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