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Energy of Computation (2)

As stated, dynamic energy arising from stray capacitance is our main energy user today.

Capacitors do not consume energy - they only store it temporarily. Only resistors dissipate energy in logic circuits, but their resistance does not feature in the energy use formula. The energy in the wiring capacitance is `wasted' on each complete transistion cycle.

As we increase voltage, dynamic power grows quadraticly. Static power grows a little better than (i.e. less than) linear since transistors may turn off more fully.

If the clock frequency is f and a net has activity ratio \alpha (the fraction of clock cycles it transitions from one to zero) then the energy used is

Energy = f * alpha * C * V^2

Note that although we divide by two as part of the standard formula for the energy in a capacitor, this quantity of energy is wasted both in the charging network on the zero-to-one transition and in the discharging network on the one-to-zero transition. So we can drop the half.