2009 Logic gates - Logic Gates Many todays applications...

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1 © 2009 A. Ganago Logic Gates Many today’s applications require logic functions such as AND, OR, NOT, etc. Even a washing machine uses logic such as “start only if the working cycle is chosen AND the lid is closed.” Here we learn about logic operations and how they can be implemented as diode and transistor circuits. 1 Logic gates © 2009 A. Ganago Digital Circuits Do Logic Operations on Signals Logic operates with “TRUE” and “FALSE” Mathematically, “TRUE” = “1” and “FALSE” = “0” In the circuits, positive logic means “1” = HIGH voltage; “0” = LOW voltage • Negative logic means “0” = HIGH voltage; “1” = LOW voltage 2 Logic gates © 2009 A. Ganago Basic Logic Operations (1) • NOT If A (input) is TRUE then C (output) is FALSE • AND If both A and B (two inputs) are TRUE then C (output) is TRUE; if any of A and B is FALSE, C is FALSE • OR If any of A and B (two inputs) is TRUE then C (output) is TRUE 3 Logic gates © 2009 A. Ganago Basic Logic Operations (2) • NAND = NOT(AND) If both A and B (two inputs) are TRUE then C (output) is FALSE; if any of A and B is FALSE then C is TRUE • NOR = NOT(OR) If any of A and B (two inputs) is TRUE then C (output) is FALSE; if both A and B are FALSE, then C is TRUE 4 Logic gates © 2009 A. Ganago Basic Logic Operations (3) There are many more operations, including OR, AND with three or more inputs, and other types as well In this course, we cover only the 5 operations listed above: OR, AND, NOT, NOR, NAND 5 Logic gates © 2009 A. Ganago Basic Logic Operations (4) Logic operations are identified by the truth tables Each logic operation has its unique truth table If two operations have identical truth tables, the operations are identical 6 Logic gates
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2 © 2009 A. Ganago Truth Tables (1) NOT Input Output A C 0 1 1 0 7 Logic gates © 2009 A. Ganago Input Input Output A B C 0 0 0 1 0 0 0 1 0 1 1 1 AND Truth Tables (2) 8 Logic gates © 2009 A. Ganago Truth Tables (3) Input Input Output A B C 0 0 0 1 0 1 0 1 1 1 1 1 OR 9 Logic gates © 2009 A. Ganago Truth Tables (4) Input Input
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