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Quine-McCluskey Method

CS341 Digital Logic & Computer Organization

Introduction

• (^) A tabular procedure.
• (^) Algorithm can be programmed.
• (^) Minimizing principles are the same as for the K-maps, i.e., based on logical adjacency property to reduce the Boolean function (x + x’ = 1).
• (^) Compares minterms with all others and combines them if possible.

Step 1: convert minterms to binary form wx’y’z’+wxy’z’+w’xyz’+wxy’z+w’x’yz+wx’y’z+w’x’y’z+w’x’y’z’ 1000 + 1100 + 0110 + 1101 + 0011 + 1001 +0001 + 0000

Step 2: Arrange the binary representations in a table with different number of 1s in each group minterm sub# Group 1: zero 1s 0 0000

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Group 2: one 1 1 0001 8 1000

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Group 3: two 1s 12 1100 6 0110 3 0011 9 1001

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Group 4: three 1s 13 1101

Step 3: Illustration: performing

matches between table entries

minterm sub# 0 x 0000

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1 x 0001 x 000- (0,1) 8 x 1000 x -000 (0,8)

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12 x 1100 PI 00-1 (1,3) PI -00- (0,1;8,9) 6 PI 0110 x -001 (1,9) 3 x 0011 x 1-00 (8,12) 9 x 1001 x 100- (8,9)

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13 x 1101 x 110- (12,13) PI 1-0- (8,9;12,13) x 1-01 (9,13)

Step 4: Construct a prime implicant

table or graph

• (^) List all minterms of the function along the x-axis.
• (^) List all prime implicants or PIs along the y- axis.
• (^) Place an x’s at intersections where a prime implicant matches the minterm in each binary value (treat dashes as don’t- cares). If a prime implicant agrees with a minter in each binary position, the PI covers the minterm.

Step 5: Choose a minimum set of PIs so that each minterm of the function is covered

• (^) In this example, all PIs are essential , because each covers at least a minterm (the x’s that are circled in the graph) that is not covered by other PIs.
• (^) So the simplified function corresponds to 0110 + 00-1 + -00- + 1-0- which corresponds to w’xyz’ + w’x’z + x’y’ + wy’

Covering problem: additional steps

may be necessary

• (^) In the previous example, all PIs are essential and were all included in the final simplified function. In general, the process of selecting a minimum number of PIs to cover a function is not as simple.
• (^) The following slides describe rules that can be used to derive the minimum number of PIs when the PI graph is complex.

Rule 1 may help reduce the

complexity of the PI graph

• (^) Because a row of the PI chart corresponds to a PI, a dominating row covers all the minterns that are covered by a row dominated by it. Hence the follow rule.
• (^) Rule 1: A row dominated by another row can be eliminated from the chart, provided the PI corresponding to the dominated row does not have fewer literals compared with the PI corresponding to the dominating row. When several identical rows are present in a chart, all but the one whose PI has the fewest literals can be eliminated.

Rule 2: may help reduce the

complexity of the PI graph

• (^) When a column dominates another column, the PIs that cover the minterm corresponding to the dominated column also cover the minterm corresponding to the dominating column. Hence:
• (^) Rule 2: A column dominating another column can be eliminated. All but one of the identical columns can be eliminated.

Terminology revisited

• (^) Implicant: a product term of a function if the function has a value of 1 for all minterms of the product term.
• (^) Prime implicant or PI: a product term that corresponds to a valid largest possible subgroup of logically adjacent 1’s. In other words, a PI cannot be contained by another PI.
• (^) Essential PI: a PI that includes a minterm not present in other PIs. It must be included as a term in the final simplified function.

Summary: general procedure for finding a minimum cover for a Boolean function

1. Select all essential PIs. If these PIs covers all the minterms, stop; otherwise, go to the next step.
2. Apply rules 1 and 2 to eliminate redundant rows and columns from the PI graph of non-essential PIs. When the chart is thus reduced, some PIs will become essential. Go back to step 1.

Optional extra point programming

project

• (^) Implement the Quine-McCluskey algorithm with C++ or other programming languages such as Visual Basic.