Sub-nW Microcontroller With Dual-Mode Logic and Self-Startup for Battery-Indifferent Sensor Nodes

In this article, a battery-indifferent microcontroller unit (MCU) with wide power-performance scaling is presented to enable continuous operation with a sub-mm 2 on-chip solar cell, even when the battery runs out of energy. The MCU is based on dual-mode standard cells to extend power-performance flexibility well beyond allowed by voltage-scaled CMOS logic. Ripple power gating is introduced to allow the processor to boot under scarce light intensity and no battery energy. The dual-mode logic is experimentally characterized at the gate level, analyzing the underlying tradeoffs among power, energy, delay, and noise margin. Compared with voltage-scaled CMOS logic, dual-mode logic is shown to reduce the minimum power/gate by 750 \times to enable purely harvested operation while having comparable energy/gate to prolong battery-powered operation. The proposed ripple power gating self-startup technique enables MSP430-compatible MCU cold start with an on-chip 0.54-mm 2 solar cell at a dim 55-lx light condition.