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COMBINATIONAL LOGIC DESIGN Chapter 2 Digital Design and Computer Architecture, 2nd Edition David Money Harris and Sarah L. Harris Chapter 2 <1>
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COMBINATIONAL LOGIC DESIGN Chapter 2 :: Topics • • • • • • • • • Introduction Boolean Equations Boolean Algebra From Logic to Gates Multilevel Combinational Logic X’s and Z’s, Oh My Karnaugh Maps Combinational Building Blocks Timing Chapter 2 <2>
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COMBINATIONAL LOGIC DESIGN Introduction A logic circuit is composed of: • Inputs • Outputs • Functional specification • Timing specification functional spec inputs outputs timing spec Chapter 2 <3>
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COMBINATIONAL LOGIC DESIGN Circuits • Nodes – Inputs: A, B, C – Outputs: Y, Z – Internal: n1 A • Circuit elements C E1 n1 B E3 E2 – E1, E2, E3 – Each a circuit Chapter 2 <4> Y Z
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Slide #5.

COMBINATIONAL LOGIC DESIGN Types of Logic Circuits • Combinational Logic – Memoryless – Outputs determined by current values of inputs • Sequential Logic – Has memory – Outputs determined by previous and current values of inputs functional spec inputs outputs timing spec Chapter 2 <5>
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COMBINATIONAL LOGIC DESIGN Rules of Combinational Composition • Every element is combinational • Every node is either an input or connects to exactly one output • The circuit contains no cyclic paths • Example: Chapter 2 <6>
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COMBINATIONAL LOGIC DESIGN Boolean Equations • Functional specification of outputs in terms of inputs • Example: S = F(A, B, Cin) Cout = F(A, B, Cin) A B Cin C L S Cout S = A  B  Cin Cout = AB + ACin + BCin Chapter 2 <7>
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Slide #8.

COMBINATIONAL LOGIC DESIGN Some Definitions • Complement: variable with a bar over it A, B, C • Literal: variable or its complement A, A, B, B, C, C • Implicant: product of literals ABC, AC, BC • Minterm: product that includes all input variables ABC, ABC, ABC • Maxterm: sum that includes all input variables (A+B+C), (A+B+C), (A+B+C) Chapter 2 <8>
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Slide #9.

COMBINATIONAL LOGIC DESIGN Sum-of-Products (SOP) Form • • • • • • All equations can be written in SOP form Each row has a minterm A minterm is a product (AND) of literals Each minterm is TRUE for that row (and only that row) Form function by ORing minterms where the output is TRUE Thus, a sum (OR) of products (AND terms) A 0 0 1 1 B 0 1 0 1 Y 0 1 0 1 minterm minterm name A B m0 m1 A B m2 A B m3 A B Y = F(A, B) = Chapter 2 <9>
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Slide #10.

COMBINATIONAL LOGIC DESIGN Sum-of-Products (SOP) Form Sum-of-Products Form • • • • • • All equations can be written in SOP form Each row has a minterm A minterm is a product (AND) of literals Each minterm is TRUE for that row (and only that row) Form function by ORing minterms where the output is TRUE Thus, a sum (OR) of products (AND terms) A 0 0 1 1 B 0 1 0 1 Y 0 1 0 1 minterm minterm name A B m0 m1 A B m2 A B m3 A B Y = F(A, B) = Chapter 2 <10>
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Slide #11.

COMBINATIONAL LOGIC DESIGN Sum-of-Products (SOP) Form Sum-of-Products Form • • • • • • All equations can be written in SOP form Each row has a minterm A minterm is a product (AND) of literals Each minterm is TRUE for that row (and only that row) Form function by ORing minterms where the output is TRUE Thus, a sum (OR) of products (AND terms) A 0 0 1 1 B 0 1 0 1 Y 0 1 0 1 minterm minterm name A B m0 m1 A B m2 A B m3 A B Y = F(A, B) = AB + AB = Σ(1, 3) Chapter 2 <11>
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Slide #12.

COMBINATIONAL LOGIC DESIGN Product-of-Sums (POS) Form All Boolean equations can be written in POS form Each row has a maxterm A maxterm is a sum (OR) of literals Each maxterm is FALSE for that row (and only that row) Form function by ANDing the maxterms for which the output is FALSE • Thus, a product (AND) of sums (OR terms) • • • • • A 0 0 1 1 B 0 1 0 1 Y 0 1 0 1 maxterm maxterm name A A A A + + + + B B B B M0 M1 M2 M3 Y = F(A, B) = (A + B)(A + B) = Π(0, 2) Chapter 2 <12>
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Slide #13.

COMBINATIONAL LOGIC DESIGN Boolean Equations Example • You are going to the cafeteria for lunch – You won’t eat lunch (E) – If it’s not open (O) or – If they only serve corndogs (C) • Write a truth table for determining if you will eat lunch (E). O 0 0 1 1 C 0 1 0 1 E Chapter 2 <13>
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Slide #14.

COMBINATIONAL LOGIC DESIGN Boolean Equations Example • You are going to the cafeteria for lunch – You won’t eat lunch (E) – If it’s not open (O) or – If they only serve corndogs (C) • Write a truth table for determining if you will eat lunch (E). O 0 0 1 1 C 0 1 0 1 E 0 0 1 0 Chapter 2 <14>
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Slide #15.

COMBINATIONAL LOGIC DESIGN SOP & POS Form • SOP – sum-of-products O 0 0 1 1 C 0 1 0 1 E minterm O C O C O C O C • POS – product-of-sums O 0 0 1 1 C 0 1 0 1 E maxterm O O O O + + + + C C C C Chapter 2 <15>
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Slide #16.

COMBINATIONAL LOGIC DESIGN SOP & POS Form • SOP – sum-of-products O 0 0 1 1 C 0 1 0 1 E 0 0 1 0 minterm O C O C O C O C E = OC = Σ(2) • POS – product-of-sums O 0 0 1 1 C 0 1 0 1 E 0 0 1 0 maxterm O O O O + + + + C C C C E = (O + C)(O + C)(O + C) = Π(0, 1, 3) Chapter 2 <16>
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Slide #17.

COMBINATIONAL LOGIC DESIGN Boolean Algebra • Axioms and theorems to simplify Boolean equations • Like regular algebra, but simpler: variables have only two values (1 or 0) • Duality in axioms and theorems: – ANDs and ORs, 0’s and 1’s interchanged Chapter 2 <17>
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Slide #18.

COMBINATIONAL LOGIC DESIGN Boolean Axioms Chapter 2 <18>
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Slide #19.

COMBINATIONAL LOGIC DESIGN T1: Identity Theorem • B 1=B • B+0=B Chapter 2 <19>
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Slide #20.

COMBINATIONAL LOGIC DESIGN T1: Identity Theorem • B 1=B • B+0=B B 1 = B B 0 = B Chapter 2 <20>
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COMBINATIONAL LOGIC DESIGN T2: Null Element Theorem • B 0=0 • B+1=1 Chapter 2 <21>
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COMBINATIONAL LOGIC DESIGN T2: Null Element Theorem • B 0=0 • B+1=1 B 0 = 0 B 1 = 1 Chapter 2 <22>
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COMBINATIONAL LOGIC DESIGN T3: Idempotency Theorem • B B=B • B+B=B Chapter 2 <23>
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COMBINATIONAL LOGIC DESIGN T3: Idempotency Theorem • B B=B • B+B=B B B = B B B = B Chapter 2 <24>
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COMBINATIONAL LOGIC DESIGN T4: Identity Theorem • B=B Chapter 2 <25>
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COMBINATIONAL LOGIC DESIGN T4: Identity Theorem • B=B B = B Chapter 2 <26>
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COMBINATIONAL LOGIC DESIGN T5: Complement Theorem • B B=0 • B+B=1 Chapter 2 <27>
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COMBINATIONAL LOGIC DESIGN T5: Complement Theorem • B B=0 • B+B=1 B B = 0 B B = 1 Chapter 2 <28>
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Slide #29.

COMBINATIONAL LOGIC DESIGN Boolean Theorems Summary Chapter 2 <29>
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COMBINATIONAL LOGIC DESIGN Boolean Theorems of Several Vars ( ) Note: T8’ differs from traditional algebra: OR (+) distributes over AND (•) Chapter 2 <30>
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Slide #31.

COMBINATIONAL LOGIC DESIGN Simplifying Boolean Equations Example 1: Y = AB + AB Chapter 2 <31>
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COMBINATIONAL LOGIC DESIGN Simplifying Boolean Equations Example 1: Y = AB + AB = B(A + A) T8 = B(1) T5’ = B T1 Chapter 2 <32>
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COMBINATIONAL LOGIC DESIGN Simplifying Boolean Equations Example 2: Y = A(AB + ABC) Chapter 2 <33>
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Slide #34.

COMBINATIONAL LOGIC DESIGN Simplifying Boolean Equations Example 2: Y = A(AB + ABC) = A(AB(1 + C)) T8 = A(AB(1)) T2’ = A(AB) T1 = (AA)B T7 = AB T3 Chapter 2 <34>
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Slide #35.

COMBINATIONAL LOGIC DESIGN DeMorgan’s Theorem • Y = AB = A + B • Y=A+B=A B A B Y A B Y A B Y A B Y Chapter 2 <35>
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Slide #36.

COMBINATIONAL LOGIC DESIGN Bubble Pushing • Backward: – Body changes – Adds bubbles to inputs A B Y A B Y Y A B Y • Forward: – Body changes – Adds bubble to output A B Chapter 2 <36>
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Slide #37.

COMBINATIONAL LOGIC DESIGN Bubble Pushing • What is the Boolean expression for this circuit? A B C D Y Chapter 2 <37>
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Slide #38.

COMBINATIONAL LOGIC DESIGN Bubble Pushing • What is the Boolean expression for this circuit? A B Y C D Y = AB + CD Chapter 2 <38>
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Slide #39.

COMBINATIONAL LOGIC DESIGN Bubble Pushing Rules • Begin at output, then work toward inputs • Push bubbles on final output back • Draw gates in a form so bubbles cancel A B C Y D Chapter 2 <39>
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Slide #40.

COMBINATIONAL LOGIC DESIGN Bubble Pushing Example A B C Y D Chapter 2 <40>
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COMBINATIONAL LOGIC DESIGN Bubble Pushing Example A B C no output bubble Y D Chapter 2 <41>
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COMBINATIONAL LOGIC DESIGN Bubble Pushing Example A B no output bubble C Y D A B C bubble on input and output Y D Chapter 2 <42>
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Slide #43.

COMBINATIONAL LOGIC DESIGN Bubble Pushing Example no output bubble A B C Y D A B bubble on input and output C Y D A B no bubble on input and output C Y D Y = ABC + D Chapter 2 <43>
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Slide #44.

COMBINATIONAL LOGIC DESIGN From Logic to Gates • Two-level logic: ANDs followed by ORs • Example: Y = ABC + ABC + ABC A B A C B C minterm: ABC minterm: ABC minterm: ABC Y Chapter 2 <44>
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Slide #45.

COMBINATIONAL LOGIC DESIGN Circuit Schematics Rules • • • • Inputs on the left (or top) Outputs on right (or bottom) Gates flow from left to right Straight wires are best Chapter 2 <45>
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Slide #46.

COMBINATIONAL LOGIC DESIGN Circuit Schematic Rules (cont.) • Wires always connect at a T junction • A dot where wires cross indicates a connection between the wires • Wires crossing without a dot make no connection wires connect at a T junction wires connect at a dot wires crossing without a dot do not connect Chapter 2 <46>
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Slide #47.

COMBINATIONAL LOGIC DESIGN Multiple-Output Circuits • Example: Priority Circuit Output asserted corresponding to most significant TRUE input A3 Y3 A2 Y2 A1 Y1 A0 Y0 PRIORITY CiIRCUIT A3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 A2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 A1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 A0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y3 Chapter 2 <47> Y2 Y1 Y0
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Slide #48.

COMBINATIONAL LOGIC DESIGN Multiple-Output Circuits • Example: Priority Circuit Output asserted corresponding to most significant TRUE input A3 Y3 A2 Y2 A1 Y1 A0 Y0 PRIORITY CiIRCUIT A3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 A2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 A1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 A0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 Chapter 2 <48> Y2 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 Y1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 Y0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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Slide #49.

COMBINATIONAL LOGIC DESIGN Priority Circuit Hardware A3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 A2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 A1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 A0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 Y2 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 Y1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 Y0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A3 A 2 A1 A 0 Y3 Y2 Y1 Y0 Chapter 2 <49>
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Slide #50.

COMBINATIONAL LOGIC DESIGN Don’t Cares A3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 A2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 A1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 A0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 Y2 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 Y1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 Y0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A3 0 0 0 0 1 A2 0 0 0 1 X A1 0 0 1 X X A0 0 1 X X X Chapter 2 <50> Y3 0 0 0 0 1 Y2 0 0 0 1 0 Y1 0 0 1 0 0 Y0 0 1 0 0 0
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Slide #51.

COMBINATIONAL LOGIC DESIGN Contention: X • Contention: circuit tries to drive output to 1 and 0 – – – – Actual value somewhere in between Could be 0, 1, or in forbidden zone Might change with voltage, temperature, time, noise Often causes excessive power dissipation A=1 Y=X • Warnings: B=0 – Contention usually indicates a bug. – X is used for “don’t care” and contention - look at the context to tell them apart Chapter 2 <51>
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Slide #52.

COMBINATIONAL LOGIC DESIGN Floating: Z • Floating, high impedance, open, high Z • Floating output might be 0, 1, or somewhere in between – A voltmeter won’t indicate whether a node is floating Tristate Buffer E Y A E 0 0 1 1 A 0 1 0 1 Y Z Z 0 1 Chapter 2 <52>
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Slide #53.

• Floating nodes are used in tristate busses – Many different drivers – Exactly one is active at once processor en1 to bus frombus video en2 to bus frombus Ethernet en3 to bus frombus memory en4 to bus frombus Chapter 2 <53> sharedbus COMBINATIONAL LOGIC DESIGN Tristate Busses
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Slide #54.

COMBINATIONAL LOGIC DESIGN Karnaugh Maps (K-Maps) • Boolean expressions can be minimized by combining terms • K-maps minimize equations graphically • PA + PA = P A 0 0 0 0 1 1 1 1 B 0 0 1 1 0 0 1 1 C 0 1 0 1 0 1 0 1 Y 1 1 0 0 0 0 0 0 Y Y AB 00 01 11 10 0 1 0 0 0 1 1 0 0 0 C AB C 00 01 11 10 0 ABC ABC ABC ABC 1 ABC ABC ABC ABC Chapter 2 <54>
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Slide #55.

COMBINATIONAL LOGIC DESIGN K-Map • Circle 1’s in adjacent squares • In Boolean expression, include only literals whose true and complement form are not in the circle A 0 0 0 0 1 1 1 1 B 0 0 1 1 0 0 1 1 C 0 1 0 1 0 1 0 1 Y 1 1 0 0 0 0 0 0 Y AB 00 01 11 10 0 1 0 0 0 1 1 0 0 0 C Y = AB Chapter 2 <55>
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Slide #56.

COMBINATIONAL LOGIC DESIGN 3-Input K-Map Y AB 00 01 11 10 0 ABC ABC ABC ABC 1 ABC ABC ABC ABC C Truth Table A 0 0 0 0 1 1 1 1 B 0 0 1 1 0 0 1 1 C 0 1 0 1 0 1 0 1 K-Map Y 0 0 1 1 0 0 0 1 Y AB C 00 01 11 0 1 Chapter 2 <56> 10
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Slide #57.

COMBINATIONAL LOGIC DESIGN 3-Input K-Map Y AB 00 01 11 10 0 ABC ABC ABC ABC 1 ABC ABC ABC ABC C Truth Table A 0 0 0 0 1 1 1 1 B 0 0 1 1 0 0 1 1 C 0 1 0 1 0 1 0 1 K-Map Y 0 0 1 1 0 0 0 1 Y AB C 00 01 11 10 0 0 1 1 0 1 0 1 0 0 Y = AB + BC Chapter 2 <57>
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Slide #58.

COMBINATIONAL LOGIC DESIGN K-Map Definitions • Complement: variable with a bar over it A, B, C • Literal: variable or its complement A, A, B, B, C, C • Implicant: product of literals ABC, AC, BC • Prime implicant: implicant corresponding to the largest circle in a K-map Chapter 2 <58>
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Slide #59.

COMBINATIONAL LOGIC DESIGN K-Map Rules • Every 1 must be circled at least once • Each circle must span a power of 2 (i.e. 1, 2, 4) squares in each direction • Each circle must be as large as possible • A circle may wrap around the edges • A “don't care” (X) is circled only if it helps minimize the equation Chapter 2 <59>
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Slide #60.

COMBINATIONAL LOGIC DESIGN 4-Input K-Map A 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 B 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 C 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y 1 0 1 1 0 1 1 1 1 1 1 0 0 0 0 0 Y CD AB 00 01 11 00 01 11 10 Chapter 2 <60> 10
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Slide #61.

COMBINATIONAL LOGIC DESIGN 4-Input K-Map A 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 B 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 C 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y 1 0 1 1 0 1 1 1 1 1 1 0 0 0 0 0 Y AB 00 01 11 10 00 1 0 0 1 01 0 1 0 1 11 1 1 0 0 10 1 1 0 1 CD Chapter 2 <61>
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Slide #62.

COMBINATIONAL LOGIC DESIGN 4-Input K-Map A 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 B 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 C 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y 1 0 1 1 0 1 1 1 1 1 1 0 0 0 0 0 Y AB 00 01 11 10 00 1 0 0 1 01 0 1 0 1 11 1 1 0 0 10 1 1 0 1 CD Y = AC + ABD + ABC + BD Chapter 2 <62>
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Slide #63.

COMBINATIONAL LOGIC DESIGN K-Maps with Don’t Cares A 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 B 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 C 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y 1 0 1 1 0 X 1 1 1 1 X X X X X X Y CD AB 00 01 00 01 11 10 Chapter 2 <63> 11 10
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Slide #64.

COMBINATIONAL LOGIC DESIGN K-Maps with Don’t Cares A 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 B 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 C 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y 1 0 1 1 0 X 1 1 1 1 X X X X X X Y AB 00 01 11 10 00 1 0 X 1 01 0 X X 1 11 1 1 X X 10 1 1 X X CD Chapter 2 <64>
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Slide #65.

COMBINATIONAL LOGIC DESIGN K-Maps with Don’t Cares A 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 B 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 C 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Y 1 0 1 1 0 X 1 1 1 1 X X X X X X Y AB 00 01 11 10 00 1 0 X 1 01 0 X X 1 11 1 1 X X 10 1 1 X X CD Y = A + BD + C Chapter 2 <65>
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Slide #66.

COMBINATIONAL LOGIC DESIGN Combinational Building Blocks • Multiplexers • Decoders Chapter 2 <66>
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Slide #67.

COMBINATIONAL LOGIC DESIGN Multiplexer (Mux) • Selects between one of N inputs to connect to output • log2N-bit select input – control input • Example: 2:1 Mux S S 0 0 0 0 1 1 1 1 D1 0 0 1 1 0 0 1 1 D0 0 D1 1 D0 0 1 0 1 0 1 0 1 Y 0 1 0 1 0 0 1 1 Y S 0 1 Y D0 D1 Chapter 2 <67>
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Slide #68.

COMBINATIONAL LOGIC DESIGN Multiplexer Implementations • Logic gates • Tristates – Sum-of-products form Y D0 D1 00 S 01 11 10 0 0 0 1 1 1 0 1 1 0 Y = D0S + D1S D0 – For an N-input mux, use N tristates – Turn on exactly one to select the appropriate input S D0 Y D1 S D1 Y 2-<68> Chapter 2 <68>
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Slide #69.

COMBINATIONAL LOGIC DESIGN Logic using Multiplexers • Using the mux as a lookup table A 0 0 1 1 B 0 1 0 1 Y 0 0 0 1 Y = AB AB 00 01 10 Y 11 Chapter 2 <69>
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Slide #70.

COMBINATIONAL LOGIC DESIGN Logic using Multiplexers • Reducing the size of the mux Y = AB A 0 0 1 1 B 0 1 0 1 Y 0 0 0 1 A Y 0 0 1 B A 0 B Chapter 2 <70> 1 Y
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Slide #71.

COMBINATIONAL LOGIC DESIGN Decoders • N inputs, 2N outputs • One-hot outputs: only one output HIGH at 2:4 once Decoder A1 A0 A1 0 0 1 1 A0 0 1 0 1 Y3 0 0 0 1 11 10 01 00 Y3 Y2 Y1 Y0 Y2 0 0 1 0 Y1 0 1 0 0 Y0 1 0 0 0 Chapter 2 <71>
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Slide #72.

COMBINATIONAL LOGIC DESIGN Decoder Implementation A1 A0 Y3 Y2 Y1 Y0 Chapter 2 <72>
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Slide #73.

COMBINATIONAL LOGIC DESIGN Logic Using Decoders • OR minterms A B 2:4 Decoder 11 10 01 00 Y = AB + AB = A  B Minterm AB AB AB AB Y Chapter 2 <73>
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Slide #74.

COMBINATIONAL LOGIC DESIGN Timing • Delay between input change and output changing • How to build fast circuits? A Y delay A Y Time Chapter 2 <74>
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Slide #75.

COMBINATIONAL LOGIC DESIGN Propagation & Contamination Delay • Propagation delay: tpd = max delay from input to output • Contamination delay: tcd = min delay from input to output A Y tpd A Y tcd Time Chapter 2 <75>
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Slide #76.

COMBINATIONAL LOGIC DESIGN Propagation & Contamination Delay • Delay is caused by – Capacitance and resistance in a circuit – Speed of light limitation • Reasons why tpd and tcd may be different: – Different rising and falling delays – Multiple inputs and outputs, some of which are faster than others – Circuits slow down when hot and speed up when cold Chapter 2 <76>
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Slide #77.

COMBINATIONAL LOGIC DESIGN Critical (Long) & Short Paths Critical Path A B n1 n2 C Y D Short Path Critical (Long) Path: tpd = 2tpd_AND + tpd_OR Short Path: tcd = tcd_AND Chapter 2 <77>
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Slide #78.

COMBINATIONAL LOGIC DESIGN Glitches • When a single input change causes an output to change multiple times Chapter 2 <78>
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Slide #79.

COMBINATIONAL LOGIC DESIGN Glitch Example • What happens when A = 0, C = 1, B falls? A B Y C Y AB 00 01 11 10 0 1 0 0 0 1 1 1 1 0 C Y = AB + BC Chapter 2 <79>
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Slide #80.

COMBINATIONAL LOGIC DESIGN Glitch Example (cont.) A=0 B=1 0 0 Critical Path 1 n1 Y=1 0 n2 C=1 1 0 Short Path B n2 n1 Y glitch Time Chapter 2 <80> 1
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Slide #81.

COMBINATIONAL LOGIC DESIGN Fixing the Glitch Y AB 00 01 11 10 0 1 0 0 0 1 1 1 1 0 C AC Y = AB + BC + AC A=0 B=1 0 Y=1 C=1 Chapter 2 <81>
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Slide #82.

COMBINATIONAL LOGIC DESIGN Why Understand Glitches? • Glitches don’t cause problems because of synchronous design conventions (see Chapter 3) • It’s important to recognize a glitch: in simulations or on oscilloscope • Can’t get rid of all glitches – simultaneous transitions on multiple inputs can also cause glitches Chapter 2 <82>
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