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Comprensión de Lectura en Ingles I para I.T.S.E.

Profa. Martha Patricia Martínez Miranda
Logic gate

Logic gates
Text taken from: https://electronics.fandom.com/wiki/Logic_gate

A logic gates performs a logical operation on one or more logic inputs and produces a single logic
output. The logic normally performed is Boolean logic and is most commonly found in digital
circuits. Logic gates are primarily implemented electronically using diodes or transistors, but can
also be constructed using electromagnetic relays, fluidics, optics, or even mechanical elements.

A logic gate takes one or more logic-level inputs and proceduces a single logic-level output. Because
the output is also a logic level, an output of one logic gate can connect to the input of one or more
other logic gates. Two outputs are never connected together as they could produce conflicting logic
values. In electronic logic gates, this would cause a short circuit.

In electronic logic, a logic level is represented by a certain voltage (this is wrong). Each logic gate
requires power so that it can source and sink currents to achieve the correct output voltage. In logic
circuit diagrams the power is not shown, but in a full electronic schematic, power connections are
required.

The simplest form of electronic logic is logic. This allows AND and OR gates to be built, but not
inverters, and so is an incomplete form of logic. To build a complete logic system, a or can be used.
The simplest family of logic gates using bipolar transistors is called meta data-rte-instance, from a
change in input of a gate to the corresponding change in its output. When gates are cascaded, the
total propagation delay is approximately the sum of the individual delays, an effect which can
become a problem in high-speed circuits.

Logic gates and hardware

NAND and NOR logic gates are the two pillars of logic, in that all other types of Boolean logic gates
(i.e., AND, OR, NOT, XOR, XNOR) can be created from a suitable network of just NAND or just NOR
gate(s). They can be built from relays or transistors, or any other technology that can create an
inverter and a two-input AND or OR gate. Hence the NAND or NOR gates are called the universal
gates.

Symbols
There are two sets of symbols in common use, both now defined by ANSI/EEE Std 91-1984 and its
supplement ANSI/EEE Std 91a-1991. The “distinctive shape” set, based on traditional schematics, is
used for simple drawing and is quicker to draw by hand. It is sometimes unofficially described as
“military”, reflecting its origin if not its modern usage. The “rectangular shape” set, based on IEC
60617-12, has rectangular
outlines for all types of gate,
and allows representation of a
much wider range of devices
than is possible with the
traditional symbols. The IEC’s
system has been adopted by
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Comprensión de Lectura en Ingles I para I.T.S.E.
Profa. Martha Patricia Martínez Miranda
Logic gate

other standards, such as EN

60617-12-1999 in Europe and BS
EN 60617-12-19999 in the
United Kingdom.
In electronics a NOT gates is
more commonly called an
inverter. The circle on the
symbol is called a bubble and is
generally used in circuit
diagrams to indicate an inverted
input or input.
Symbolically, a NAND gate can
also be shown using the OR
shape with bubbles on its
inputs, and a NOR gate can be
shown as an NAD gate with
bubbles on its inputs. This
reflects the equivalency due to
De Morgans law, but it also
allows a diagram to be read
more easily, or a circuit to ne
mapped onto available physical
gates in packages easily, since
any circuit node that has
bubbles at both ends can be
replaced by a simple bubble-less connection and a suitable change of gate. If the NAND is drawn
as OR with input (effectively, bubbles “cancel”). This is commonly seen in real logic diagrams -thus
the reader must not get into the habit of associating the shapes exclusively as OR or AND shapes,
but also take into account the bubbles at both inputs and outputs or determine the “true” logic
function indicated.
Two more gates are the exclusive -OR or XOR function and its inverse, exclusive -NOR or XNOR.
The two input Exclusive-OR is true only when the two input values are different, false is they are
equal, regardless of the value. If there are more than two inputs, the gate generates a true at its
output if the number of trues at its input is odd. In practice, these gates are built from
combinations of simpler logic gates.

De Morgan equivalent symbols

By use of De Morgan´s theorem, an AND gate can be
turned into an OR gate by inverting the sense of the
logic at its inputs and outputs. This leads to separate
set of symbols with inverted inputs and the outputs.
This leads to a separate set of symbols with inverted
inputs and the opposite core symbol. These symbols
can make circuit diagrams for circuits using active low
signals much clearer and help to show accidental connection of an active high output to an active
low input or vice-versa.
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Comprensión de Lectura en Ingles I para I.T.S.E.
Profa. Martha Patricia Martínez Miranda
Logic gate

Storage of bits.

Related to the concept of logic gates (and also built from them) is the idea of storing a bit of
information. The gates discussed up to here cannot store a value: when the inputs change, the
outputs immediately react. It is possible to make a storage element either through a capacitor (which
stores charge due to its physical properties) or by feedback. Connecting the output of a gate to the
input causes it to be put through the logical again, and chossing the feedback correctly allows it to be
perversed or modified through the use of other inputs. A set of gates arranged in this fashion is
known as a latch , and more complicated designs that utilise clocks (signals that oscillate with a
known perioid) and change only on the rising edge are called edge-triggered “flip-flops”. The
combination of multiple flip-flops in parallel, to store a multiple-bit value, is known as a register.
When using any of these gate setups the overall system has memory, it is then called a sequencial
system since its output can be influenced by its previous state(s).

Three-state logic gates.

Three-state, or 3-state, logic gates have three states of the output: high (H), low (L) and high-
impedance (Z). The high-impedance state plays no role in the logic, which remains strictly binary.
These devices are used on buses to allow multiple chips to send data. A group of three-states driving
a line with a suitable control circuit is basically equivalent to a multiplexer, which may be physically
distribuited over separate devices or plug-in cards.

Miscellaneous.

Logic circuit include such devices as multiplexers, registers, arithmetic logic unitarithmetic units
(ALUs), and computer memory, all the way up through complete microprocessors which can contain
more than a 100 million gates. In practice, the gates are made from field effect transsistors (FETs),
particulary metal-oxide-semiconductor FETs (MOSFETs).

In reversible logic, Toffoli gates are used.