Embedded Systems Development

Bare-metal firmware runs directly on hardware without an OS the main function contains a super-loop (infinite while loop) that checks state flags and calls functions, supplemented by interrupts for time-critical events. Minimal overhead, deterministic timing, appropriate for devices with one or two concurrent tasks. RTOS firmware uses a scheduler to run multiple tasks with distinct priorities each task has its own stack and execution context. RTOS is appropriate when: three or more activities run concurrently with different timing requirements (a communication task must respond in 100ms but a logging task can wait 1 second), or the bare-metal super-loop grows unmaintainably complex as features are added. FreeRTOS adds approximately 5-10KB flash and 500 bytes RAM overhead trivial on modern MCUs like STM32 with 64KB+ RAM.

For a CR2032 coin cell battery with 220mAh capacity, 1-year battery life requires average current <25µA (220mAh / 8760h ≈ 25µA). For safety margin and typical battery self-discharge, target <10µA average current. Achieving this requires: careful sleep mode selection (Stop mode (20µA) vs Standby (2µA) vs Shutdown (<1µA) trade-off between wake latency and current draw), peripheral power gating (disable SPI clock when idle, gate sensor VCC between measurements inactive sensors still draw quiescent current), wake-up sources (RTC alarm for periodic measurement (once per minute), external interrupt for user event), current profiling (Nordic Power Profiler Kit II measures current vs time identify unexpected active periods, unexpected high sleep current from un-gated peripherals). ClickMasters achieves <10µA for most BLE sensor applications, <5µA for simple periodic measurement devices.