introduction to chapter 9 - computer engineeringjgallaher/download/etem212/...– the 74147 decimal...

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Introduction to Chapter 9

• Basic operation and specific ICs that perform:– Decoding and encoding– Multiplexing– Demultiplexing– Comparison– Code conversion– Data busing

Ronald Tocci/Neal Widmer/Gregory MossDigital Systems: Principles and Applications, 9e

Copyright ©2004 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

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9-1 Decoders• Decoder – logic circuit that activates an output

that corresponds to a binary number on the input.– ENABLE inputs– The 74ALS138 decoder– BCD to decimal decoders (the 7442)– BCD to decimal decoder/driver (the 7445)– Decoder applications




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FIGURE 9-2 Three-line-to-8-line (or 1-of-8) decoder.




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FIGURE 9-3 (a) Logic diagram for the 74ALS138 decoder; (b) truth table; (c) logic symbol. (Courtesy of Fairchild, a Schlumberger company)




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FIGURE 9-4 Four 74ALS138s forming a 1-of-32 decoder.




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FIGURE 9-5 (a) Logic diagram for the 7442 BCD-to-decimal decoder; (b) logic symbol; (c) truth table. (Courtesy of Fairchild, a Schlumberger company)




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FIGURE 9-6 Example 9-3: counter/decoder combination used to provide timing and sequencing operations.




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9-2 BCD to 7 Segment Decoder/Drivers

• The 7 segment display is a common way to display decimal or hexadecimal characters.

• BCD to 7 segment decoder/driver• Common-anode versus common-cathode

LED displays• 7446/47 activate specific segment patterns

in response to input codes




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FIGURE 9-7 (a) 7-segment arrangement; (b) active segments for each digit.




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FIGURE 9-8 (a) BCD-to-7 segment decoder/driver driving a common-anode 7-segment LED display; (b) segment patterns for all possible input codes.




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BCD-to-7 Segment Decoder/Driver

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9-3 Liquid Crystal Displays

• How LCD and LED displays differ.• LCD operation• Driving an LCD• Types of LCDs• Active matrix TFT LCDs




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FIGURE 9-9 Liquid-crystal display: (a) basic arrangement; (b) applying a voltage between the segment and the backplane turns ON the segment. Zero voltage turns the segment OFF.




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FIGURE 9-10 (a) Method for driving an LCD segment; (b) driving a 7-segment display.




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FIGURE 9-11 A passive matrix LCD panel.




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9-4 Encoders• The general encoding and decoding process




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9-4 Encoders

• Priority encoders– The 74148, 74LS148, and 74HC148 octal to

binary priority encoders– The 74147 decimal to BCD priority encoder– The 74147 as a switch encoder

• Operation of the keyboard entry circuit described in figure on the next page.




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FIGURE 9-14 74147 decimal-to-BCD priority encoder.




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FIGURE 9-15 Decimal-to-BCD switch encoder.




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FIGURE 9-16 Circuit for keyboard entry of three-digit number into storage registers.




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9-5 Troubleshooting

• More complex circuitry increases possible reasons for failure

• Applying observation and analysis will narrow the focus and simplify testing

• After using observation and analysis to determine the possible faults, repeatedly use the divide and conquer technique to reduce possible causes by half




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9-5 Troubleshooting• Example 9-7

– A technician tests the circuit by using a set of switches to apply the input code at A4 through A0. She runs through each possible input code and checks the corresponding decoder output to see if it is activated. She observes that all of the odd-numbered outputs respond correctly, but all of the even-numbered outputs fail to respond when their code is applied. What are the most probable faults?

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9-5 Troubleshooting

• Solution:– Since all of the even numbers have a problem, we look

for some common fault. Look at the truth table.– The A input is common input– Look for problem in that area. (driving circuitry, bad

chip, bad pc board, etc.)

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FIGURE 9-17 Troubleshooting circuitry in Example 9-7.




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9-6 Multiplexers (Data Selectors)

• A multiplexer (MUX) selects one of multiple input signals and passes it to the output.

• The basic two input multiplexer• The four input multiplexer• The eight input multiplexer• The quad two input MUX

(74ALS157/HC157)




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FIGURE 9-18 Functional diagram of a digital multiplexer (MUX).




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FIGURE 9-19 Two-input multiplexer.




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FIGURE 9-20 Four-input multiplexer.




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FIGURE 9-21 (a) Logic diagram for the 74ALS151 multiplexer; (b) truth table; (c) logic symbol. (Courtesy of Fairchild, a Schlumberger company)




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FIGURE 9-22 Example 9-9: two 74HC151s combined to form a 16-input multiplexer.




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FIGURE 9-23 (a) Logic diagram for the 74ALS157 multiplexer; (b) logic symbol; (c) truth table. (Courtesy of Fairchild, a Schlumberger company)




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9-7 Multiplexer Applications

• Applications include data selection, data routing, operation sequencing, parallel to serial conversion, waveform generation, and logic function generation.– Data routing– Parallel to serial conversion– Operation sequencing– Logic function generation




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FIGURE 9-24 System for displaying two multidigit BCD counters one at a time.




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FIGURE 9-25 (a) Parallel-to-serial converter; (b) waveforms for X7X6X5X4X3X2X1X0 = 10110101.




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FIGURE 9-26 Seven-step control sequencer.




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FIGURE 9-27 Multiplexer used to implement a logic function described by the truth table.




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9-8 Demultiplexers (Data Distributors)

• A demultiplexer (DEMUX) distributes a single input to multiple outputs.

• 1 line to 8 line demultiplexer• Clock demultiplexer• A security monitoring system application

using the 74ALS138 decoder/DEMUX




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FIGURE 9-28 General demultiplexer.




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FIGURE 9-29 A 1-line-to-8-line demultiplexer.




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FIGURE 9-30 (a) The 74ALS138 decoder can function as a demultiplexer with E1 used as the data input; (b) typical waveforms for a select code of A2A1A0 = 000 show that O0 is identical to the data input I on E1.




__ _

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FIGURE 9-31 A clock demultiplexer transmits the clock signal to a destination determined by the select code inputs.




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FIGURE 9-32 Security monitoring system.




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9-8 Demultiplexers (Data Distributors)• A synchronous data transmission system application

– The receiver– Complete operation




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FIGURE 9-34 Waveforms during one complete transmission cycle.




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9-9 More Troubleshooting

• Troubleshooting skills are developed through exercise. Apply observation and analysis to the following examples to determine the probable fault:– Example 9-12– Example 9-13




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• Example 9-12– A test performed on this circuit yields the result

shown below:

– What is wrong?

ActualCount

DisplayedCount

Case 1 Counter 1 25 25

Counter 2 37 35


Counter 2 72 79


Counter 2 14 16

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• Example 9-12 (continued)– Solution:

• Counter 1 appears to be working correctly.• Counter 2’s tens digit is working – it’s the units digit that is incorrect.• Note that the units digit of counter 2 is always the same as the units digit of

counter 1.

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• Example 9-13– The security monitoring system shown is tested and

the results are recorded in the table:

Condition LEDs

All doors closed All LEDs off

Door 0 open Led 4 flashing







Door 7open Led 7 flashing

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• Example 9-13 (continued)– Note that doors 4 through 7 function properly.– Look at truth tables:

– Note that the door indicator is always 4 or more. This indicates that the MSB input is probably stuck high keeping the most significant digit (4) on.

741HC51

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9-10 Magnitude Comparator

• The magnitude comparator compares the magnitude of two binary input quantities and indicates which is greater.

• The 74HC85– Data inputs– Outputs– Cascading inputs

• Applications – comparison of position, speed, or temperature to a reference value




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FIGURE 9-36 Logic symbol and truth table for a 74HC85 (7485, 74LS85) four-bit magnitude comparator.




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FIGURE 9-37 (a) 74HC85 wired as a four-bit comparator; (b) two 74HC85s cascaded to perform an eight-bit comparison.




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FIGURE 9-38 Magnitude comparator used in a digital thermostat.




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9-11 Code Converters

• The code converter changes data between different binary codes– BCD to 7-segment– BCD to binary– Binary to BCD– Binary to Gray code– Gray code to binary– ASCII to EBCDIC– EBCDIC to ASCII

• The conversion process




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FIGURE 9-39 Basic idea of a two-digit BCD-to-binary converter.




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9-11 Code Converters• Circuit implementation with two 74HC83

four bit adders




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9-12 Data Busing

• The data bus is a common set of connecting lines for data transfers

• Many devices are connected to the data bus– Microprocessors– Semiconductor memory– Digital to analog converters– Analog to digital converters




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9-12 Data Busing• An example of data busing




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9-13 The 74ALS173/HC173 Tristate Register

• Devices connected to a data bus contain registers to hold the device data

• Registers are usually tied to the bus through a tristate buffer

• 74ALS173 – TTL 4 bit register with PIPO capability

• 74HC173 – CMOS version




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FIGURE 9-42 Truth table and logic diagram for the 74ALS173 tristate register. (Courtesy of Fairchild, a Schlumberger company)




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9-13 The 74ALS173/HC173 Tristate Register

• The logic symbol




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9-14 Data Bus Operation

• Data transfer operation• Bus signals• Simplified bus timing diagram• Expanding the bus• Simplified bus representation• Bi-directional busing




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FIGURE 9-44 Tristate registers connected to a data bus.




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FIGURE 9-45 Signal activity during the transfer of the data 1011 from register A to register C.




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FIGURE 9-46 Simplified way to show signal activity on data bus lines.




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FIGURE 9-47 A 74HC541 octal bus driver connects the outputs of an analog-to-digital converter (ADC) to an eight-line data bus. The D0 output connects directly to the bus showing the capacitive effects.




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FIGURE 9-48 Simplified representation of bus arrangement.




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FIGURE 9-49 Bundle method for simplified representation of data bus connections. The “/8” denotes an eight-line data bus.




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FIGURE 9-50 Bidirectional register connected to data bus.




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9-15 Decoders Using HDL

• Macrofunctions exist to describe standard devices– AHDL

• The AHDL equivalent to the 74138 decoder (figure 9-52)

• The AHDL equivalent using truth table description (figure 9-53)

– VHDL• The VHDL equivalent to the 74138 decoder using a

truth table (figure 9-54)




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9-16 The HDL 7 Segment Decoder Driver

• The standard part number being described is a 7447– The AHDL 7 segment BCD display decoder

(figure 9-55)– The VHDL 7 segment BCD display decoder

(figure 9-56)




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9-17 Encoder Using HDL

• Priority encoders in HDL• Tristate devices in HDL• The 74147 priority encoder with active

HIGH tristate outputs will be described– The AHDL priority encoder (figure 9-58)– The AHDL priority encoder using IF/ELSE

(figure 9-59)– The VHDL priority encoder (figure 9-60)




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9-18 HDL Multiplexers and Demultiplexers

• A multiplexer acts as a selector switch for digital signals

• A demultiplexer distributes a digital signal to one of its outputs

• Select inputs are used to determine which signal goes through the pipeline

• AHDL MUX and DEMUX• VHDL MUX and DEMUX




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9-19 HDL Magnitude Comparators

• The magnitude of two binary numbers is compared and an output indicates: greater than, less than, or equal to.

• The IF/ELSE construct is useful in this application.– The AHDL comparator using the IF/ELSE

construct.– The VHDL comparator using the IF/ELSE

construct.




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9-20 HDL Code Converters

• HDL description of code converters is very intuitive.– AHDL BCD to binary code converter (figure 9-

68)– VHDL BCD to binary code converter (figure 9-

69)




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