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Published byCalvin Ray Modified over 3 years ago

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Why? What’s Boolean algebra used for? –“The purpose of Boolean algebra is to facilitate the analysis and design of digital circuits.” Express a truth table relationship in algebraic (symbolic) notation Express a logic diagram’s input/output relationships in algebraic notation Provides a means for finding simpler circuits to implement a given function

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Axioms 1.x + 0 = x 2.x + 1 = 1 3.x + x = x 4.x + x’ = 1 5.x + y = y + x 6.x + (y + z) = (x + y) + z 7.x(y + z) = xy + xz 8.(x + y)’ = x’y’ 9.(x’)’ = x 10.x · 1 = x 11.x · 0 = 0 12.x · x = x 13.x · x’ = 0 14.xy = yx 15.x(yz) = (xy)z 16.x + yz = (x + y)(x + z) 17.(xy)’ = x’ + y’

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Axioms 1, 10 – identity laws 2, 11 – one, zero laws 3, 12 – idempotence laws 4, 13 – inverse laws 5, 14 – commutative laws 6, 15 – associative laws 7, 16 – distributive laws 8, 17 – De’ Mogan’s laws 9 – double negation law (?? – I made that name up)

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Logic Circuits (Gates) These are the things computers (and other digital devices) are made of Circuit designers use Boolean algebra to design circuits drawn on schematic drawings Fabrication facilities use schematic drawings to produce silicon chips AND OR NOT Schematic Symbols

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Axioms Axioms are one way to simplify a Boolean expression –Consider the expression F(A,B,C) = (A+B)(A+C) –Requires 3 logic gates AND OR B A C A

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Axioms Using the axioms one can “prove” that this expression simplifies F(A,B,C) = (A+B)(A+C) = A + BC (this is axiom 14 but it can be proved from the others) –Resulting in a two gate solution –We shaved 33% off of our design! A C B AND OR

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Simplification Why bother? –A simplified expression means a smaller circuit –A smaller circuit is easier to build –A smaller circuit is easier to debug –A smaller circuit means more money in your pocket

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Another Example F(A,B,C) = ABC + ABC’ + A’C –How many gates are required as specified? –What does the circuit look like? –Can the expression be simplified based on the axioms? If so, how?

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Axioms/Gates Based on the axioms, the AND and OR gates may be created with more two inputs –Which axioms allow for this?

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More Gates NAND –Shortened form of “not and” –Truth table? –Symbol NAND

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More Gates NOR –Shortened form of “not or” –Truth table? –Symbol NOR

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NAND/NOR So, what’s so special about NAND and NOR? NAND and NOR are considered “universal gates” –That is, anything that can be done with AND/OR/NOT can be done with only NAND or NOR gates (one or the other, not both)

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NAND/NOR The universality of NAND/NOR is important because it means you can make many copies of a single gate type on a single piece of silicon and then use it to create complex circuits on a single chip Programmable (FPGA) devices use this technology

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Simplification Using the axioms to “prove” that a simplified version of a circuit is equivalent to the complex version takes a special kind of person… –…of which I’m not one Fortunately, there’s another way…

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Homework Chapter 1 –1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7 –Create an AND gate using only NAND gates –Create an AND gate using only NOR gates –Create an OR gate using only NAND gates –Create an OR gate using only NOR gates –Create a NOT gate using only NAND gates –Create a NOT gate using only NOR gates –Due next lecture

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