BOOLEAN ALGEBRA AND LOGIC SIMPLIFICATION

Boolean Analysis of Logic Circuits, evaluating of Boolean expressions, representing the operation of Logic circuits and Boolean expressions in terms of Function tables and representing Boolean expressions in SOP and POS forms are inter-related. Boolean laws, rules and theorems are used to readily change from one form of representation to the other.

digital logic design  BOOLEAN ALGEBRA AND LOGIC SIMPLIFICATION

Two examples are considered which illustrate the analysis, simplification and representation of Logic Circuits and Boolean expressions. In both the examples, a Boolean expression representing the Logic Circuit is developed, the Boolean expression is evaluated and a function table is implemented that represents the Boolean expression and the function of the Logic Circuit. Each Boolean expression is also simplified into SOP or POS form, the simplified expression is presented in a function table format. The original and the simplified expressions are verified to show identical functions.

Example 1

1. Finding the Boolean Expression


digital logic design  BOOLEAN ALGEBRA AND LOGIC SIMPLIFICATION

The circuit can be represented by a Boolean Expression. Starting from the left hand

side

  • Output of NOT gate 1 isB
  • Output of NOT gate 2 is A
  • Output of two input AND gate 3 is A.B (product of A and B
  • Output of two input AND gate 4 is C.D (product of C and D)
  • Output of three input NAND gate 5 is A.B.C.D NOT(product of A , B and CD)
  • Output of two input NOR gate 6 is A.B +A.B.C.D NOT(sum of A.B and A.B.C.D)

2. Evaluating the Expression

Considering that X =A.B and Y = A.B.C.D . The expression A.B +A.B.C.D can be represented by X + Y.

The output of the logic circuit is 1 when X = 0 and Y = 0

  • X=0 NOR Y=0 Output = 1
  • X=0 NOR Y=1 Output = 0
  • X=1 NOR Y=0 Output = 0
  • X=1 NOR Y=1 Output = 0

X= A.B = 0 when any literal is zero. That is, A =0 or B =0 (B=1)

Y= A.B.C.D = 0 whenA.B.C.D= 1

A.B.C.D = 1 when all literals are one. That is A =1 (A=0), B =1 (B=0), C=1 and D=1

The expression output is 1 for the input conditions (A=0 OR B=1) AND (A=0 AND B=0 AND C=1 AND D=1) That is, A=0, B=0, C=1and D=1.

3. Putting the Results in Truth Table Format

Input Output
A B C D F
0 0 0 0 0
0 0 0 1 0
0 0 1 0 0
0 0 1 1 1
0 1 0 0 0
0 1 0 1 0
0 1 1 0 0
0 1 1 1 0
1 0 0 0 0
1 0 0 1 0
1 0 1 0 0
1 0 1 1 0
1 1 0 0 0
1 1 0 1 0
1 1 1 0 0
1 1 1 1 0

Table 9.1 Truth Table representing function of Logic Circuit (fig. 9.1)

4. Simplification of Boolean Expression

The A.B +A.B.C.D expression can be simplified by applying Demorgan’s second theorem A + B = A.B.

= (A.B +A.B.C.D = (A.B).(A.B.C.D) Apply Demorgan’s first theorem to the first term and Rule 9 to the second term

= (A + B).(A.B.C.D)

= (A + B).(A.B.C.D) Using the Distributive Law = (A.A.B.C.D) + A.B.B.C.D Applying Rule 8 to the second term = A.B.C.D

expression =1 when all literals are one that is A =1 AND B =1 AND C=1 AND D=1 or A=0 AND B=0 AND C=1 AND D=1

5. Putting the result in Truth Table format

Input Output
A B C D F
0 0 0 0 0
0 0 0 1 0
0 0 1 0 0
0 0 1 1 1
0 1 0 0 0
0 1 0 1 0
0 1 1 0 0
0 1 1 1 0
1 0 0 0 0
1 0 0 1 0
1 0 1 0 0
1 0 1 1 0
1 1 0 0 0
1 1 0 1 0
1 1 1 0 0
1 1 1 1 0

Table 9.2 Truth Table representing function of simplified expression
6. Implementing Logic Circuit from Simplified Boolean expression

F = A.B.C.D

The expression F represents a product term having four literals. Product term is implemented using AND gates. Since, the product has four literals therefore a 4-input AND gate is used. The literalsA andBare implemented using NOT gates.

digital logic design  BOOLEAN ALGEBRA AND LOGIC SIMPLIFICATION

Figure 9.2 Simplified Logic Circuit

Example 2

1. Finding the Boolean Expression


digital logic design  BOOLEAN ALGEBRA AND LOGIC SIMPLIFICATION

The circuit can be represented by a Boolean Expression. Starting from the left hand side

  • Output of NOT gate 1 is A
  • Output of NOT gate 2 isC
  • Output of three input AND gate 4 is A.B.C (product of A , B and C)
  • Output of two input OR gate 5 is C +D (sum of C and D)
  • Output of NAND gate 6 is (A.B.C).(C +D) NOT(product of A.B.C and C + D)

2. Evaluating the Expression

Considering that X =A.B.C and Y = C + D . The expression (A.B.C).(C +D) can be

represented by X.Y. The output of the logic circuit is 1 when X=0 or Y=0

  • X=0 NAND Y=0 Output = 1
  • X=0 NAND Y=1 Output = 1
  • X=1 NAND Y=0 Output = 1
  • X=1 NAND Y=1 Output = 0

X= A.B.C = 0 when any literal is zero. That is, A =0 (A=1) or B =0 or C =0 (C=1)

Y=C +D = 0 when both literals are zero. That is C =0 (C=1) and D=0

The expression output is 1 for the input conditions (A=1 OR B=0 OR C=1) OR (C=1 AND D=0)

3. Putting the Results in Truth Table Format

Input Output
A B C D F
0 0 0 0 1
0 0 0 1 1
0 0 1 0 1
0 0 1 1 1
0 1 0 0 0
0 1 0 1 0
0 1 1 0 1
0 1 1 1 1
1 0 0 0 1
1 0 0 1 1
1 0 1 0 1
1 0 1 1 1
1 1 0 0 1
1 1 0 1 1
1 1 1 0 1
1 1 1 1 1

Table 9.2 Truth Table representing function of Logic Circuit (fig. 9.3)

The truth table shows that the variable D has no effect on the output of the circuit. The truth table reduces to a three variable truth table. Table 9.3

4. Simplification of Boolean Expression

The (A.B.C).(C +D) expression can be simplified by applying Demorgan’s first theorem A.B = A + B.

= (A.B.C).(C + D) = (A.B.C) + (C + D)

Input Output
A B C F
0 0 0 1
0 0 1 1
0 1 0 0
0 1 1 1
1 0 0 1
1 0 1 1
1 1 0 1
1 1 1 1

Table 9.3 Alternate Truth Table representing function of Logic Circuit (fig. 9.3) Apply Demorgan’s first and second theorems to the first and second terms respectively

digital logic design  BOOLEAN ALGEBRA AND LOGIC SIMPLIFICATION

= (A + B + C) + (C.D)

Applying Rule 9 = (A + B + C) + (C.D) = A + B + C(1+ D)

= A + B + C expression =1 when any one literal is one that is A=1 OR B=1 OR C=1

or A=1 OR B=0 OR C=1

5. Putting the result in Truth Table format

Input Output
A B C F
0 0 0 1
0 0 1 1
0 1 0 0
0 1 1 1
1 0 0 1
1 0 1 1
1 1 0 1
1 1 1 1

Table 9.4 Truth Table representing function of simplified expression

6. Implementing Logic Circuit from Simplified Boolean expression

F = A + B + C

The expression F represents a sum term having three literals. Sum term is implemented using OR gates. Since, the sum has three literals therefore a 3-input OR gate is used. The literal B is implemented using NOT gate.

Figure 9.4 Simplified Logic Circuit

Standard SOP form

A standard SOP form has product terms that have all the variables in the domain of the expression. The SOP expression AC +BC is not a standard SOP as the domain of the expression has variables A, B and C.

A non-standard SOP is converted into a standard SOP by using the rule A + A = 1

AC + BC

= AC(B + B) + (A + A)BC

= ABC + ABC + ABC + ABC

= ABC + ABC + ABC

Standard POS form

A standard POS form has sum terms that have all the variables in the domain of the expression. The POS expression (A +B + C)(A + B + D)(A + B + C + D) is not a standard POS as the domain of the expression has variables A, B, C and D.

A non-standard POS is converted into a standard POS by using the rule AA = 0

(A + B + C)(A + B + D)(A + B + C + D)

= (A + B + C + D)(A + B + C + D)(A + B + C + D)(A + B + C + D)(A + B + C + D)

Converting to Standard SOP and POS forms

There are several reasons for converting SOP and POS forms into standard SOP and POS forms respectively.

Any logic circuit can be implemented by using either the SOP, AND-OR combination of gates or POS, OR-AND combination of gates. It is very simple to convert from standard SOP to standard POS or vice versa. This helps in selecting an implementation that requires the minimum number of gates. Secondly, the simplification of general Boolean expression by applying the laws, rules and theorems does not always result in the simplest form as the ability to apply all the rules depends on ones experience and knowledge of all the rules.

A simpler mapping method uses Karnaugh maps to simplify general expressions. Mapping of all the terms in a SOP form expression and the sum terms in a POS form can be easily done if standard forms of SOP and POS expressions are used. Karnaugh maps will be discussed latter in the chapter.

Lastly, the PLDs are implemented having a general purpose structure based on ANDOR arrays. A function represented by an expression in Standard SOP form can be readily programmed.

Minterms and Maxterms

The Product terms in the Standard SOP form are known as Minterms and the Sum terms in the Standard POS form are known as Maxterms.

A B C Minterms Maxterms
0 0 0 A B C. . CBA ++
0 0 1 A B C. . CBA ++
0 1 0 A B C. . CBA ++
0 1 1 A B C. . CBA ++
1 0 0 A B C. . CBA ++
1 0 1 A B C. . CBA ++
1 1 0 A B C. . CBA ++
1 1 1 A B C. . CBA ++

Table 9.5 Table of Minterms and Maxterms

Binary representation of a standard Product term or Minterm

A standard product term is equal to one for only one combination of variable values. For all other variable values the standard product term is equal to zero.

For the expression ABC + ABC + ABC ABC =1 if A=1, B=1 and C=0 ABC =1 if A=1, B=0 and C=0 ABC =1 if A=0, B=1 and C=0

An SOP expression is equal to 1 when one or more product terms in the expression are equal to 1.

Binary representation of a standard Sum term or Maxterm

A standard sum term is equal to zero for only one combination of variable values. For all other variable values the standard sum term is equal to one.

For the expression (A + B + C + D)(A + B + C + D)(A + B + C + D)(A + B + C + D)(A + B + C + D) (A + B + C + D) =0 if A=0, B=1, C=0 and D=1 (A + B + C + D) =0 if A=0, B=1, C=0 and D=0 (A + B + C + D) =0 if A=0, B=0, C=1 and D=1 (A + B + C + D) =0 if A=0, B=0, C=0 and D=1 (A + B + C + D) =0 if A=0, B=1, C=1 and D=0

A POS expression is equal to 0 when one or more product terms in the expression are equal to 0.

Converting Standard SOP into Standard POS

The binary values of the product terms in a given standard SOP expression are not present in the equivalent standard POS expression. Also, the binary values that are not represented in the SOP expression are present in the equivalent POS expression.

ABC + ABC + ABC + ABC + ABC has the binary values 000, 010,011,101 and 111 Canonical Sum = ∑A,B,C (0,2,3,5,7) = ABC + ABC + ABC + ABC + ABC The missing binary values are 001, 100 and 110. The POS expression is (A + B + C)(A + B + C)(A + B + C)

Canonical Product = ∏A,B,C (1,4,6)

Verifying POS expression is equivalent to SOP expression

(A + B + C)(A + B + C)(A + B + C)

= (A.A + A.B + A.C + A.B + B.B + B.C + A.C + B.C + C.C).(A + B + C)

= (A.B + A.C + A.B + B + B.C + A.C + B.C).(A + B + C)

= (A.C + B + A.C).(A + B + C)

=A.A.C + A.B.C + A.C.C + A.B + B.B + B.C + A.A.C + A.B.C + A.C.C

=A.B.C + A.C + A.B + B.C + A.C + A.B.C Converting into standard SOP =A.B.C + A.C(B + B) + A.B(C + C) + B.C(A + A) + A.C(B + B) + A.B.C

=A.B.C + A.B.C + A.B.C + A.B.C + A.B.C + A.B.C + A.B.C + A.B.C + A.B.C + A.B.C

Simplifies to = ABC + ABC + ABC + ABC + ABC Therefore ∑(0,2,3,5,7) = ∏(1,4,6)

Boolean Expressions and Truth Tables

All standard Boolean expressions can be easily converted into truth table format using binary values for each term in the expression. Standard SOP or POS expressions can also be determined from a truth table.

Converting SOP expression to Truth Table format

A truth table is a list of possible input variable combinations and their corresponding output values. An SOP expression having a domain of 2 variables will have a truth table having 4 combinations of inputs and corresponding output values.

To convert an SOP expression in a Truth table format, a truth table having input combinations proportional to the domain of variables in the SOP expression is written. Next the SOP expression is written in a standard SOP form. In the last step all the sum terms present in the standard SOP expression are marked as 1 in the output.

AB + BC has a domain of three variables thus a truth table having 8 input and output combinations is required. The SOP expression is converted into standard SOP expression

AB(C + C) + BC(A + A) = ABC + ABC + ABC + ABC = ABC + ABC + ABC + ABC

Marking the outputs in the truth table as 1 for sum terms that are present in the standard SOP.

Input Output
A B C F
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 1
1 0 0 1
1 0 1 1
1 1 0 0
1 1 1 1

Table 9.6 Mapping SOP expression to Truth Table

Canonical Sum F= ∑A,B,C (3,4,5,7) =A.B.C + A.B.C + A.B.C + A.B.C

Converting POS expression to Truth Table format

An POS expression having a domain of 2 variables will have a truth table having 4 combinations of inputs and corresponding output values. To convert a POS expression in a Truth table format, a truth table having input combinations proportional to the domain of variables in the POS expression is written. Next the POS expression is written in a standard POS form. In the last step all the product terms present in the standard POS expression are marked as 0 in the output.

(A +B)(B + C) has a domain of three variables thus a truth table having 8 input and output combinations is required. The POS expression is converted into standard POS expression

(A + B + CC)(AA + B + C) = (A + B + C)(A + B + C)(A + B + C)(A + B + C)

Marking the outputs in the truth table as 0 for product terms that are present in the standard POS

Input Output
A B C F
0 0 0 1
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 0
1 1 0 1
1 1 1 1

Table 9.7 Mapping POS expression to Truth Table Canonical Product F= ∏A,B,C (1,2,3,5) = (A + B + C)(A + B + C)(A + B + C)(A + B + C)

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