94 from Ch 23 Logic Gates

94 from Ch 23 Logic Gates - Making sense of EE / 2nd...

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Logic operations such as AND, OR, NOT, NAND (NOT AND), NOR (NOT OR) are used in nearly all applications of today’s electronics. Here we learn about these basic functions and their definitions based on the truth tables, which contain TRUE (1) and FALSE (0) logic, or Boolean values. Also, we learn how the logic gates can be combined. Next, we translate the truth tables into voltages in digital circuits. There are two opposite types called the “positive” and “negative” logic. Recall that digital circuits produce only HIGH voltages (for example, around +5 V) and LOW voltages, typically around 0 V. In “positive” logic, HIGH voltage means TRUE and LOW voltage means FALSE. In “negative” logic, HIGH voltage means FALSE and LOW voltage means TRUE. Logic gate functions are implemented in circuits with diodes and transistors (both BJT and MOSFET can be used). Here we learn how to determine the logic function of a given circuit. First of all, do the circuit analysis to find the output voltage from the inputs; this step does not depend on the choice of logic. Then, translate each of the voltages (input and output) into the logic TRUE or FALSE values; this step certainly depends on the chosen type of logic. Finally, from the combinations of TRUE and FALSE inputs and outputs, reconstruct the truth table, and from the truth table determine the type of logic function. Finally, we take a look at a combination of logic gates that has memory and can serve as a simple memory cell. Making sense of EE // 2nd edition Logic Gates © 2008 A. Ganago Page 1 of 12
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© 2008 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. © 2008 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
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This note was uploaded on 12/06/2010 for the course EECS 314 taught by Professor Ganago during the Spring '07 term at University of Michigan.

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94 from Ch 23 Logic Gates - Making sense of EE / 2nd...

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