Computationally Enabled Minimum Total Energy Tracking for a Performance Regulated Sub-Threshold Microprocessor in 65-nm CMOS

This article describes an architecture that auto- nomously sets the operating supply voltage ( $V_{dd}$ ) of a digital system for minimum total energy dissipation, inclusive of regulator loss. The technique directly computes the energy per cycle (EPC) delivered by the supply to the system, to guide $V_{dd}$ adjustment for minimum energy-per-cycle point (MEP; voltage: $V_{MEP}$ and frequency: $f_{MEP}$ ) tracking. The proposed approach avoids significant $V_{dd}$ margins typically required for system operation near-threshold and at sub-threshold $V_{dd}$ levels by employing a unified clock and power ( UniCaP ) architecture that regulates target performance across process, voltage, and temperature (PVT) variation with minimal guardband requirements. Measurement results from a 65-nm CMOS test chip demonstrate the $V_{MEP}$ tracking with less than 5 mV of error within the operating voltage range of 0.38–0.58 V. Measurement results also demonstrate the MEP tracking with temperature variation from −30 °C to 105 °C across varied loading conditions over 20 different test chips.