2.1 LOGIC GATES

Digital Logic

Logic Gates

•             Logic circuits are represented by Boolean algebra using variables and operators. The function and variables have only one value, 0 and 1.

•       The complement of a variable is shown by an apostrophe (X’) or a bar over the letter such as . Table below summarizes logic gates as the symbol of the functions in Boolean expressions. 

•      Gate is the fundamental building block of all digital logic circuits.

•      The basic gates used in digital logic are :

§   AND

§   OR

§   NOT

§   NAND

§   NOR

§   XOR

•  Gates are the implementation of logical functions by the interconnection.

•  A gate is an electronic circuit that produces an output signal that is a simple Boolean operation on  its input signals.

•  Each gate is defined in three ways: graphic symbol, algebraic notation, and truth table.

•    The symbology used here and throughout the appendix is the IEEE standard, IEEE Std 91.

•  Note that the inversion (NOT) operation is indicated by a circle.

•    Each gate shown in the above table has one or two inputs and one output.

•    However, as indicated in table all of the gates except NOT can have more than two inputs.

•  Thus, can be implemented with a single OR gate with three inputs.

•    Gate delay is when one or more of the values at the input are changed, the correct output signal appears almost instantaneously, delayed only by the propagation time of signals through the gate.

•    In some cases, a gate is implemented with two outputs, one output being the negation of the other output.

•    Here we introduce a common term: we say that to assert a signal is to cause signal line to make a transition from its logically false (0) state to its logically true state.

•    The true state is either a high or low voltage state, depending on the type of electronic circuitry.

•    Design and fabrication are simpler if only one or two types of gates are used.

•    Thus, it is important to identify functionally complete sets of gates.

•    This means that any Boolean function can be implemented using only the gates in the set.