This work details the development of an ultra-low-power firmware for an IoT system, aiming for battery-less operation powered by ambient energy (Energy Harvesting). Using the nRF52840 SoC and the Zephyr RTOS, an eventdriven architecture was developed that manages high-consumption resources, such as the high-frequency clock, on an on-demand basis. The project overcame the lack of documentation for the ProMicro nRF52840 board by creating a custom hardware configuration. Experimental validation quantified the impact of debugging tools, as disabling them reduced the idle current from 1.94 mA to 1.10 mA. A fixed and high energy cost at startup, caused by the board’s bootloader, was also identified. It is concluded that although the software strategy is effective, the prototyping hardware limits performance, making the design of a custom PCB a requirement for a viable final application.