The fanout-of-4 inverter delay metric


David Harris, Ron Ho, Gu Wei, and Horowitz Mark. 1997. “The fanout-of-4 inverter delay metric.” Unveröffentlichtes Manuskript: http://odin. ac. hmc. edu/harris/research/FO4. pdf.


Digital circuit delays vary with feature size, process corner, operating voltage, and junction temperature. Delays are steadily decreasing with advances in process technology, so comparing results reported in nanoseconds between process generations is difficult. This paper proposes using the delay of a fanout-of-4 inverter (FO4) to normalize process and operating condition variations and quantifies how well this normalization works. A novel application of this correlation is a power-reduction technique. Power supply and operating frequency can be regulated on the fly to minimize power while a chip is performing non-critical operations while allowing full-speed operation when necessary. Proposed implementations [1,2,3] rely on a good correlation between ring-oscillator frequency and critical path latency. The tracking of chip delays with FO4 delay determines the necessary extra margin for functionality over process and environmental variation.
Last updated on 05/06/2022