Charles Kime & Thomas Kaminski © 2008 Pearson Education, Inc. (Hyperlinks are active in View Show mode) Chapter 2 – Combinational Logic Circuits Part 2 – Circuit Optimization Logic and Computer Design Fundamentals

Chapter 2 - Part 2 2 Overview Part 1 – Gate Circuits and Boolean Equations Binary Logic and Gates Boolean Algebra Standard Forms Part 2 – Circuit Optimization Two-Level Optimization Map Manipulation Practical Optimization (Espresso) Multi-Level Circuit Optimization Part 3 – Additional Gates and Circuits Other Gate Types Exclusive-OR Operator and Gates High-Impedance Outputs

Chapter 2 - Part 2 3 Circuit Optimization Goal: To obtain the simplest implementation for a given function Optimization is a more formal approach to simplification that is performed using a specific procedure or algorithm Optimization requires a cost criterion to measure the simplicity of a circuit Distinct cost criteria we will use: Literal cost (L) Gate input cost (G) Gate input cost with NOTs (GN)

Chapter 2 - Part 2 4 D Literal – a variable or its complement Literal cost – the number of literal appearances in a Boolean expression corresponding to the logic circuit diagram Examples: F = BD + A C + A L = 8 F = BD + A C + A + AB L = F = (A + B)(A + D)(B + C + )( + + D) L = Which solution is best? Literal Cost D B C B B DC B C

Chapter 2 - Part 2 5 Gate Input Cost Gate input costs - the number of inputs to the gates in the implementation corresponding exactly to the given equation or equations. (G - inverters not counted, GN - inverters counted) For SOP and POS equations, it can be found from the equation(s) by finding the sum of: all literal appearances the number of terms excluding single literal terms,(G) and optionally, the number of distinct complemented single literals (GN). Example: F = BD + A C + A G = 11, GN = 14 F = BD + A C + A + AB G =, GN = F = (A + )(A + D)(B + C + )( + + D) G =, GN = Which solution is best? D B C B B DC B DB C

Chapter 2 - Part 2 6 Example 1: F = A + B C + Cost Criteria (continued) A B C F B C L = 5 L (literal count) counts the AND inputs and the single literal OR input. G = L + 2 = 7 G (gate input count) adds the remaining OR gate inputs GN = G + 2 = 9 GN(gate input count with NOTs) adds the inverter inputs

Chapter 2 - Part 2 7 Example 2: F = A B C + L = 6 G = 8 GN = 11 F = (A + )( + C)( + B) L = 6 G = 9 GN = 12 Same function and same literal cost But first circuit has better gate input count and better gate input count with NOTs Select it! Cost Criteria (continued) B C A A B C F C B F A B C A

Chapter 2 - Part 2 8 Boolean Function Optimization Minimizing the gate input (or literal) cost of a (a set of) Boolean equation(s) reduces circuit cost. We choose gate input cost. Boolean Algebra and graphical techniques are tools to minimize cost criteria values. Some important questions: When do we stop trying to reduce the cost? Do we know when we have a minimum cost? Treat optimum or near-optimum cost functions for two-level (SOP and POS) circuits first. Introduce a graphical technique using Karnaugh maps (K-maps, for short)

Chapter 2 - Part 2 9 Karnaugh Maps (K-map) A K-map is a collection of squares Each square represents a minterm The collection of squares is a graphical representation of a Boolean function Adjacent squares differ in the value of one variable Alternative algebraic expressions for the same function are derived by recognizing patterns of squares The K-map can be viewed as A reorganized version of the truth table A topologically-warped Venn diagram as used to visualize sets in algebra of sets

Chapter 2 - Part 2 10 Some Uses of K-Maps Provide a means for: Finding optimum or near optimum SOP and POS standard forms, and two-level AND/OR and OR/AND circuit implementations for functions with small numbers of variables Visualizing concepts related to manipulating Boolean expressions, and Demonstrating concepts used by computer- aided design programs to simplify large circuits

Chapter 2 - Part 2 11 Two Variable Maps A 2-variable Karnaugh Map: Note that minterm m0 and minterm m1 are adjacent and differ in the value of the variable y Similarly, minterm m0 and minterm m2 differ in the x variable. Also, m1 and m3 differ in the x variable as well. Finally, m2 and m3 differ in the value of the variable y y = 0 y = 1 x = 0 m 0 = m 1 = x = 1 m 2 = m 3 = yx yx yx yx

Chapter 2 - Part 2 12 K-Map and Truth Tables The K-Map is just a different form of the truth table. Example – Two variable function: We choose a,b,c and d from the set {0,1} to implement a particular function, F(x,y). Function Table K-Map y = 0 y = 1 x = 0 a b x = 1 c d

Chapter 2 - Part 2 13 K-Map Function Representation Example: F(x,y) = x For function F(x,y), the two adjacent cells containing 1s can be combined using the Minimization Theorem: F = x y = 0 y = 1 x = x = xyxyx)y,x(F

Chapter 2 - Part 2 14 K-Map Function Representation Example: G(x,y) = x + y For G(x,y), two pairs of adjacent cells containing 1s can be combined using the Minimization Theorem: G = x+y y = 0 y = 1 x = x = yxyxxyyxyx)y,x(G Duplicate x y

Chapter 2 - Part 2 15 Three Variable Maps A three-variable K-map: Where each minterm corresponds to the product terms: Note that if the binary value for an index differs in one bit position, the minterms are adjacent on the K-Map yz=00 yz=01 yz=11 yz=10 x=0 m0m0 m1m1 m3m3 m2m2 x=1 m4m4 m5m5 m7m7 m6m6 yz=00 yz=01 yz=11 yz=10 x=0 x=1 zyx zyx zyxzyx zyx zyx zyx zyx

Chapter 2 - Part 2 16 Alternative Map Labeling Map use largely involves: Entering values into the map, and Reading off product terms from the map. Alternate labelings are useful: y z x x y z z y y z z x x

Chapter 2 - Part 2 17 Example Functions By convention, we represent the minterms of F by a "1" in the map and leave the minterms of blank Example: Learn the locations of the 8 indices based on the variable order shown (x, most significant and z, least significant) on the map boundaries y x z x y z F

Chapter 2 - Part 2 18 Combining Squares By combining squares, we reduce number of literals in a product term, reducing the literal cost, thereby reducing the other two cost criteria On a 3-variable K-Map: One square represents a minterm with three variables Two adjacent squares represent a product term with two variables Four adjacent terms represent a product term with one variable Eight adjacent terms is the function of all ones (no variables) = 1.

Chapter 2 - Part 2 19 Example: Combining Squares Example: Let Applying the Minimization Theorem three times: Thus the four terms that form a 2 × 2 square correspond to the term "y". y zyyz zyxzyxzyxzyx)z,y,x(F x y z

Chapter 2 - Part 2 20 Three-Variable Maps Reduced literal product terms for SOP standard forms correspond to rectangles on K-maps containing cell counts that are powers of 2. Rectangles of 2 cells represent 2 adjacent minterms; of 4 cells represent 4 minterms that form a pairwise adjacent ring. Rectangles can contain non-adjacent cells as illustrated by the pairwise adjacent ring above.

Chapter 2 - Part 2 21 Three-Variable Maps Topological warps of 3-variable K-maps that show all adjacencies: Venn Diagram Cylinder YZ X

Chapter 2 - Part 2 22 Three-Variable Maps Example Shapes of 2-cell Rectangles: Read off the product terms for the rectangles shown y x z

Chapter 2 - Part 2 23 Three-Variable Maps Example Shapes of 4-cell Rectangles: Read off the product terms for the rectangles shown y x z

Chapter 2 - Part 2 24 Three Variable Maps z)y,F(x, y 11 x z 11 1 z z yx yx K-Maps can be used to simplify Boolean functions by systematic methods. Terms are selected to cover the 1sin the map. Example: Simplify

Chapter 2 - Part 2 25 Three-Variable Map Simplification Use a K-map to find an optimum SOP equation for

Chapter 2 - Part 2 26 Four Variable Maps Map and location of minterms: X Y Z W Variable Order

Chapter 2 - Part 2 27 Four Variable Terms Four variable maps can have rectangles corresponding to: A single 1 = 4 variables, (i.e. Minterm) Two 1s = 3 variables, Four 1s = 2 variables Eight 1s = 1 variable, Sixteen 1s = zero variables (i.e. Constant "1")

Chapter 2 - Part 2 28 Four-Variable Maps Example Shapes of Rectangles: X Y Z W

Chapter 2 - Part 2 29 Four-Variable Maps Example Shapes of Rectangles: X Y Z W

Chapter 2 - Part 2 30 Four-Variable Map Simplification )8,10,13,152,4,5,6,7, (0, Z)Y,X,F(W, m

Chapter 2 - Part ,14,15 Four-Variable Map Simplification )(3,4,5,7,9,1 Z)Y,X,F(W, m

Chapter 2 - Part 2 32 Systematic 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.

Chapter 2 - Part 2 33 DB CB B D A Example of Prime Implicants Find ALL Prime Implicants ESSENTIAL Prime Implicants C BD CD BD Minterms covered by single prime implicant DB B C D A AD BA

Chapter 2 - Part 2 34 Prime Implicant Practice Find all prime implicants for:

Chapter 2 - Part 2 35 Another Example Find all prime implicants for: Hint: There are seven prime implicants!

Chapter 2 - Part 2 36 Sometimes 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 dont cares. Don't Cares in K-Maps

Chapter 2 - Part 2 37 Example 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 combinations of A, B, and C such A = 1 and B = 1 or C = 0, otherwise ignoring it. Thus, Z is specified only for those combinations, and for all other combinations of A, B, and C, Z is a dont care. Specifically, Z must be specified for AB + C = 1, and is a dont care for : AB + C = (A + B)C = AC + BC = 1 Ultimately, each dont care x entry may take on either a 0 or 1 value in resulting solutions For example, an x may take on value 0 in an SOP solution and value 1 in a POS solution, or vice-versa. Any minterm with value x need not be covered by a prime implicant. Don't Cares in K-Maps

Chapter 2 - Part 2 38 Example: BCD 5 or More The map below gives a function F1(w,x,y,z) which is defined as "5 or more" over BCD inputs. With the 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." G = 12 For this particular function, cost G for the POS solution for F 1 (w,x,y,z) is not changed by using the don't cares. z w XXX XX X x y y x w yx w z x w z) y,x,F 2 (w,

Chapter 2 - Part 2 39 Product of Sums Example Find the optimum POS solution: Hint: Use and complement it to get the result. F

Chapter 2 - Part 2 40 Optimization 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 - More on Optimization Obtaining a good simplified solution: Use the Selection Rule

Chapter 2 - Part 2 41 Prime 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.

Chapter 2 - Part 2 42 Selection Rule Example Simplify F(A, B, C, D) given on the K- map B D A C B D A C 1 1 Essential Minterms covered by essential prime implicants Selected

Chapter 2 - Part 2 43 Selection 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 x x x x x 1 B D A C 1 1 Essential

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