9.1 LESSON FROM THE REAL WORLD: THE MANAGER’S PERSPECTIVE AND THE ENGINEER’S PERSPECTIVE
All circuits will burn power in order to accomplish the functions for which they are designed. This is true for digital circuitry as well as for other circuits. Digital circuits will burn power in order to carry out the logical functions they are designed to accomplish. Although there are exceptions to every rule, larger circuits generally burn more power than do smaller circuits. In addition, faster circuits will burn more power than slower circuits. And, generally speaking, circuits that drive heavy loads will burn more power than circuits that drive lighter loads. This all seems to be inherently obvious, but experience can suggest differently.
Unfortunately, because chip-design engineers sometimes misinterpret the power number with in a liberty file, the amount of power that a given circuit will actually consume is seriously overstated. This is unfortunate because in order to care for such a supposed large amount of power consumption, chip designers add significant amounts of power-rail routing. They thereby significantly reduce the percentage of the core area budgeted for stdcell routing, thus increasing the size of the core, which pushes the stdcells farther apart than required, and causing the need for larger stdcells that in turn consume even more power. Further, the chip designers supply that power-rail routing with increased numbers of power and ground inputs–outputs ...