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Power Estimation Formula

Power is measured in Watts and P = V x I = E x f

Gate current I = Static Current (leakage) + Dynamic Current.

Early CMOS (VCC 5 volts): negligible static current,

but today at VCC of 1.3 volts it's up to 30 percent of consumption.

Dynamic current = Short circuit current + Dynamic charge current.

Dynamic charge current computation:

  • All energy in a net/gates is wasted each time it goes from one to zero.
  • The energy in a capacitor is E = CV^2/2.
  • Dominant capacitance is proportional to net length.
  • Gate input and output capacitance also contribute to C.

Activity ratio, a: percentage of clock cycles that see a transition from 0 to 1.

The net toggle rate = Clock frequency x activity ratio = f x a.

For leakage and short circuit power see cell library.

Page 30 or so of this cell library has real-word examples: »90nm Cell Library

See also the power formula on the 7400A data sheet: »74LVC00A.pdf

Further details: »Power Management in CPU Design.

11: (C) 2008-13, DJ Greaves, University of Cambridge, Computer Laboratory.