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Digital Logic Design Lecture 13

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Announcements HW5 up on course webpage. Due on Tuesday, 10/21 in class. Upcoming: Exam on October 28. Will cover material from Chapter 4. Details to follow soon.

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Agenda Last time – Using 3,4 variable K-Maps to find minimal expressions (4.5) This time – Minimal expressions for incomplete Boolean functions (4.6) – 5 and 6 variable K-Maps (4.7) – Petrick’s method of determining irredundant expressions (4.9)

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Minimal Expressions of Incomplete Boolean Functions Recall an incomplete Boolean function has a truth table which contains dashed functional entries indicating don’t-care conditions. Idea: Can replace don’t-care entries with either 0s or 1s in order to form the largest possible subcubes.

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Example 0132 4576 12131514 891110 00011110 00 01 11 10

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Example 1101 01-- 0010 100 00011110 00 01 11 10

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Example Step 1: Find prime implicants (pretend don’t care cells set to 1) 1101 01-- 0010 100 00011110 00 01 11 10

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Example Step 2: Find essential prime implicants (discount don’t care cells) 1101 01-- 0010 100 00011110 00 01 11 10

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Example Step 3: Add prime implicants to cover all 1-cells (discount don’t care cells) 1101 01-- 0010 100 00011110 00 01 11 10

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Example Step 3: Add prime implicants to cover all 1-cells (discount don’t care cells) 1101 01-- 0010 100 00011110 00 01 11 10

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Five and Six Variable K-Maps

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Five Variable K-Maps We can visualize five-variable map in two different ways:

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Five Variable K-Maps 0132 891110 24252726 16171918 000001011010 00 01 11 10 6754 14151312 30312928 22232120 110111101100

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Five Variable K-Maps 0132 4576 12131514 891110 16171918 20212322 28293130 24252726

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Example 0100 0100 0100 0110 0000 1100 1111 0011

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Step 1: Find all Prime Implicants. 0100 0100 0100 0110 0000 1100 1111 0011

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Example Step 2: Find all Essential Prime Implicants. 0100 0100 0100 0110 0000 1100 1111 0011 00 01 11 10

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Example Step 2: Find all Essential Prime Implicants. 0100 0100 0100 0110 0000 1100 1111 0011

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Example Step 2: Find all Essential Prime Implicants. 0100 0100 0100 0110 0000 1100 1111 0011

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Six Variable K-Maps We can visualize a six-variable map in two different ways:

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Six Variable K-Maps 0132 891110 24252726 16171918 6754 14151312 30312928 22232120 48495150 56575958 40414342 32333534 54555352 62636160 46474544 38393736 000001011010 000 001 011 010 110111101100 110 111 101 100

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Six Variable K-Maps 0132 4576 12131514 891110 16171918 20212322 28293130 24252726 48495150 52535554 60616362 56575958 32333534 36373938 44454746 40414342 Subcubes: Subcubes occurring in corresponding positions on all four layers collectively form a single subcube.

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An Algorithm for the Final Step in Expression Minimization

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Petrick’s Method of Determining Irredundant Expressions AX BXXX CXXX DXXX EXX FX GXX HXX IXX The covering problem: Determine a subset of prime implicants that covers the table. A minimal cover is an irredundant cover that corresponds to a minimal sum of the function.

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Petrick’s Method of Determining Irredundant Expressions AX BXXX CXXX DXXX EXX FX GXX HXX IXX p-expression: (G+H)(F+G)(A+B)(B+C)(H+I)(D+I)(C+D)(B+C+E)(D+E) The p-expression equals 1 iff a sufficient subset of prime implicants is selected.

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P-expressions If a p-expression is manipulated into its sum-of- products form using the distributive law, duplicate literals deleted in each resulting product term and subsuming product temrms deleted, then each remaining product term represents an irredundant cover of the prime implicant table. Since all subsuming product terms have been deleted, the resulting product terms must each describe an irredundant cover. The irredundant DNF is obtained by summing the prime implicants indicated by the variables in a product term.

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Simplifying p-expressions

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Finding Minimal Sums

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