Reported work on minimum-energy (ME) computing for low-power applications has focused entirely on tracking solely the microprocessor ME voltage supply. However, the use of low-power systems requires accounting for regulator losses, voltage monitors, biasing, peripheral, clock sources, and start-up energies to adapt the correct ME supply to different operation modes. Here we demonstrate a 32-bit RISC-V IMAC based microcontroller (MCU) in 180nm CMOS technology featuring a low-energy always-on (AON) subsystem extending on ME adaption by including peripherals. AON peripherals enable the MCU for low-duty-cycle sensor node applications. Low-energy clock sources and voltage monitors enable 32.768kHz to 55MHz operation and power-gate the MCU into three power states adjusted to work at the ME supply operation. Measured start-up energies using integrated RC-based oscillators show restarting energies down to 6pJ, which is 1000X less than the energy required in MCUs that apply crystal oscillators.