CS 121 Digital Logic Design. Chapter 2. Teacher Assistant. Hanin Abdulrahman

Transcription

1 CS 121 Digital Logic Design Chapter 2 Teacher Assistant Hanin Abdulrahman 1

2 2 Outline 2.2 Basic Definitions 2.3 Axiomatic Definition of Boolean Algebra. 2.4 Basic Theorems and Properties 2.5 Boolean Functions 2.6 Canonical and Standard Forms 2.7 Other Logic Operations 2.8 Digital Logic Gates

3 3 2.2 Basic Definitions (1-2) Boolean Algebra: Formulated by E.V. Huntington, A set of elements B={0,1} and two binary operators + and Closure x, y B x+y B x, y B x y B Commutative x + y = y + x x y = y x Associative (x+y)+z = x + (y + z) (x y) z = x (y z) Identity element 0+x = x+0 = x 1 x = x 1=x

4 4 2.2 Basic Definitions (2-2) Complement x B, x' B (complement of x) x+x'=1; x x'=0 Distributive Law Over +: x (y+z)=(x y)+(x z). Over : x+ (y.z)=(x+ y) (x+z).

5 2.3 Axiomatic Definition of Boolean Algebra (1-2) Two- Valued Boolean Algebra: 5 x y x y x y x+y = AND + = OR = NOT x x

6 2.3 Axiomatic Definition of Boolean Algebra (2-2) Two- Valued Boolean Algebra: 6 Distributive law: x (y+z)=(x y)+(x z)

7 2.4 Basic Theorems and Properties (1-7) Duality Principle Using Huntington rules, one part may be obtained from the other if the binary operators and the identity elements are interchanged. Duality done by: 1- Interchange OR and AND Operations 2- Replace the identity elements 1 s by 0 s and 0 s by 1 s. 7

8 2.4 Basic Theorems and Properties (2-7) Table 2.1 Postulates and Theorems of Boolean algebra Postulates 2 (a) X + 0 = X (b) Postulates 5 Theorem 1 (a) X + X = 1 (a) X + X = X (b) (b) Theorem 2 (a) X + 1 = 1 (b) Theorem 3, involution (X ) = X X. 1 X. X X. X = X = 0 = X X. 0 0 = Postulate 3,commutative(a) X + Y Theorem 4, associative Postulate 4, distributive Theorem 5, demorgan s Theorem 6,absorption (b) (a)(x + Y) + Z = X + (Y + Z) (b) (a) = Y + X X(Y + Z) = XY + XZ (a)(x + Y) = X. Y (a) X + XY = X (b) (b) (b) XY = YX (XY) Z X(Y Z) = X + YZ = (X + Y) (X + Z) (X. Y) X + Y = X(X+Y) = X Operator Precedence 1. Parentheses. 2. NOT. 3. AND. 4. OR.

9 2.4 Basic Theorems and Properties (3-7) Basic Theorems 9

10 2.4 Basic Theorems and Properties (4-7) Basic Theorems 10

11 2.4 Basic Theorems and Properties (5-7) Basic Theorems 11

12 2.4 Basic Theorems and Properties (6-7) Truth Table A table of all possible combinations of x and y variables showing the relation between the variable values and the result of the operation. 12 Theorem 6(a) Absorption The theorems of Boolean algebra can be proven by means of truth table.

13 2.4 Basic Theorems and Properties (7-7) Truth Table Theorem 5: DeMorgan theorem, (x + y) = x y 13

14 Boolean Functions (1-11) Example Logic Circuit Boolean Function List the truth table for the follwoing functions and draw their implementation gates. F 1 = x + (y z). F 2 = x y z + x yz + xy

15 2.5 Boolean Functions (1-11) F 1 = x + (y z) X Y Z Y Y *Z X + (y z)

16 2.5 Boolean Functions (1-11) Truth table 16

17 2.5 Boolean Functions (2-11) 17 Gate Implementation of F 1 F1= x + (y z).

18 2.5 Boolean Functions (3-11) 18 Gate Implementation of F 2 F2= x y z + x yz + xy

19 Boolean Functions (4-11) Gate Implementation F 2 after simplifying it F 2 = x y z + x yz + xy. F 2 = x (y z + yz) + xy. F 2 = x (z (y + y)) + xy. F 2 = x (z (1)) + xy. F 2 = x z + xy.

20 Boolean Functions (5-11) Algebraic Manipulation - Simplification Example Simplify the following Boolean functions to a minimum number of literals: x(x +y) =xx + xy =0+xy = xy x+x y =(x+x )(x+y) =1(x+y) = x+y

21 Boolean Functions (6-11) DeMorgan s Theorem (x+y)(x+y ) =x+ xy + xy +yy =x (1+ y + y ) =x xy + x z + yz = xy + x z + yz(x+x ) = xy + x z + xyz + x yz =xy(1+z) + x z (1+y) = xy + x z (x+y)(x +z)(y+z) =(x+y)(x +z) by duality function 4

22 2.5 Boolean Functions (7-11) Complement of a Function Complement of a variable x is x (0 1 and 1 0). The complement of a function F is x and is obtained from an interchange of 0 s for 1 s and 1 s for 0 s in the value of F The dual of a function is obtained from the interchange of AND and OR operators and 1 s and 0 s. Finding the complement of a funtion F Applying DeMorgan s theorem as many times as necessary. Complementing each literal of the dual of F 22

23 Boolean Functions (8-11) DeMorgan s Theorem 2-variable DeMorgan s Theorem (x + y) = x y. (xy) = x + y. 3-variable DeMorgan s Theorem

24 2.5 Boolean Functions (9-11) DeMorgan s Theorem Generalized DeMorgan s Theorem 24

25 Boolean Functions (10-11) Example A simpler procedure for deriving the complement of a function is to take the dual of the function and complement each literal. This method follows from the Generalized DeMorgan s Theorem

26 2.5 Boolean Functions (11-11) Example 26

27 Canonical and Standard Forms (1-23) Logic circuit Boolean function Truth table Any Boolean function can be expressed as a sum of minterms. Any Boolean function can be expressed as a product of maxterms

28 Canonical and Standard Forms (2-23) Minterms and Maxterms Minterm (or standard products): Any binary variable may appear either in its normal form (x) or in its complement form (x'). n variables combined with AND. n variables can be combined to form 2 n minterms. two Variables: x y, x y, xy, and xy A variable of a minterm is: Prime: if the corresponding bit of the binary number is a 0. Unprime: if the corresponding bit of the binary number is a => x y z (m 1 ) 100 => xy z (m 4 ) 111 => xyz (m 7 )

29 Canonical and Standard Forms (3-23) Minterms and Maxterms Maxterm (or standard sums): n variables combined with OR. A variable of a maxterm is: Prime: if the corresponding bit of the binary number is a 1. Unprime: if the corresponding bit of the binary number is a 0. Each maxterm is the complement of its corresponding minterm and vice versa. 001 => x + y + z (M 1 ) 100 => x + y + z (M 4 ) 111 => x + y + z (M 7 )

30 2.6 Canonical and Standard Forms (4-23) Minterms and Maxterms 30

31 Canonical and Standard Forms (5-23) Expressing Truth Table in Boolean Function Any Boolean function can be expressed as: Sum of minterms. Product of maxterms. (either 0 or 1 for each term) Said to be in a canonical form. Expressing Truth Table in Boolean Function Forming a minterm for each combination of the variables that produces a one in the function. Taking the OR of all those terms. n variables: 2 2n possible functions.

32 2.6 Canonical and Standard Forms (6-23) Expressing Truth Table in Boolean Function Example: 32

33 2.6 Canonical and Standard Forms (7-23) Expressing Truth Table in Boolean Function

34 Canonical and Standard Forms (8-23) Expressing Boolean Function in Sum of Minterms (Method 1 - Supplementing) Rule If the function is not in Minterm form Expanding the expression into a sum of AND terms. Each term is then inspected to see if it contains all the variables. If it misses one or more variables, it is ANDed with an expression such as x + x', where x is one of the missing variables.

35 2.6 Canonical and Standard Forms (9-23) Expressing Boolean Function in Sum of Minterms (Method 1 - Supplementing) Example Express the Boolean function F = A + B'C as a sum of minterm. 35

36 Canonical and Standard Forms (10-23) Expressing Boolean Function in Sum of Minterms (Method 1 - Supplementing) Example Short Notation: F(A, B, C ) = (1,4,5,6,7)

37 Canonical and Standard Forms (11-23) Expressing Boolean Function in Sum of Minterms (Method 1 - Supplementing) Short Notation: F(A, B, C ) = (1,4,5,6,7) Stands for the ORing of terms The minterms of the function List of the variables in the order taken when the minterm is converted to an AND term

38 Canonical and Standard Forms (12-23) Expressing Boolean Function in Sum of Minterms (Method 2 Truth Table) Example Express the Boolean function F = A + B'C as a sum of minterm, using the truth table F(A, B, C) = (1, 4, 5, 6,7) = (0, 2, 3) F (A, B, C) = (0, 2, 3) = (1, 4, 5, 6, 7)

39 Canonical and Standard Forms (13-23) Expressing Boolean Function in Product of Maxterms Rule If the function is not in Maxterm form Use distributive law e.g. x + yz = (x + y)(x + z) Each term is then inspected to see if it contains all the variables. If it misses one or more variables, it is ORed with an expression such as xx', where x is one of the missing variables.

40 2.6 Canonical and Standard Forms (14-23) Expressing Boolean Function in Product of Maxterms Example Express the Boolean function F = xy + x z as a product of maxterms. 40

41 2.6 Canonical and Standard Forms (15-23) Expressing Boolean Function in Product of Maxterms Example 41 Short Notation: F(A, B, C ) = (0,2,4,5) The minterms of the function List of the variables in the order taken when the maxterm is converted to an OR term Stands for the ANDing of terms

42 2.6 Canonical and Standard Forms (16-23) Conversion Between Canonical Forms Canonical conversion procedure Consider: F(A, B, C) = (1, 4, 5, 6, 7). F : complement of F = F (A, B, C) = (0, 2, 3) =m 0 +m 2 + m 3 (Missing minterms) Compute complement of F by DeMorgan s Theorem: m j =M j. Summary 42 F = (F ) = (m 0 +m 2 + m 3 ) = (m 0. m 2. m 3 ) = M 0. M 2. M 3 = (0, 2,3). (Obtain F in different form) Interchange the symbols and. List numbers missing from the original form. E.g. (1, 4, 5, 6, 7) = (0, 2,3). Shown by truth table (Table 2-5)

43 2.6 Canonical and Standard Forms (17-23) Two Canonical Forms of Boolean Algebra from Truth Table Example: Boolean exprexsion: x(x, y, z) = xy + x z 43 Truth Table Canonical Forms x(x, y, z) = (1, 3, 6, 7) = (0, 2, 4, 5)

44 2.6 Canonical and Standard Forms (18-23) 44 Standard Forms Canonical Forms: each minterm or maxterm must contain all the variables. Can be obtained from the truth table. Standard Forms: the terms that form the function may contain one, two, or any number of literals (variables). Canonical forms Standard forms Sum of Minterms, Product of Maxterms. Sum of Products, Product of Sums.

45 Canonical and Standard Forms (19-23) Two types of standard forms (2-level) Sum of Products Boolean expression containing AND terms, called product terms with one or more literals each. Sum denotes ORing of these term. E.g. F 1 = y + xy + x yz. Product of Sums Boolean expression containing OR terms, called sum terms with one or more literals each. Product denotes ANDing of these term. E.g. F 2 = x (y + z)(x + y + z )

46 2.6 Canonical and Standard Forms (20-23) Standard Form and Logic Circuit 46 Two level Implementation F 1 = y + xy + x yz

47 2.6 Canonical and Standard Forms (21-23) Standard Form and Logic Circuit 47 Two level Implementation F 2 = x (y + z)(x + y + z )

48 2.6 Canonical and Standard Forms (22-23) NonStandard Form and Logic Circuit 48 Three level Implementation NonStandard Form: F 3 = AB + C(D + E) Can be changed to a standard form by using distributive law

49 Canonical and Standard Forms (23-23) NonStandard Form and Logic Circuit Two level Implementation Standard Form: F 4 = AB + CD + CE A two-level implementation is preferred: produces the least amount of delay through the gates when the signal propagates from the inputs to the output

50 Other Logic Operations (1-3) There are 2 2n functions for n binary variables. For n =2: Where are 16 possible functions. AND and OR operators are two of them: x. y and x + y. Subdivided into Three Categories Two functions that produce a constant 0 or I. Four functions with unary operation: complement and transfer. Ten functions with binary operators that define eight different operations: AND, OR NAND, NOR, exclusive-or, equivalence (XNOR), inhibition, and implication.

51 2.7 Other Logic Operations (2-3) 51 Possible functions for the two n variables = 2 2n

52 2.7 Other Logic Operations (3-3) 52

53 Digital Logic Gates (1-9) Standard Gates Eight gates are: AND. OR. Complement (NOT) (Inverter = Buffer + Bubble) Transfer (Buffer) NAND. NOR. XOR. Equivelance. Each gate has one or two binary input variables, designated by x and y, and one binary output variable designated by F.

54 2.8 Digital Logic Gates (2-9) 54

55 2.8 Digital Logic Gates (3-9) 55

56 Digital Logic Gates (4-9) Mulitple Inputs Inverter and buffere accepts ONLY one variable. AND and OR functions are Communicative and Associative.

57 Digital Logic Gates (5-9) Mulitple Inputs NAND and NOR functions are Communicative but not Associative. Solution Define multiple NOR (or NAND) gate as a complemented OR (or AND) gate (Section 3-6)

58 2.8 Digital Logic Gates (6-9) Mulitple Inputs 58

59 2.8 Digital Logic Gates (7-9) 59

60 Digital Logic Gates (8-9) Mulitple Inputs XOR and equivalence gates are both Communicative and Associative. Uncommon, usually constructed with other types of gates. XOR is an odd function (Section 3.9).

61 2.8 Digital Logic Gates (9-9) Mulitple Inputs 61