Power Management

In the CMOS dynamic power formula P = αCV²f, what does α represent?

  1. AThe number of transistors
  2. BThe fraction of signals switching per clock cycle (the activity factor)
  3. CThe chip temperature
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Answer: B. The fraction of signals switching per clock cycle (the activity factor)

Dynamic power is consumed charging and discharging the capacitance of wires and gates. C is the switched capacitance, V the supply voltage, f the clock frequency and α the activity factor, the fraction of signals that switch per clock. Parts whose data does not change, or whose clocks are stopped, do not switch, so reducing α is also an important power-saving lever.

  • A. Transistor count is folded into the capacitance C.
  • B. Correct. Circuits that do not switch use no dynamic power.
  • C. Temperature mainly affects static power.
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More Power Management questions

  1. What is the dynamic power of a circuit with activity factor α = 0.1, switched capacitance C = 1 nF, supply voltage V = 1 V and frequency f = 1 GHz?
  2. With the supply voltage unchanged, only the clock frequency is halved. What happens to the dynamic energy needed to finish a fixed amount of computation?
  3. Assume DVFS can lower the supply voltage in proportion to frequency. When frequency is halved, roughly what happens to dynamic power and to the dynamic energy for a fixed amount of computation?
  4. Which describes static (leakage) power in CMOS circuits?
  5. How does leakage power change as chip temperature rises, and what problem can result?