Gate Delay
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Propagation delay is the time duration taken for a signal to reach its destination. It can relate to
networking Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematic ...
,
electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
or
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
. ''Hold time'' is the minimum interval required for the logic level to remain on the input after triggering edge of the clock pulse.


Networking

In
computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
s, propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver. It can be computed as the ratio between the link length and the propagation speed over the specific medium. Propagation delay is equal to ''d / s'' where ''d'' is the distance and ''s'' is the
wave propagation speed The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or of a transmission medium is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, a light pulse in an optical fibr ...
. In wireless communication, ''s''=''c'', i.e. the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
. In
copper wire Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor. Copper is the electri ...
, the speed ''s'' generally ranges from .59c to .77c. This delay is the major obstacle in the development of high-speed computers and is called the
interconnect bottleneck The interconnect bottleneck comprises limits on integrated circuit (IC) performance due to connections between components instead of their internal speed. In 2006 it was predicted to be a "looming crisis" by 2010. Improved performance of compute ...
in IC systems.


Electronics

In
electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, digital circuits and
digital electronics Digital electronics is a field of electronics involving the study of digital signals and the engineering of devices that use or produce them. This is in contrast to analog electronics and analog signals. Digital electronic circuits are usual ...
, the propagation delay, or gate delay, is the length of time which starts when the input to a
logic gate A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, ...
becomes stable and valid to change, to the time that the output of that logic gate is stable and valid to change. Often on manufacturers'
datasheets A datasheet, data sheet, or spec sheet is a document that summarizes the performance and other characteristics of a product, machine, component (e.g., an electronic component), material, subsystem (e.g., a power supply), or software in suffici ...
this refers to the time required for the output to reach 50% of its final output level from when the input changes to 50% of its final input level. This may depend on the direction of the level change, in which case separate fall and rise delays tPHL and tPLH or tf and tr are given. Reducing gate delays in digital circuits allows them to process data at a faster rate and improve overall performance. The determination of the propagation delay of a combined circuit requires identifying the longest path of propagation delays from input to output and by adding each propagation delay along this path. The difference in propagation delays of logic elements is the major contributor to
glitch A glitch is a short-lived fault in a system, such as a transient fault that corrects itself, making it difficult to troubleshoot. The term is particularly common in the computing and electronics industries, in circuit bending, as well as among ...
es in
asynchronous circuit Asynchronous circuit (clockless or self-timed circuit) is a sequential digital logic circuit that does not use a global clock circuit or signal generator to synchronize its components. Instead, the components are driven by a handshaking circuit ...
s as a result of
race condition A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of t ...
s. The principle of logical effort utilizes propagation delays to compare designs implementing the same logical statement. Propagation delay increases with
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
, as resistance of conductive materials tends to increase with temperature. Marginal increases in supply voltage can increase propagation delay since the upper switching threshold voltage, VIH (often expressed as a percentage of the high-voltage supply rail), naturally increases proportionately. Increases in output load capacitance, often from placing increased fan-out loads on a wire, will also increase propagation delay. All of these factors influence each other through an
RC time constant The RC time constant, also called tau, the time constant (in seconds) of an RC circuit, is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads), i.e. : \tau = RC econds It is the time required to c ...
: any increase in load capacitance increases C, heat-induced resistance the R factor, and supply threshold voltage increases will affect whether more than one time constants are required to reach the threshold. If the output of a logic gate is connected to a long trace or used to drive many other gates (high
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 ...
) the propagation delay increases substantially. Wires have an approximate propagation delay of 1 ns for every of length. Logic gates can have propagation delays ranging from more than 10 ns down to the picosecond range, depending on the technology being used.


Physics

In
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
, particularly in the
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
field, the propagation delay is the length of time it takes for a signal to travel to its destination. For example, in the case of an electric signal, it is the time taken for the signal to travel through a wire. See also
velocity factor The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or of a transmission medium is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, a light pulse in an optical fib ...
and
radio propagation Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affecte ...
.


See also

*
Contamination delay In digital circuits, the contamination delay (denoted as tcd) is the minimum amount of time from when an input changes until any output starts to change its value. This change in value does not imply that the value has reached a stable condition ...
*
Delay calculation Delay calculation is the term used in integrated circuit design for the calculation of the gate delay of a single logic gate and the wires attached to it. By contrast, static timing analysis computes the delays of entire paths, using delay calc ...
*
Latency (engineering) Latency, from a general point of view, is a time delay between the cause and the effect of some physical change in the system being observed. Lag, as it is known in gaming circles, refers to the latency between the input to a simulation and t ...
*
Time of flight Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a w ...
*
Transmission delay In a network based on packet switching, transmission delay (or store-and-forward delay, also known as packetization delay) is the amount of time required to push all the packet's bits into the wire. In other words, this is the delay caused by the ...


References

{{DEFAULTSORT:Propagation Delay Digital circuits Digital electronics Electronics concepts Timing in electronic circuits