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.

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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.

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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

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Comparators

• Evaluating two 4-bit numbers – Figure 8-1

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Comparators• Magnitude

Comparators– A = B– A > B– A < B

• 7485 4 bit comparator

• Figure 8-2

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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

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Decoding• Complete Octal Decoder (active LOW out)

• Figure 8-6

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Decoding• Octal Decoder

– Also known as 1-of-8 decoder– Also known as 3-line-to-8-line decoder

• Decoder ICs

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Decoding• Octal Decoder IC

– 74138 pin configuration and logic symbol.

Figure 8-7

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Decoding• Octal Decoder IC

– 74138 logic diagram and function table

– Don’t- Care level

– Figure 8-7 (continued)

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Decoding• BCD Decoder IC

– 7442 1-of-10 decoder pin configuration and logic symbol – Figure 8-10

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Decoding• BCD Decoder IC

– 7442 1-of-10 decoder logic diagram and function table – Figure 8-10 (continued)

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Decoding• Hexadecimal Decoder IC

– 74154 1-of16 Decoder pin configuration and logic symbol – Figure 8-11

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Decoding• Hexadecimal Decoder IC

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

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Encoding• Opposite process from decoding

• Used to generate a coded output

• Decimal-to-BCD encoder block diagram:

Figure 8-12

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Encoding• Octal to binary encoder – Figure 8-12

(continued)

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Encoding

• The truth table can be used to design encoders using combinational logic.

• See Table 8-3 in your text

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Encoding• Combinational logic for decimal to BCD

encoder based on truth table – Figure 8-13

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Encoding

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

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Encoding

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

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Encoding• Octal-to-Binary Encoder

• 74148

• Eight active-low inputs

• Three active-low outputs

• Priority encoder

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Encoding• Octal-to-Binary Encoder

• 74148 logic symbol and function table – Figure 8-17

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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?

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Code Converters

• Convert a coded input into another form– Computer program (software)– MSI integrated circuits (hardware)

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Code Converters

• BCD-to-Binary conversion– weighting factor of 10

• Figure 8-20

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Code Converters

• 74184 BCD-to-Binary Converter logic symbol – Figure 8-21

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Code Converters• Six-bit BCD-to-Binary Converter using 74184 –

Figure 8-22

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

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Code Converters• BCD to binary for three BCD

decades – Figure 8-23(b)

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Code Converters• 6 bit binary to BCD and 8 bit binary to BCD converters –

Figure 8-23 (c) and (d)

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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.

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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

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Code Converters• Gray Code

– Comparison between regular binary and Gray code:

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Code Converters• Conversion between binary and Gray code using

XOR gates• Figure 8-26 and 8-27

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Multiplexers

• Funneling several data lines into a single one for transmission to another point

• Data selector

• Figure 8-30

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Multiplexers

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Multiplexers• Logic diagram for a four-line multiplexer:

Figure 8-31

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

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Multiplexers• 74151 Eight-Line Multiplexer logic diagram –

Figure 8-32(continued)

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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

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Demultiplexers

• Opposite procedure from multiplexing

• Data distributor

• Single data input routed to one of several outputs

• Figure 8-37

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Demultiplexers

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

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Demultiplexers• 74139 connected to route an input signal to

the 2a output – Figure 8-39

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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

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Demultiplexers

• Analog Multiplexer/Demultiplexer

• 4051, 4052, 4053 CMOS devices

• Both functions

• Bidirectional

• Analog and digital

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

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

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

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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

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Figure 8-44

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Figure 8-45

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System Design Applications

• Multiplexed Display Application– Share common ics, components and conductors– Digital bus and display bus– See Figure 8-46

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Figure 8-46

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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

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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.

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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.

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Summary

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

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