SleepRider: a 5.5μW/MHz Cortex-M4 MCU in 28nm FD-SOI with ULP SRAM, Biomedical AFE and Fully-Integrated Power, Clock and Back-Bias Management

Abstract

Ultra-low-power microcontrollers (ULP MCUs) face a performance trade-off between energy-efficient computing during activity periods and low sleep power, associated with limited wake-up time and energy. Adaptive back-biasing in FD-SOI, along with near-threshold operation at ultra-low voltage, has brought significant improvements by dynamically shifting the minimum energy point (MEP) along the frequency axis. This work introduces a highly-integrated 64-MHz ULP Cortex-M4 MCU with 96-kB SRAM in 28nm FD-SOI. A clock and power management unit (CPMU) generates all internal supplies and clocks from a 1.8-V supply, while unified frequency and back-bias regulation (UFBBR) performs PVT compensation. Custom 16-kB ULP SRAMs achieve low read/write access energy, 1.2/0.84pJ/32-bit access respectively, and provide 0.98nW/kB ultra-low-leakage data retention. A low-power biomedical analog front-end enables biopotential monitoring. The MEP is 5.5μW/MHz (8.2μW/MHz including conversion losses). Sleep power is 7.7μW with retention of logic state and 32-kB memory.