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Chapter 2 - Part 1 - PPT - Mano & Kime - 2nd EdSlide 1 Lecture 8 Systematic SimplificationLecture 8 Systematic Simplification
Slide 2 OutlineOutline Four-Variable Karnaugh Maps Systematic Simplification Prime Implicants Essential Prime Implicants Don t Care Entries Prime Implicant Selection
Slide 3 Four Variable MapsFour Variable Maps Map and location of minterms: 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 X Y Z W
Slide 4 Four Variable MapsFour Variable Maps Map and location of minterms: 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 X Y Z W
Slide 5 Four Variable TermsFour Variable Terms Four variable maps can have rectangles corresponding to: A single 1 = 4 variables (e.g., WXYZ) A single one corresponds to a minterm Two 1s = 3 variables (e.g., X YZ ) Four 1s = 2 variables (e.g., XZ) Eight 1s = 1 variable (e.g., W ) Sixteen 1s = zero variables (i.e., Constant "1")
Slide 6 Four-Variable MapsFour-Variable Maps Example Shapes of 2-Cell Rectangles: WYZ W X Y W XZ 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 X Y Z W
Slide 7 Four-Variable MapsFour-Variable Maps Example Shapes of 4-Cell Rectangles: 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 X Y Z W X Z XZ W Y
Slide 8 Four-Variable MapsFour-Variable Maps Example Shapes of 8-Cell Rectangles: X Y Z 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 W W Z X
Slide 9 Four-Variable Map SimplificationFour-Variable Map Simplification ) 8,10,13,15 2,4,5,6,7, (0, Z) Y, X, F(W, m S = X Y 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 W Z 1 1 1 1 1 1 1 1 1 1 XZ W X X Z F(W,X,Y,Z) = XZ + X Z + W X
F = XZ + X'Z'
Slide 10 Four-Variable Map Simplification3,14,15 Four-Variable Map Simplification ) (3,4,5,7,9,1 Z) Y, X, F(W, m S = X Y 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 W Z 1 1 1 1 1 1 1 1 XZ WXY F(W,X,Y,Z) = XZ + W XY + WXY + WY Z + W YZ W XY W YZ WY Z
F = W' X Y' + W' Y Z + WXY + WY'Z
Slide 11 Systematic SimplificationSystematic Simplification A Prime Implicant is a product term obtained by combining the maximum possible number of adjacent squares in the map into a rectangle with the number of squares a power of 2. A prime implicant is called an Essential Prime Implicant if it is the only prime implicant that covers (includes) one or more minterms. Prime Implicants and Essential Prime Implicants can be determined by inspection of a K-Map. A set of prime implicants "covers all minterms" if, for each minterm of the function, at least one prime implicant in the set of prime implicants includes the minterm.
Slide 12 Example of Prime ImplicantsD B C B 1 1 1 1 1 1 B D A 1 1 1 1 1 Example of Prime Implicants Find ALL Prime Implicants C BD CD BD Minterms covered by single prime implicant D B ESSENTIAL Prime Implicants 1 1 1 1 1 1 B C D A 1 1 1 1 1 AD B A
Slide 13 Prime Implicant PracticePrime Implicant Practice Find all prime implicants for: B C 8 9 10 11 12 13 14 15 0 1 3 2 5 6 4 7 A D 1 1 1 1 1 1 1 1 1 1 1 B D A B C Prime implicants are: A, B'C, and B'D All prime implicants are essential
Prime implicants are: A, B'C, and B'D'
Slide 14 Five Variable or More K-MapsFive Variable or More K-Maps For five variable problems, we use two adjacent K-maps. It becomes harder to visualize adjacent minterms for selecting PIs. You can extend the problem to six variables by using four K-Maps. X Y Z W V = 0 X Z W V = 1 Y V XZ VW WX
Slide 15 Don't Cares in K-MapsSometimes a function table or map contains entries for which it is known: The input values for the minterm will never occur, or The output value for the minterm is not used In these cases, the output value need not be defined Instead, the output value is defined as a don't care By placing don't cares ( an x entry) in the function table or map, the cost of the logic circuit may be lowered. Example 1: A logic function having the binary codes for the BCD digits as its inputs. Only the codes for 0 through 9 are used. The six codes, 1010 through 1111 never occur, so the output values for these codes are x to represent don t cares. Don't Cares in K-Maps
Slide 16 Don't Cares in K-MapsExample 2: A circuit that represents a very common situation that occurs in computer design has two distinct sets of input variables: A, B, and C which take on all possible combinations, and Y which takes on values 0 or 1. and a single output Z. The circuit that receives the output Z observes it only for (A,B,C) = (1,1,1) and otherwise ignores it. Thus, Z is specified only for the combinations (A,B,C,Y) = 1110 and 1111. For these two combinations, Z = Y. For all of the 14 remaining input combinations, Z is a don t care. Ultimately, each x entry may take on either a 0 or 1 value in resulting solutions Any minterm with value x need not be covered by a prime implicant. Don't Cares in K-Maps
Slide 17 Example: BCD 5 or MoreExample: BCD 5 or More Determine a function F1(w,x,y,z) which is defined as "5 or more" over BCD inputs, with don't cares used for the 6 non-BCD combinations: F1 (w,x,y,z) = w + x z + x y G = 7 This is much lower in cost than F2 where the don't cares were treated as "0s." z w 0 1 3 2 4 5 7 6 12 13 15 14 8 9 11 10 1 1 1 1 1 X X X X X X 0 0 0 0 0 x y y x w y x w z x w z) y, x, F2(w, + + = xy xz w G = 12
Slide 18 Optimization AlgorithmOptimization Algorithm Find all prime implicants. Include all essential prime implicants in the solution Select a minimum cost set of non-essential prime implicants to cover all minterms not yet covered: Obtaining an optimum solution: See Reading Supplement 1 - More on Optimization Obtaining a good simplified solution: Use the Selection Rule
Slide 19 Prime Implicant Selection RulePrime Implicant Selection Rule Minimize the overlap among prime implicants as much as possible. In particular, in the final solution, make sure that each prime implicant selected includes at least one minterm not included in any other prime implicant selected.
Slide 20 Selection Rule ExampleSelection Rule Example Simplify F(A, B, C, D) given on the K-map. 1 1 1 1 1 1 1 B D A C 1 1 1 1 1 1 1 1 1 B D A C 1 1 Essential Minterms covered by essential prime implicants Selected F(A, B, C, D) = A B + A CD + AC D + B C D
Slide 21 Selection Rule Example with Don't CaresSelection Rule Example with Don't Cares Simplify F(A, B, C, D) given on the K-map. Selected Minterms covered by essential prime implicants 1 1 x x x x x 1 B D A C 1 1 1 1 x x x x x 1 B D A C 1 1 Essential F(A, B, C, D) = A B + AB D + B C
Slide 22 SummarySummary Four-Variable Karnaugh Maps Systematic Simplification Prime Implicants Essential Prime Implicants Don t Care Entries Prime Implicant Selection
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