The design of ultra-low-voltage microcontroller (MCU) systems with high energy-efficiency operations is a key concept to achieve fully autonomous energy-harvesting powered Internet-of-Things applications. In this paper, a system-on-chip (SoC) is presented, embedding an ARM® Cortex®-M0+ MCU, 2 × 4 KB SRAM, an ultra-low power frequency synthesizer, a custom power switch, and a power management unit enabling the active and sleep modes. The 28 nm fully depleted silicon-on-insulator (FD-SOI) technology has been used to fabricate the device. The whole system operates at a fixed voltage of 0.5 V, and can switch from active and sleep/deep sleep modes, adjusting its frequency from 16 to 8 MHz or 32 kHz in one cycle upon energy availability. Silicon measurements report an SoC's power consumption of 2.7 pJ/cycle at 16 MHz during active mode, and a total power consumption of 0.7 μW during deep sleep mode. By combining frequency and power modes switching with extra reverse body-biasing, the system power consumption is drastically reduced by 2× and 61× in, respectively, sleep and deep sleep modes.