Chapter 8 Code Converters Multiplexers and Demultiplexers 1

Chapter 8 Code Converters, Multiplexers and Demultiplexers 1

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

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

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

Comparators n Evaluating two 4 -bit numbers 5

Comparators n Magnitude Comparators A=B n. A>B n. A<B n n 7485 TTL 4 -bit comparator 6

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

Comparators n Magnitude comparison of two 8 -bit strings 7

VHDL Comparator using IF-THEN-ELSE

PROCESS Flowchart

8 -bit Comparator Simulation

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

Decoding n 3 -Bit Binary-to-Octal Decoder Truth Tables 9

Decoding n Complete Octal Decoder (active LOW outputs) 10

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

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

Decoding n 74138 logic diagram and function table 13

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

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

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

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

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

VHDL Octal Decoder Boolean Equations and Block Diagram

VHDL Octal Decoder Implemented with Vectors

VHDL Octal Decoder With Enable Input

VHDL Octal Decoder IF-THEN-ELSE and CASE Statements

VHDL Octal Decoder Simulation with Enable Control

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

Encoding n Octal-to-binary and decimal-to-BCD encoders 19

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

Encoding n A basic decimal-to-BCD encoder 21

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

Encoding n 74147 Decimal-to-BCD Encoder logic symbol and function table 23

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

Encoding n 74148 Octal-to-Binary Encoder logic symbol and function table 25

VHDL Octal Priority Encoder n VHDL listing and block symbol file

VHDL Octal Priority Encoder n Octal encoder simulated waveforms

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

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

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

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

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

Code Converters n BCD-to-binary for two BCD decades 31

Code Converters n BCD-to-binary for three BCD decades 32

Code Converters n 6 bit binary-to-BCD and 8 bit binary-to. BCD converters 33

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

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

Code Converters n Gray Code 36

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

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

Multiplexers n S 0 and S 1 determine which data is transmitted 39

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

Multiplexers n Some popular TTL and CMOS multiplexers 40

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

Multiplexers n 74151 Eight-Line Multiplexer logic diagram 42

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

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

VHDL 4 -Line Multiplexer n Simulated waveforms 43

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

Demultiplexers n In this simulation file, the D waveform is routed to the Y outputs as determined by the data select inputs (S 0 and S 1). 45

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

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

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

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

Demultiplexers n 4051 CMOS analog multiplexer /demultiplexer 50

System Design Application n The 74138 as a memory address decoder 51

System Design Application n The 74148 used to encode an active alarm 52

Serial Data Multiplexing for a Microcontroller 8051 micro controller n One serial receive line n One serial transmit line n 74 HCT 151 is the multiplexer n 74 HCT 238 is the demultiplexer n n HCT versions used to match the microcontroller`s speed 53

54

74 HCT 4051 Staircase Generator 55

74 HCT 4051 Staircase Generator n The voltage divider at the Y-inputs creates outputs at 8 different voltage levels.

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

Multiplexed Display Application 57

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

FPGA Design Using LPMs n Simulation waveforms for the LPM comparator 58

FPGA Design Using LPMs n LPM module for an octal decoder 58

FPGA Design Using LPMs n Simulation waveforms for the LPM decoder 58

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

FPGA Design Using LPMs n Simulation waveforms for the LPM multiplexer 58

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

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

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