FO4
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Fan-out of 4 is a process independent delay metric used in digital CMOS technologies.
Fan out = Cload / Cin
Cload = total MOS gate capacitance driven by the logic gate under consideration
Cin = the MOS gate capacitance of the logic gate under consideration
As a delay metric, one FO4 is the delay of an inverter, driven by an inverter 4x smaller than itself, and driving an inverter 4x larger than itself. Both conditions are necessary since input signal rise/fall time affects the delay as well as output loading.
A fan out of 4 is the answer to the canonical problem stated as follows: Given a fixed size inverter, small in comparison to a fixed large load, minimize the delay in driving the large load. After some math, it can be shown that the minimum delay is achieved when the load is driven by a chain of N inverters, each successive inverter ~4x larger than the previous; N ~ log4(Cload/Cin).
Interestingly, in the absence of parasitic capacitances (drain diffusion capacitance and wire capacitance), the result is "a fan out of e" (now N ~ ln(Cload/Cin).
If the load itself is not large, then using a fan out of 4 scaling in successive logic stages does not make sense. In these cases, minimum sized transistors may be faster.
Because scaled technologies are inherently faster (in absolute terms), circuit performance can be more fairly compared using the fan out of 4 as a metric. For example, given two 64-bit adders, one implemented in a 0.5um technology and the other in 90nm technology, it would be unfair to say the 90nm adder is better from a circuits and architecture standpoint just because it has less latency. The 90nm adder might be faster only due to its inherently faster devices. To compare the adder architecture and circuit design, it is more fair to normalize each adder's latency to the delay of one FO4 inverter.
See also
External links
- Logical Effort Revisited
- Revisiting the FO4 Metric
- David Harris, Slides on Logical Effort – with a succinct example of design using FO4 inverters (p. 19).
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