Power Management

What is commonly used to wake a battery-powered device from sleep at a set time?

  1. AKeeping the CPU running at maximum frequency
  2. BAn alarm from a real-time clock (RTC) that keeps running on tiny power during sleep
  3. CRemoving and reinserting the battery
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Answer: B. An alarm from a real-time clock (RTC) that keeps running on tiny power during sleep

Even during sleep, a small real-time clock driven by a 32.768 kHz crystal or similar keeps time on a microampere or less. At the set time it raises an alarm interrupt that wakes the CPU. Other wake sources include button or sensor signal changes and incoming communications. Configuring which sources may wake the device lets it sleep deeply the rest of the time.

  • A. Then it is not sleeping at all.
  • B. Correct. Pin-change interrupts and the like can also wake it.
  • C. Removing the battery loses the time and other state.
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More Power Management questions

  1. If only the supply voltage is lowered by 10% (all else equal), by roughly what percentage does dynamic power fall?
  2. In the CMOS dynamic power formula P = αCV²f, what does α represent?
  3. 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?
  4. 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?
  5. 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?