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Low-Power Design interview questions

Low-Power Design questions a Embedded / hardware can be asked, from first job to senior. Open one to see the strongest answer and why it works.

  1. Your MCU offers several sleep modes. What actually differs between them?

    The strongest answer: How much stays alive: deeper modes stop more clocks and power down more peripherals, may lose RAM contents, leave fewer sources able to wake you, and take longer to come back

    Why it works: Right — the choice is a set of trade-offs: retention, wake sources, wake latency, and what your peripherals need.

    • Deeper sleep costs retention and wake speed.
    • List the wake sources you need.
    • Check which RAM is retained.
    • Budget the wake latency.

  2. Battery dies in hours, not months. What do you measure first?

    The strongest answer: Sleep current and what you left awake — clocks, pull-ups, a chatty radio — I do not start by blaming the cell chemistry slide

    Why it works: Exactly. Quiescent first. Bigger cells hide leaks. BOD-off on empty is data corruption.

    • Measure sleep current.
    • Find what you left awake.
    • Do not start with a bigger cell.
    • Do not disable BOD to sip the last mV.

  3. How do you talk power without a fake “months of battery” slide?

    The strongest answer: Sleep current, duty cycle I measured, and what radio/sensor policy I chose — I do not invent a month count

    Why it works: Exactly. Measured pieces. Invented months and theoretical mAh are ads.

    • Measure sleep and duty.
    • Name radio/sensor policy.
    • No invented month count.
    • Do not sack BOD for a slogan.

  4. A sensor should sleep between readings, but the average current says it is barely sleeping at all.

    How do you find what is waking it?

    The strongest answer: Read the wake-cause register the moment it comes back, and toggle a spare GPIO on sleep entry and exit so a scope shows every wake lined up against the current trace

    Why it works: Exactly — measure before you guess. An interrupt flag left pending before sleeping wakes you instantly, and only the cause register shows that.

    • Instrument the wake before changing anything.
    • Read the wake-cause register.
    • Toggle a GPIO on entry and exit.
    • Clear pending flags before sleeping.

  5. A product team wants a coin-cell sensor to run for years, and the first prototype is nowhere close.

    Which levers do you actually pull?

    The strongest answer: How often we wake and how long we stay awake — the radio dominates, so fewer and shorter transmissions beat any sleep-current tuning; then check the cell survives the transmit pulse, usually with a reservoir capacitor

    Why it works: Right — duty cycle is the lever, and a coin cell has enough internal resistance that the pulse, not the average, is what kills it.

    • Duty cycle beats sleep-current tuning.
    • Wake less often; stay awake less.
    • Radio airtime is the budget.
    • Check the cell survives the pulse.

  6. Who owns battery life in a device company?

    The strongest answer: A named owner across radio, firmware, and mechanics — I do not let each team quote a month in isolation

    Why it works: Exactly. One throat. Isolated month-quotes are how the box lies.

    • Name an owner across radio/FW/mech.
    • No isolated month quotes.
    • Marketing does not own the mA.
    • The cell vendor is not the product.