CSE 370 – Winter 2002 - Combinational Implementation - 1 Overview Last lecture Examples of two-level combinational logic designs Implicants, prime implicants, Essential prime implicants Today Two-level logic using NANDs and NORs Multilevel logic 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 1

Combinational logic implementation Two-level logic implementations of two-level logic NAND/NOR Multi-level logic factored forms and-or-invert gates Time behavior gate delays hazards Regular logic multiplexors decoders PAL/PLAs ROMs 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 2

Implementations of two-level logic Sum-of-products AND gates to form product terms (minterms) OR gate to form sum Product-of-sums OR gates to form sum terms (maxterms) AND gates to form product F = AB’C’ + A’B + BC G = (A+B)(B+C’)(A’+B’+C) 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 3

Two-level logic using NAND gates Step 1: Replace minterm AND gates with NAND gates and place compensating inversion at inputs of OR gate 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 4

Two-level logic using NAND gates (cont’d) Step 2: OR gate with inverted inputs is a NAND gate de Morgan's: A' + B' = (A • B)' Two-level NAND-NAND network inverted inputs are not counted in a typical circuit, inversion is done once and signal distributed 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 5

Two-level logic using NOR gates Step 1: Replace maxterm OR gates with NOR gates and place compensating inversion at inputs of AND gate 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 6

Two-level logic using NOR gates (cont’d) Step 2: AND gate with inverted inputs is a NOR gate de Morgan's: A' • B' = (A + B)' Two-level NOR-NOR network inverted inputs are not counted in a typical circuit, inversion is done once and signal distributed 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 7

Two-level logic using NAND and NOR gates NAND-NAND and NOR-NOR networks de Morgan's law: (A + B)' = A' • B' (A • B)' = A' + B' written differently: A + B = (A' • B')' (A • B) = (A' + B')' In other words –– OR is the same as NAND with complemented inputs AND is the same as NOR with complemented inputs NAND is the same as OR with complemented inputs NOR is the same as AND with complemented inputs OR OR AND AND NAND NAND NOR NOR 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 8

Conversion between forms Convert from networks of ANDs and ORs to networks of NANDs and NORs introduce appropriate inversions ("bubbles") Each introduced "bubble" must be matched by a corresponding "bubble" conservation of inversions do not alter logic function Example: AND/OR to NAND/NAND A B C D Z A B C D NAND OR with complemented inputs is the same as NAND 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 9

Conversion between forms (cont’d) Example: verify equivalence of two forms A B C D Z A B C D Z NAND Z = [ (A • B)' • (C • D)' ]' = [ (A' + B') • (C' + D') ]' = [ (A' + B')' + (C' + D')' ] = (A • B) + (C • D)  12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 10

Conversion between forms (cont’d) Example: map AND/OR network to NOR/NOR network A B C D Z NOR A B C D Z NOR \A \B \C \D Z Step 1 Step 2 conserve "bubbles" conserve "bubbles" 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 11

Conversion between forms (cont’d) Example: verify equivalence of two forms NOR \A \B \C \D Z A B C D Z Z = { [ (A' + B')' + (C' + D')' ]' }’ /* ((X’ + Y’ )’)’ */ = { (A' + B') • (C' + D') }’ /* (X • Y)’*/ = (A' + B')' + (C' + D')’ /* X’ + Y’ */ = (A • B) + (C • D)  12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 12

CSE 370 – Winter 2002 - Combinational Implementation - 13 Multi-level logic x = A D F + A E F + B D F + B E F + C D F + C E F + G reduced sum-of-products form – already simplified 6 x 3-input AND gates + 1 x 7-input OR gate (that may not even exist!) 25 wires (19 literals plus 6 internal wires) x = (A + B + C) (D + E) F + G factored form – not written as two-level S-o-P 1 x 3-input OR gate, 2 x 2-input OR gates, 1 x 3-input AND gate 10 wires (7 literals plus 3 internal wires) A B C D E F G X 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 13

Choosing the “best” realization Two-level logic: usually has minimal delay Usually means faster circuits But with more gates (more transistors) and more wires (larger circuit area); hence requires more power. Easy to eliminate static hazards (see later) Sometimes requires large fan-ins Multilevel logic usually requires less gates Usually means smaller circuits Less gates, less wires, shorter wires Harder to eliminate hazards 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 14

Issues with multilevel design No global definition of an “optimal” multilevel circuit Depends on user-defined goals (number of gates, delay etc.) Use of synthesis (starting with a formal description and proceeding to a logic diagram) to meet the design goals Synthesis requires CAD-tool help No simple hand methods like Karnaugh maps CAD tools manipulate Boolean expressions with some “heurisitics” to achieve goals 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 15

Conversion of multi-level logic to NAND gates Level 1 Level 2 Level 3 Level 4 F = A (B + C D) + B C' original AND-OR network A C D B \C F introduction and conservation of bubbles A C D B \C F redrawn in terms of conventional NAND gates A C D \B B \C F 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 16

Conversion of multi-level logic to NORs Level 1 Level 2 Level 3 Level 4 F = A (B + C D) + B C' A C D B \C F original AND-OR network introduction and conservation of bubbles A C D B \C F redrawn in terms of conventional NOR gates \A \C \D B \B C F 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 17

Conversion between forms Example A X B C D F (a) original circuit A X B C D F (b) add double bubbles at inputs \D A X B C F \X (d) insert inverters to fix mismatches \D A \X B C F (c) distribute bubbles some mismatches 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 18

CSE 370 – Winter 2002 - Combinational Implementation - 19 AND-OR-Invert gates AOI function: conceptually three stages of logic — AND, OR, Invert multiple gates "packaged" as a single circuit block AND OR Invert logical concept A B C D Z NAND NAND Invert possible implementation A B C D Z & + 2x2 AOI gate symbol & + 3x2 AOI gate symbol 3 inputs, 2 stacks 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 19

Conversion to AOI forms General procedure to place in AOI form compute the complement of the function in sum-of-products form by grouping the 0s in the Karnaugh map Example: XOR implementation –– A xor B = A' B + A B' AOI form: F = (A' B' + A B)' 0 1 1 0 A B & + A' B' A B F 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 20

Examples of using AOI gates F = B C' + A C' + A B F' = A' B' + A' C + B' C Implemented by 2-input 3-stack AOI gate F = (A + B) (A + C') (B + C') F' = (B' + C) (A' + C) (A' + B') Implemented by 2-input 3-stack OAI gate Example: 4-bit equality function Z = (A0 B0 + A0' B0')(A1 B1 + A1' B1')(A2 B2 + A2' B2')(A3 B3 + A3' B3') 0 1 0 0 1 1 1 0 C B A each implemented in a single 2x2 AOI gate; then NOR them together 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 21

Examples of using AOI gates (cont’d) Example: AOI implementation of 4-bit equality function high if A0  B0 low if A0 = B0 A0 B0 A1 B1 A2 B2 A3 B3 & + NOR Z conservation of bubbles if all inputs are low then Ai = Bi, i=0,...,3 output Z is high 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 22

Summary for multi-level logic Advantages circuits may be smaller gates have smaller fan-in circuits may be faster Disadvantages more difficult to design tools for optimization are not as good as for two-level analysis is more complex 12/5/2018 CSE 370 – Winter 2002 - Combinational Implementation - 23