Chapter 8

MSI Logic

The complexity of a chip

Scale of integration:

• SSI 1 - 10 gates

• MSI 10 - 100 gates

• LSI 100 - 1000 gates

• VLSI > 1000 gates

Specialized MSI components

• Encoders

• Decoders

• Multiplexers / Demultiplexer

• BCD-to-7-Segment Decoder and Liquid Crystal Displays

Encoders - Decoders

• Main function of encoder and decoder• The purpose is to reduce the number of wires required for

interconnection.

Decoder – logic circuit that activates an output that corresponds to a binary number on the input (set of inputs).

May not use all of the possible input codes– ENABLE inputs– The 74ALS138 decoder– BCD to decimal decoders (the 7442)– BCD to decimal decoder/driver (the 7445)– Decoder applications

Decoders

A 2n-to-n-line encoder symbol

Symbol for an n-to-2n-line decoder

Binary Decoders, N-to-2N decoders

• Accept a N-bit binary input code (number)

• Activate a 1-out-of-2Noutput code corresponding to the input code (number)

• Some decoders do not use all 2Ninput codes

3-Line to 8-Line Decoder

Example

Example 74138 Decoder W/Enable

74138 Decoder

Example BCD-to-Decimal Decoders 74LS42 /

74HC42

BCD-to-7-SEGMENT

• 7-segment configuration- to form the decimal characters 0 -9 and sometimes the hex characters A –F.

• BCD-to-7-segment decoder/driver is used to take 4-bit BCD input and provide the outputs that will pass current through the appropriate segment to display the decimal digit.

BCD-to-7-SEGMENT

(a) BCD-to-7-segment decoder/driver driving a common-anode 7-segment LED display;

(b) segment patterns for all possible input codes.

Encoder

• The opposite of this decoding process is called encoding and is performed by a logic circuit called an encoder.

• Encoder has a number of input lines, only one of which is activated at a given time, and produces an N-bit output code depending on which input is activated.

• An 8-line-to-3-line encoder accepts 8 input lines and produces a 3-bit output code corresponding to the activated input.

Encoder

Following through the logic, verify that a LOW

at any input will produce the output binary code

corresponding to that input.

Note that A0 is not internally connected (A1…A7=1111111, then Q2Q1Q0=000

Only one input should be low. Example: If A3 = A5 =0, and all other are High, then Q2Q1Q0=0112 (=310), NOT ACCEPTABLE

Priority Encoders

• Include the necessary logic to ensure that when

2 or more inputs are activated, the output code

will correspond to the HIGHEST-NUMBERED

input.

• Example: A’3, and A’5 are LOW, the output code

will be 101(5).

• When A’6, A’2, and A’0 are all LOW, the output

code is 110(6).

• The 74148, 74LS148, and 74HC148 are all octal-

to-binary priority encoders

Encoder

74147 Decimal-to-BCD Priority Encoder

• Nine active low inputs representing decimal 1 thru 9• Output: inverted BCD code corresponding to the highest numbered

activated input. Outputs can be converted to normal BCD by putting each through an inverter.

• No A0. When all inputs are high, it corresponds to decimal 0

Switch Encoder

Multiplexers (Data Selector)

• Multiplexer (MUX)- it selects one of the several input

signal sand passes it on to the output.

• Multiplexing- multiplexer selects 1 out of N input data

sources and transmits the selected data to a single

• Digital multiplexer or data selector is a logic circuit

that accepts several digital data inputs and selects

one of them at any given time to pass on to the

output.

• The routing of the desired data input to the output is

controlled by SELECT inputs

Basic Two Input Multiplexer• The logic level applied to the S input determines

which AND gate is enable so that its data input passes through the OR gate to output Z.


Four Input Multiplexer


Four-input multiplexer: (a) using sum of products logic; (b) using tristate buffers.

Multiplexers

• Main function of multiplexer and demultiplexer• The purpose is to reduce the number of wires required

for interconnection• by making the signals to time-share the link.

A multiplexer/demultiplexer arrangement

for information transmission

Demultiplexer

A 2n-to-1-line multiplexer symbol

MUX implementation of a Boolean function

• Any Boolean function of n variables can be implemented by a multiplexer with n control inputs in a straightforward manner.

Eight Input Multiplexer- 74LS151(74HC151)

This multiplexer has an enable input E’ and provides both the normal and the inverted outputs.


E’=0, Select input S2S1S0 will select one data inputE’=1, multiplexer is disable

Quad Two Input MUX (74ALS157/HC157)


1. Data routing

2. Parallel-to-serial conversion

3. Operation Sequencing

4. Logic Function Generation

Multiplexer Applications

Logic Function Generation

1-Line-to-8-Line Demultiplexer

Clock Demultiplexer• 74ALS138 demultiplexer being used as a

clock demultiplexer • Under control of the SELECT lines, the clock

signal is routed to the desired destination.

• If S2S1S0 = 000, the clock signal applied to I will appear at output O’0.

• If S2S1S0 = 101, the clock signal applied to I will appear at output O’5.

Demux (Data Distributer)

Magnitude Comparator

• Magnitude comparator – combinational logic that compares two input binary quantities and generates outputs to indicate which one has the greater magnitude.

• Data inputs – 74HC85 compares two unsigned 4-bit binary numbers.– A3A2A1A0 which is called word A; the other B3B2B1B0

which is called word B• Outputs

– Has 3 active-HIGH outputs– Output OA>B will be HIGH when the magnitude of word A

is greater than the magnitude of word B– Output OA<B will be HIGH when magnitude of word A is

less than the magnitude of word B.– Output OA=B will be HIGH when magnitude of word A is

identical with the magnitude of word B.

Magnitude Comparator

• Cascading Inputs– Expanding the

comparison operation to more than 4 bits by cascading two or more 4-bit comparators.

– When 2 comparators are to be cascaded, the outputs of the lower-order comparator are connected to the corresponding inputs of higher-order comparator.

Code Converters

• Code converter is a logic circuit that changes data presented in one type of binary code to another type of binary code.

• BCD-to-7 segment decoder is a code converter because it changes a BCD input code to the 7 segment code needed by the LED.

• BCD-to-binary converter– 2 digit decimal values ranging from 00 to 99

can be represented in BCD by 2 4-bit code groups.

BCD-to-binary converter

Code Converters

Try this....

0,1;0,1 201123 AAAEEE

0,1;0,1 102123 AAAEEE

What’s the output, if 74ALS138

….

Figure 1

Determine the states of the 74147 decimal-to-BCD priority encoder outputs when are LOW and the other are HIGH. 762 ,, AAA

Try this....

Basic Adders

Half-Adder

0 + 0 = 00 + 0 = 0

0 + 1 = 10 + 1 = 1

1 + 0 = 11 + 0 = 1

1 + 1 = 101 + 1 = 10

Zero plus zero equals zeroZero plus zero equals zero

Zero plus one equals oneZero plus one equals one

One plus zero equals oneOne plus zero equals one

One plus one equals zero with a carry One plus one equals zero with a carry of oneof one

Simple Binary Addition

Half-Adder

Full-Adder

Full-Adder

• Full adder from two half-adder circuits

Parallel Binary Adders


• Two-bit parallel binary adder


• Four-bit parallel binary adder

Chapter 8

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