Page 41 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes 2.3 Summary

• Logic gates are the basic components in digital electronics. They are used to create digital
circuits and even complex integrated circuits.
• The AND logic gate performs an”AND” logic operation, which is a multiplication.
• An OR logic gate performs an”OR” logic operation, which is an addition.

• A NAND logic gate is an AND gate with an inverter attached.
• A NOR logic gate is an OR gate with an inverter attached. So, its output is the opposite
from OR.

• The XOR stands for exclusive OR. XOR gate compares two values and if they are different
its output will be “1.”
• The XNOR stands for exclusive NOR and is an XOR gate with its output inverted. So, its
output is at ”1” when the inputs have the same value and ”0” when they are different.
• Fan-out is the maximum number of gates a given integrated circuit is capable of being
connected to.

• A Tri-state Gate can be thought of as an input controlled switch which has an output that
can be electronically turned “ON” or “OFF” by means of an external “Control” or “Enable”
signal input.
• The control signal of tri-state gate can be either a logic “0” or a logic “1” type signal resulting
in the Tri-state gate being in one state allowing its output to operate normally giving either
logic “0” or logic “1” output.

2.4 Keywords

AND gate: The AND gate is so named because, if 0 is called “false” and 1 is called “true”, the gate
acts in the same way as the logical “and” operator.
Inverter: A logical inverter, sometimes called a NOT gate to differentiate it from other types of
electronic inverter devices, has only one input. It reverses the logic state.
Logic gate: A logic gate is an elementary building block of a digital circuit. Most logic gates have
two inputs and one output.
NAND gate: The NAND gate operates as an AND gate followed by a NOT gate. It acts in the
manner of the logical operation “and” followed by negation. The output is “false” if both inputs
are “true”. Otherwise, the output is “true”.
NOR gate: The NOR gate is a combination OR gate followed by an inverter. Its output is “true”
if both inputs are “false”. Otherwise, the output is “false”.
OR gate: The OR gate gets its name from the fact that it behaves after the fashion of the logical
inclusive “or”. The output is “true” if either or both of the inputs are “true”. If both inputs are
“false”, then the output is “false”.
XNOR gate: The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter.
Its output is “true” if the inputs are the same and “false” if the inputs are different.
XOR gate: The XOR (exclusive-OR) gate acts in the same way as the logical “either/or.” The
output is “true” if either, but not both, of the inputs are “true”. The output is “false” if both inputs
are “false” or if both inputs are “true”.

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