Millimeter-scale nearly perpetual sensor system with stacked battery and solar cells

An 8.75 mm
³
sensor system is implemented with a near-threshold ARM Cortex-M3 core, custom 3.3 fW leakage-per-bit SRAM, two 1 mm
²
solar cells, a thin-film Li-ion battery, and an integrated power management unit. The 2.1 ¿W system enters a 100 pW data-retentive sleep state between sensor measurements and harvests energy from the solar cells to enable nearly perpetual operation.