Presentation on theme: "Logic Gates A logic gate is an elementary building block of a digital circuit Most logic gates have two inputs and one output At any given moment, every."— Presentation transcript:
2 Logic GatesA logic gate is an elementary building block of a digital circuitMost logic gates have two inputs and one outputAt any given moment, every terminal is in one of the two binary conditions low (0) or high (1) represented by different voltage levelsThe logic state of a terminal can, and generally does, change often, as the circuit processes dataIn most logic gates, the low state is approximately zero volts (0 V)while the high state is approximately five volts positive (+5 V).
3 Truth TablesA truth table shows how a logic circuit's output responds to various combinations of the inputsusing logic 1 for true and logic 0 for false.All permutations of the inputs are listed on the leftoutput of the circuit is listed on the rightThe desired output can be achieved by a combination of logic gatesA truth table for two inputs is shown, but it can be extended to any number of inputs.The input columns are usually constructed in the order of binary counting with a number of bits equal to the number of inputs.
4 NOT Gate or InverterThe output of a not gate is always opposite to it's inputAn X with a vertical bar over it means 'NOT X' .
5 AND gateAn AND gate always has one output, but it can have many inputs (at least 2).The output is high only when both inputs A and B are high.
7 OR gateAs with the AND gate, the OR gate has one output and several inputs. X+Y is equivilent to X OR Y. The output is high when either or both of inputs A or B is high.Below we have pictured a 2-input OR gate.
9 NAND gatesA NAND gate is equivalent to an AND gate followed by a NOT gate. Notice that the bar stretches over all of the expression, to say that the whole thing is negatedThe output is high when either of inputs A or B is high, or if neither is high. In other words, it is normally high, going low only if both A and B are high.
11 NOR gatesA NOR gate is an OR gate followed by a not gate. Notice that the bar stretches over all of the expression, to say that the whole thing is negatedThe output is high only when neither A nor B is high. That is, it is normally high but any kind of non-zero input will take it low.
13 EXOR gatesEXOR (or EXclusive OR) gives TRUE if either but not both inputs are TRUENotice that the symbol for EXOR is + (as in OR) in a circleThe output is high when either of inputs A or B is high, but not if both A and B are high.
15 Negative Logic Gates Each of the basic gates has a negative logic equivalent as shown. The equivalence is shownby the application of DeMorgan's theorem. Itamounts to changing AND's to OR's or viceversa and inverting all inputs.
16 A practical operational way to look at DeMorgan's Theorem is that the inversion bar of an expression may be broken at any point and the operation at that point replaced by its opposite (i.e., AND replaced by OR or vice versa
28 Setting the NAND LatchAfter being set to Q=1 by the low pulse at S (NAND gate function), the restored normal value S=1 is consistent with the Q=1 state, so it is stable. Another negative pulse on S gives which does not switch the flip-flop, so it ignores further input.
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