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hardEmbedded Systems Engineer interview question

Design the firmware for a battery-powered sensor that must run for 5 years on a coin cell and report data every 15 minutes.

What the interviewer wants to hear

  • Power budget calculation from battery capacity and duty cycle
  • Deep sleep most of the time, wake by RTC
  • Minimise radio on-time: batch data, efficient protocol
  • Turn off peripherals and configure pins correctly
  • Measure real current with a power profiler
  • Handle battery voltage drop, brownout and robust recovery

Model answer

I start with a power budget. A CR2032 has about 220 mAh, and 5 years is roughly 43,800 hours, so the average current must be under about 5 uA, minus margin for self-discharge and temperature. That means the device sleeps almost always. I pick an MCU and radio with sub-microamp sleep and an RTC wake-up. Every 15 minutes it wakes, powers the sensor, takes a reading, and goes back to sleep. The radio is the biggest consumer, so I batch several readings and transmit less often if allowed, use a low-power protocol like BLE advertising, LoRaWAN or Zigbee, and keep packets short. Before sleep, I turn off peripheral clocks, put unused GPIOs in analog mode and cut sensor power with a load switch. I measure the real current profile with a power analyser like a Nordic PPK2, check the coin cell can handle radio current peaks, often with a capacitor, and handle brownout and low-battery reporting gracefully.

Can you answer it under time pressure?

Take a timed hard round and get scored on every answer.

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