Chapter 5

Boolean Algebra and Reduction Techniques

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Objectives

• You should be able to: Write Boolean equations for combinational

logic applications. Use Boolean algebra laws and rules to simplify

combinational logic circuits. Apply DeMorgan’s theorem to complex

Boolean equations to arrive at simplified equivalent equations.

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Objectives

• You should be able to: Design single-gate logic circuits by using the

universal capability of NAND and NOR gates. Troubleshoot combinational logic circuits. Implement sum-of-products expressions using

AND-OR-INVERT gates.

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Objectives

• You should be able to: Use the Karnaugh mapping procedure to

systematically reduce complex Boolean equations to their simplest form.

Describe the steps involved in solving a complete system design application.

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Combinational Logic

• Using two or more logic gates to form a more useful, complex function

• A combination of logic functionsB = KD + HD

• Boolean ReductionB = D(K+H)

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Discussion Point• Write the Boolean equation for the circuit

below:

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Boolean Laws and Rules• Commutative law of addition and

multiplication A + B = B + A ABC = BCA

Figures 5-7 and 5-8

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Boolean Laws and Rules• Associative law of addition and

multiplication A + (B + C) = (A + B) + C A(BC) = (AB)C

Figures 5-9 and 5-10

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Boolean Laws and Rules• Distributive law

A(B + C) = AB + AC (A + B)(C + D) = AC + AD + BC + BD

Figures 5-11 and 5-12

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Boolean Laws and Rules

• Rule 1: Anything ANDed with a 0 is equal to 0

• Rule 2: Anything ANDed with a 1 is equal to itself

• Figure 5-14

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Boolean Laws and Rules• Rule 3: Anything ORed with a 0 is equal to itself

• Figure 5-15

• Rule 4: Anything ORed with a 1 is equal to 1

• Figure 5-16

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Boolean Laws and Rules• Rule 5: Anything ANDed with itself is equal to

itself• Figure 5-17

• Rule 6: Anything ORed with itself is equal to itself

• Figure 5-18

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Boolean Laws and Rules• Rule 7: Anything ANDed with its own

complement equals 0• Figure 5-19

• Rule 8: Anything ORed with its own complement equals 1

• Figure 5-20

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Boolean Laws and Rules

• Rule 9: Anything complemented twice will return to its original logic level

• Figure 5-21

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Boolean Laws and Rules

• Rule 10: A + AB = A + B A + AB = A + B

See Table 5-1 in your text

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Discussion Point

• Which Boolean laws are illustrated below? B + (D + E) = (B + D) + E AB = BA A + B + C = B + C + A A(C + D) = AC + AD

• What are some strategies for remembering the 10 Boolean rules?

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Simplification of Combinational Logic Circuits Using Boolean Algebra

• Equivalent circuits can be formed with fewer gates

• Cost is reduced

• Reliability is improved

• Use laws and rules of Boolean Algebra

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Simplification of Combinational Logic Circuits Using Boolean Algebra

• Simplify the logic circuit shown by using the appropriate laws and rules.

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Simplification of Combinational Logic Circuits Using Boolean Algebra

• Simplify the logic circuit shown by using the appropriate laws and rules.

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DeMorgan’s Theorem

• To simplify circuits containing NAND and NOR gates

• A B = A + B

• A + B = A B

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DeMorgan’s Theorem

• Break the bar over the variables and change the sign between them

• Inversion bubbles - used instead of inverters to show inversion.

• Use parentheses to maintain proper groupings

• Results in Sum-of-Products (SOP) form• Use of the MultiSIM logic converter

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DeMorgan’s Theorem• Bubble Pushing

• Figure 5-46

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DeMorgan’s Theorem

• Bubble Pushing shortcut method of forming equivalent gates change the logic gate

• (AND to OR or OR to AND)

Add bubbles to the inputs and outputs where there were none and remove original bubbles

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The Universal Capability of NAND and NOR Gates

• The NAND as an inverter.

• Figure 5-49(a)

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The Universal Capability of NAND and NOR Gates

• Forming an AND with two NANDs

• Figure 5-49(b)

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The Universal Capability of NAND and NOR Gates

• Forming an OR with three NANDs

• Figure 5-53

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The Universal Capability of NAND and NOR Gates

• Forming a NOR with three NANDs

• Figure 5-54

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Discussion Point• The technique used to form all gates from NANDs can also be

used with NOR gates.• Here is an inverter – Figure 5-55

• Form the other logic gates using only NORs.

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AND-OR-INVERT Gates for Implementing Sum-of-Products

Expressions

• Product-of-sums (POS) form• Sum-of-products (SOP) form

Can easily be implemented using an AOI gate

• Programmable Logic Devices (PLDs) Can also be used

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Karnaugh Mapping

• To minimize the number of gates

• Reduce circuit cost

• Reduce physical size

• Reduce gate failures

• Requires SOP form

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Karnaugh Mapping

• Graphically shows output level for all possible input combinations

• Moving from one cell to an adjacent cell, only one variable changes

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Karnaugh Mapping

• Steps for K-map reduction: Transform the Boolean equation into SOP form Fill in the appropriate cells of the K-map Encircle adjacent cells in groups of 2, 4 or 8

• watch for the wraparound

Find terms by determining which variables remain constant within circles

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Discussion Point

• Use a K-map to simplify the circuit.

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System Design Applications

• Use Karnaugh Mapping to reduce equations

• Use AND-OR-INVERT gates to implement logic

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System Design Applications

• Use a K-map to simplify a circuit that will use an AOI and inverters to output a HIGH when a 4 bit hexadecimal input is an odd number from 0 to 9

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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 5-1 and 5-2

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Figures 5-80 and 5-81

Summary

• Several logic gates can be connected together to form combinational logic.

• There are several Boolean laws and rules that provide the means to form equivalent circuits.

• Boolean algebra is used to reduce logic circuits to simpler equivalent circuits that function identically to the original circuit.

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Summary

• DeMorgan’s theorem is required in the reduction process whenever inversion bars cover more than one variable in the original Boolean equation.

• NAND and NOR gates are sometimes referred to as universal gates, because they can be used to form any of the other gates.

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Summary

• AND-OR-INVERT (AOI) gates are often used to implement sum-of-products (SOP) equations

• Karnaugh mapping provides a systematic method of reducing logic circuits.

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