Integrated circuits for wireless sensor nodes (WSNs) targeting the Internet-of-Things (IoT) paradigm require ultralow-power consumption for energy-harvesting operation and low die area for low-cost nodes. As the IoT calls for the deployment of trillions of WSNs, minimizing the carbon footprint for WSN chip manufacturing further emerges as a third target in a design-for-the-environment (DfE) perspective. The SleepWalker microcontroller is a 65-nm ultralow-voltage SoC based on the MSP430 architecture capable of delivering increased speed performances at 25 MHz for only 7 μW/MHz at 0.4 V. Its sub-mm² die area with low external component requirement ensures a low carbon footprint for chip manufacturing. SleepWalker incorporates an on-chip adaptive voltage scaling (AVS) system with DC/DC converter, clock generator, memories, sensor and communication interfaces, making it suited for WSN applications. An LP/GP process mix is fully exploited for minimizing the energy per cycle, with power gating to keep stand-by power at 1.7 μW. By incorporating a glitch-masking instruction cache, system power can be reduced by up to 52%. The AVS system ensures proper 25-MHz operation over process and temperature variations from -40 °C to +85 °C, with a peak efficiency of the DC/DC converter above 80%. Finally, a multi-V_t clock tree reduces variability-induced clock skew by 3 × to ensure robust timing closure down to 0.3 V.