Reconvergent fan-out is a technique to make
VLSI
Very large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining millions or billions of MOS transistors onto a single chip. VLSI began in the 1970s when MOS integrated circuit (Metal Oxide Semiconductor) c ...
logic simulation
Logic simulation is the use of simulation software to predict the behavior of digital circuits and hardware description languages. Simulation can be performed at varying degrees of physical abstraction, such as at the transistor level, gate lev ...
less pessimistic.
Static timing analysis
Static timing analysis (STA) is a simulation method of computing the expected timing of a Synchronous circuit, synchronous digital circuit without requiring a simulation of the full circuit.
High-performance integrated circuits have tradition ...
tries to figure out the best and
worst case
In computer science, best, worst, and average cases of a given algorithm express what the resource usage is ''at least'', ''at most'' and ''on average'', respectively. Usually the resource being considered is running time, i.e. time complexity, b ...
time estimate for each signal as they pass through an electronic device. Whenever a signal passes through a node, a bit of uncertainty must be added to the time required for the signal to transit that device. These uncertain delays add up so, after passing through many devices, the worst-case timing for a signal could be unreasonably pessimistic.
It is common for two signals to share an identical path, branch and follow different paths for a while, then converge back to the same point to produce a result. When this happens, you can remove a fair amount of uncertainty from the total delay because you know that they shared a common path for a while. Even though each signal has an uncertain delay, because their delays were identical for part of the journey the total uncertainty can be reduced. This tightens up the worst-case estimation for the signal delay, and usually allows a small but important speedup of the overall device.
This term is starting to be used in a more generic sense as well. Any time a signal splits into two and then reconverges, certain optimizations can be made. The term reconvergent fan-out has been used to describe similar optimizations in
graph theory
In mathematics, graph theory is the study of ''graphs'', which are mathematical structures used to model pairwise relations between objects. A graph in this context is made up of '' vertices'' (also called ''nodes'' or ''points'') which are conne ...
and
static code analysis
In computer science, static program analysis (or static analysis) is the analysis of computer programs performed without executing them, in contrast with dynamic program analysis, which is performed on programs during their execution.
The term i ...
.
See also
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Fan-out
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 ...
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Fan-in
Fan-in is the number of inputs a logic gate can handle. For instance the fan-in for the AND gate shown in the figure is 3. Physical logic gates with a large fan-in tend to be slower than those with a small fan-in. This is because the complexity o ...
External links
An example of reconvergent fan-out
{{DEFAULTSORT:Reconvergent Fan-Out
Logic gates