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This letter presents an energy efficient, temperature-compensated frequency-locked loop (FLL) for use as an on-chip clock source. We first present a fully integrated FLL architecture that significantly improves energy efficiency by using a loop divider to boost the output frequency without requiring increased static power dissipation. We develop models for the FLL energy-per-cycle and temperature stability and use them to implement an energy-optimized and highly temperature-stable FLL design in 65-nm CMOS that achieves 20.3-ppm/◦C temperature stability from −20 ◦C to 60 ◦C and an energy efficiency of 44.6-fJ/cycle at 23 ◦C (45.3 nW at 1.016 MHz), which is the highest energy efficiency reported to date for a fully on-chip oscillator, regardless of architecture, operating frequency, or temperature stability.