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In digital circuits, a logic level is one of a finite number of states that a digital signal can inhabit. Logic levels are usually represented by the voltage difference between the signal and
ground Ground may refer to: Geology * Land, the surface of the Earth not covered by water * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical c ...
, although other standards exist. The range of voltage levels that represent each state depends on the logic family being used. A '' logic-level shifter'' can be used to allow compatibility between different circuits.


2-level logic

In binary logic the two levels are logical high and logical low, which generally correspond to
binary number A binary number is a number expressed in the base-2 numeral system or binary numeral system, a method of mathematical expression which uses only two symbols: typically "0" (zero) and "1" ( one). The base-2 numeral system is a positional notation ...
s 1 and 0 respectively. Signals with one of these two levels can be used in
boolean algebra In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas i ...
for digital circuit design or analysis.


Active state

The use of either the higher or the lower voltage level to represent either logic state is arbitrary. The two options are active high and active low. Active-high and active-low states can be mixed at will: for example, a read only memory integrated circuit may have a chip-select signal that is active-low, but the data and address bits are conventionally active-high. Occasionally a logic design is simplified by inverting the choice of active level (see
De Morgan's laws In propositional logic and Boolean algebra, De Morgan's laws, also known as De Morgan's theorem, are a pair of transformation rules that are both valid rules of inference. They are named after Augustus De Morgan, a 19th-century British mathem ...
). The name of an active-low signal is historically written with a bar above it to distinguish it from an active-high signal. For example, the name Q, read "Q bar" or "Q not", represents an active-low signal. The conventions commonly used are: * a bar above () * a leading slash (/Q) * a lower-case n prefix or suffix (nQ or Q_n) * a trailing # (Q#), or * an "_B" or "_L" suffix (Q_B or Q_L). Many control signals in electronics are active-low signals (usually reset lines, chip-select lines and so on). Logic families such as TTL can sink more current than they can source, so
fanout In digital electronics, the fan-out is the number of gate inputs driven by the output of another single logic gate. In most designs, logic gates are connected to form more complex circuits. While no logic gate input can be fed by more than one ...
and
noise immunity Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
increase. It also allows for wired-OR logic if the logic gates are open-collector/ open-drain with a pull-up resistor. Examples of this are the
I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous, multi-controller/multi-target (master/slave), packet switched, single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is wid ...
bus and the
Controller Area Network A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol, designed originally for m ...
(CAN),and the
PCI Local Bus Peripheral Component Interconnect (PCI) is a local computer bus for attaching hardware devices in a computer and is part of the PCI Local Bus standard. The PCI bus supports the functions found on a processor bus but in a standardized format ...
. Some signals have a meaning in both states and notation may indicate such. For example, it is common to have a read/write line designated R/W, indicating that the signal is high in case of a read and low in case of a write.


Logic voltage levels

The two logical states are usually represented by two different voltages, but two different
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are used in some logic signaling, like digital current loop interface and
current-mode logic Current mode logic (CML), or source-coupled logic (SCL), is a digital design style used both for logic gates and for board-level digital signaling of digital data. The basic principle of CML is that current from a constant current generator is s ...
. High and low thresholds are specified for each logic family. When below the low threshold, the signal is "low". When above the high threshold, the signal is "high". Intermediate levels are undefined, resulting in highly implementation-specific circuit behavior. It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid and occur only in a fault condition or during a logic level transition. However, few logic circuits can detect such a condition, and most devices will interpret the signal simply as high or low in an undefined or device-specific manner. Some logic devices incorporate
Schmitt trigger In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input s ...
inputs, whose behavior is much better defined in the threshold region and have increased resilience to small variations in the input voltage. The problem of the circuit designer is to avoid circumstances that produce intermediate levels, so that the circuit behaves predictably. Nearly all digital circuits use a consistent logic level for all internal signals. That level, however, varies from one system to another. Interconnecting any two logic families often required special techniques such as additional
pull-up resistor In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. It is typically used in combination with components such as switches and transistors, which physically int ...
s or purpose-built interface circuits known as level shifters. A
level shifter In digital electronics, a level shifter, also called logic-level shifter or voltage level translator, is a circuit used to translate signals from one logic level or voltage domain to another, allowing compatibility between integrated circuits ...
connects one digital circuit that uses one logic level to another digital circuit that uses another logic level. Often two level shifters are used, one at each system: A
line driver A line driver is an electronic amplifier circuit designed for driving a load such as a transmission line. The amplifier's output impedance may be matched to the characteristic impedance of the transmission line. Line drivers are commonly used ...
converts from internal logic levels to standard interface line levels; a line receiver converts from interface levels to internal voltage levels. For example, TTL levels are different from those of
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...
. Generally, a TTL output does not rise high enough to be reliably recognized as a logic 1 by a CMOS input, especially if it is only connected to a high-input-impedance CMOS input that does not source significant current. This problem was solved by the invention of the 74HCT family of devices that uses CMOS technology but TTL input logic levels. These devices only work with a 5 V power supply.


3-level logic

In
three-state logic In digital electronics, a tri-state or three-state buffer is a type of digital buffer that has three stable states: a high output state, a low output state, and a high-impedance state. In the high-impedance state, the output of the buffer is discon ...
, an output device can be in one of three possible states: 0, 1, or Z, with the last meaning high impedance. This is not a logic level, but means that the output is not controlling the state of the connected circuit.


4-level logic

4-level logic adds a fourth state, X ("don't care"), meaning the value of the signal is unimportant and undefined. It means that an input is undefined, or an output signal may be chosen for implementation convenience (see ).


9-level logic

IEEE 1164 The IEEE 1164 standard (''Multivalue Logic System for VHDL Model Interoperability'') is a technical standard published by the IEEE in 1993. It describes the definitions of logic values to be used in electronic design automation, for the VHDL har ...
defines 9 logic states for use in electronic design automation. The standard includes strong and weakly driven signals, high impedance and unknown and uninitialized states.


Multi-level cells

In solid-state storage devices, a
multi-level cell In electronics, a multi-level cell (MLC) is a memory cell capable of storing more than a single bit of information, compared to a single-level cell (SLC), which can store only one bit per memory cell. A memory cell typically consists of a single ...
stores data using multiple voltages. Storing n bits in one cell requires the device to reliably distinguish 2n distinct voltage levels.


Line coding

Digital
line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained ...
s may use more than two states to encode data more efficiently. Examples include MLT-3 encoding and
pulse-amplitude modulation Pulse-amplitude modulation (PAM) is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulse ...
variants used by modern Ethernet.


See also

* Logic family * Digital current loop interface


References


External links


Positive Logic (active-high) and Negative logic (active-low )
{{DEFAULTSORT:Logic Level Digital electronics