FO4

Fan-out of 4 is a process-dependent delay metric used in digital CMOS technologies. Fan-out of 4 is a process-dependent delay metric used in digital CMOS technologies. Fan out = Cload / Cin, where 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. FO4 is generally used as a delay metric because such a load is generally seen in case of tapered buffers driving large loads, and approximately in any logic gate of a logic path sized for minimum delay. Also, for most technologies the optimum fanout for such buffers generally varies from 2.7 to 5.3. 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). 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).