In
digital circuit In theoretical computer science, a circuit is a model of computation in which input values proceed through a sequence of gates, each of which computes a function. Circuits of this kind provide a generalization of Boolean circuits and a mathematical ...
s, a logic level is one of a finite number of
states that a
digital signal
A digital signal is a signal that represents data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents continuous values; at ...
can inhabit. Logic levels are usually represented by the
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
difference between the signal and
ground, although other standards exist. The range of voltage levels that represent each state depends on the
logic family
In computer engineering, a logic family is one of two related concepts:
* A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compati ...
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 notatio ...
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 in e ...
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
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
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
TTL may refer to:
Photography
* Through-the-lens metering, a camera feature
* Zenit TTL, an SLR film camera named for its TTL metering capability
Technology
* Time to live, a computer data lifespan-limiting mechanism
* Transistor–transistor lo ...
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
A wired logic connection is a logic gate that implements boolean algebra (logic) using only passive components such as diodes and resistors. A wired logic connection can create an AND or an OR gate. The limitations are the inability to create a ...
logic if the logic gates are
open-collector
An open collector is a common type of output found on many integrated circuits (IC), which behaves like a switch that is either connected to ground or disconnected. Instead of outputting a signal of a specific voltage or current, the output sig ...
/
open-drain
An open collector is a common type of output found on many integrated circuits (IC), which behaves like a switch that is either connected to ground or disconnected. Instead of outputting a signal of a specific voltage or current, the output sig ...
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 wide ...
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 mu ...
(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
currents
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
are used in some logic signaling, like
digital current loop interface
For serial communications, a current loop is a communication interface that uses current instead of voltage for signaling. Current loops can be used over moderately long distances (tens of kilometres), and can be interfaced with optically isolat ...
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 si ...
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 wit ...
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
TTL may refer to:
Photography
* Through-the-lens metering, a camera feature
* Zenit TTL, an SLR film camera named for its TTL metering capability
Technology
* Time to live, a computer data lifespan-limiting mechanism
* Transistor–transistor lo ...
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 MOSFE ...
. 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
In electronics, high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current and potentially high voltage, whereas l ...
. 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
Electronic design automation (EDA), also referred to as electronic computer-aided design (ECAD), is a category of software tools for designing Electronics, electronic systems such as integrated circuits and printed circuit boards. The tools wo ...
. 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 2
n 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 c ...
s may use more than two states to encode data more efficiently. Examples include
MLT-3 encoding
MLT-3 encoding (Multi-Level Transmit) is a line code (a signaling method used in a telecommunication system for transmission purposes) that uses three voltage levels. An MLT-3 interface emits less electromagnetic interference and requires less band ...
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
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
.
See also
*
Logic family
In computer engineering, a logic family is one of two related concepts:
* A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compati ...
*
Digital current loop interface
For serial communications, a current loop is a communication interface that uses current instead of voltage for signaling. Current loops can be used over moderately long distances (tens of kilometres), and can be interfaced with optically isolat ...
References
External links
Positive Logic (active-high) and Negative logic (active-low )
{{DEFAULTSORT:Logic Level
Digital electronics