EC-20.6 Low Power Design, and Measuring It

Work through these

• Where the energy goes: switching, leakage, and the radio

  Dynamic power depends on how often things change state, leakage flows whenever the part is powered at all, and a radio dwarfs both while it is transmitting. Knowing which of the three dominates decides which optimisations are worth doing.

  NPTEL: Low Power VLSI Circuits and Systems (IIT Kharagpur) · Course

• Sleep modes, wake sources, and what each transition costs

  Deeper sleep saves more and takes longer to leave, and the wake-up itself burns energy and time. A duty cycle that wakes too often can use more than one that stays awake, which is a result worth deriving once.

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• The duty cycle, and building an energy budget from it

  Multiplying each state's current by the fraction of time spent in it gives an average from which battery life follows directly. Doing this on paper before the board exists is what makes a battery choice defensible.

• Voltage and frequency scaling, and when it actually helps

  Running slower at a lower voltage saves energy per operation but takes longer, so it helps only when the work is not fixed. Racing to sleep is frequently the better answer, and which one wins depends on the leakage of the part.

  NPTEL: Low Power VLSI Circuits and Systems (IIT Kharagpur) · Course

• Measuring current from microamperes to hundreds of milliamperes at once

  The interesting behaviour spans five orders of magnitude and lasts milliseconds, which defeats an ordinary meter. A shunt with a wide dynamic range instrument, or a dedicated analyser, is what makes the sleep current and the radio burst visible together.

  NPTEL: Design for Internet of Things (IISc Bangalore) · Course

• Battery chemistry, self-discharge and the cold end of the temperature range

  A cell's rated capacity assumes a temperature and a discharge rate you may not have, and self-discharge alone can dominate a device that sleeps for years. The budget has to be checked against the cell's own curves, not its headline number.