This paper presents a CMOS 40nm microcontroller where for the first time, stacked voltage domains are used. The system features an ARM Cortex M0+ processor, 4kB ROM, 16kB SRAM, peripherals, and an on-chip switched-capacitor voltage regulator (SCVR). By using voltage stacking the test chip achieves state-of-the-art (96%) power-conversion efficiency and observed power savings run from 23% to 63% depending upon the payload current, while supply voltage variations are reduced from 5.6mV to 3.8mV (RMS).@en