Lecture (04) Boolean Algebra and Logic...
Transcript of Lecture (04) Boolean Algebra and Logic...
Lecture (04)Boolean Algebra and
Logic GatesBy:
Dr. Ahmed ElShafee
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design١
Boolean algebra properties
basic assumptions and properties:
• Closure law
– A set S is closed with respect to a binary operator, for every pair of elements of S,
– the binary operator specifies a rule for obtaining a unique element of S.
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٢
• Commutative law.
– A binary operator * on a set S is said to be commutative whenever
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• Associative law.
– A binary operator * on a set S is said to be associative whenever
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٤
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٥
• Identity element.
– A set S is said to have an identity element with respect to a binary operation * on S
– if there exists an element e S with the property that
• Example: The element 0 is an identity element with respect to the binary operator + on the set of integers I = {…, ‐3, ‐2, ‐1, 0, 1, 2, 3,…}, since
• x + 0 = 0 + x = x for any x I
• Example: The element 1 is an identity element with respect to the binary operator . on the set of integers I = {…, ‐3, ‐2, ‐1, 0, 1, 2, 3,…}, since
• x . 1 = 1 . x = x for any x I
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• Inverse.
– A set S having the identity element e with respect to a binary operator * is said to have an inverse whenever, for every x S, there exists an element y S such that
• Example: In the set of integers, I, and the operator +, with e = 0, the inverse of an element a is (‐a), since a + (‐a) = 0.
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٨
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٩
• Distributive law
– If * and . are two binary operators on a set S, * is said to be distributive over . Whenever
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design١١
Summery
• The binary operator + defines addition (OR).
• The additive identity is 0.
• The additive inverse defines subtraction.
• The binary operator . defines multiplication (AND).
• The multiplicative identity is 1.
• For a 0, the multiplicative inverse of a = 1/a defines division (i.e., a . 1/a = 1 ).
• The only distributive law applicable is that of . over +:
a . (b + c) = (a . b) + (a .. c)
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Ordinary and Boolean Algebra
• George Boole developed an algebraic system now called Boolean algebra.
• Claude E. Shannon introduced a two‐valued Boolean algebra called switching algebra that represented the properties of bistable electrical switching circuits.
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1. (a) The structure is closed with respect to the operator +.
(b) The structure is closed with respect to the operator . .
2. (a) The element 0 is an identity element with respect to +; that is, x + 0 = 0 + x = x.
(b) The element 1 is an identity element with respect to . ; that is, x . 1 = 1 . x = x.
3. (a) The structure is commutative with respect to +; that is, x + y = y + x.
(b) The structure is commutative with respect to . ; that is, x . y = y . x.
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4. (a) The operator . is distributive over +; that is, x . (y + z) = (x . y) + (x . z).
(b) The operator + is distributive over . ; that is, x + (y . z) = (x + y) . (x + z).
5. For every element x B, there exists an element x B (called the complement of x) such that
(a) x + x = 1 and
(b) x . x = 0 ,
6. There exist at least two elements x, y B such that x y.
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design١٥
Comparing Boolean algebra with arithmetic and ordinary algebra:
• The distributive law of + over . (i.e., x + (y . z) = (x + y) . (x + z) ) is valid for Boolean algebra, but not for ordinary algebra.
• Boolean algebra does not have additive or multiplicative inverses; therefore, there are no subtraction or division operations.
• defines an operator called the complement that is not available in ordinary algebra.
• Ordinary algebra deals with the real numbers, which constitute an infinite set of elements. Boolean algebra deals with the as yet undefined set of elements, B, but in the two‐valued Boolean algebra defined next (and of interest in our subsequent use of that algebra), B is defined as a set with only two elements, 0 and 1.
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Two‐Valued Boolean Algebra
• defined on a set of two elements, B = {0, 1}, with rules for the two binary operators + and .
• rules are exactly the same as the AND, OR, and NOT operations
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Truth table
• the structure is closed with respect to the two operators, each operation is either 1 or 0 and 1, 0 B.
• From the tables, we see that
(a) 0 + 0 =0 0+ 1 = 1 + 0 = 1;
(b) 1 . 1 = 1 1. 0 = 0 . 1 = 0.
This establishes the two identity elements, 0 for + and 1 for . ,
• The commutative laws are obvious from the symmetry of the binary operator table (A+B = B+A) & (A.B = B.A)
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• The distributive law x . (y + z) = (x . y) + (x . z) can be shown to hold from the operator tables
• The distributive law of + over . can be shown to hold by means of a truth table
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٢٠Truth table
• the complement table(a) x + x = 1, since 0 + 0 = 0 + 1 = 1 and 1 + 1 = 1 + 0 = 1.
(b) x . x = 0, since 0 . 0 = 0 . 1 = 0 and 1 . 1 = 1 . 0 = 0.
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٢١
Postulates and Basic theorem of Boolean algebraPostulates and Theorems of Boolean Algebra
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Duality principle
• postulates were listed in pairs and designated by part (a) and part (b).
• One part may be obtained from the other if the binary operators and the identity elements are interchanged
• we simply interchange OR and AND operators and replace 1’s by 0’s and 0’s by 1’s.
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Theorem 5.a
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Truth table
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٢٥
Truth table
• Operator Precedence
– (1) parentheses,
– (2) NOT,
– (3) AND, and
– (4) OR.
• expressions inside parentheses must be evaluated before all other operations. The next operation that holds precedence is the complement, and then follows the AND and, finally, the OR.
• example: demorgan’s
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Boolean functions
• Boolean function described by an algebraic expression consists of binary variables
• A Boolean function can be represented in a truth table. The number of rows in the truth table is 2n, where n is the number of variables in the function.
• The interconnection of gates will dictate the logic expression.
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٢٧
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٢٨
Logic diagram
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٢٩
F1xy‘ zY'
0001
1011
0000
0000
1101
1111
1100
1100
zyx
000
100
010
110
001
101
011
111
• it is sometimes possible to obtain a simpler expression for the same function and thus reduce the number of gates in the circuit and the number of inputs to the gate.
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣٠
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣١
Logic diagram
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣٢
Logic diagram
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣٣
zyx
000
100
010
110
001
101
011
111
f2X’zzX’Xy’Y’X
0001010
1111010
0001000
1111000
1000111
1010111
0000001
0010001
Truth table
Algebraic Manipulation
• When a Boolean expression is implemented with logic gates, each term requires a gate and each variable within the term designates an input to the gate.
• We define a literal to be a single variable within a term, in complemented or un‐complemented form.
• By reducing the number of terms, the number of literals, or both in a Boolean expression, it is often possible to obtain a simpler circuit
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣٤
Example 01
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣٦
Complement of a Function
• The complement of a function may be derived algebraically through DeMorgan’s theorems
• DeMorgan’s theorems can be extended to three or more variables.
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣٧
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٣٨
Example 02
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٤٠
Example 03
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Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٤٢
Thanks,..
See you next week (ISA),…
Dr. Ahmed ElShafee, ACU : Spring 2016, Logic Design٤٣