Figure 8-45 53

Figure 8-44 54

Figure 8-45 55

System Design Applications Multiplexed Display Application Share common ics, components and conductors Digital bus and display bus See Figure 8-46 56

Figure 8-46 57

CPLD Design Applications Used to simulate combinations of inputs and observe the resulting output to check for proper design operation. See CPLD Applications 8-1 and 8-2 58

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

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

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