A 510-pW 32-kHz Crystal Oscillator With High Energy-to-Noise-Ratio Pulse Injection

This article introduces a 32-kHz crystal oscillator (XO) with high energy-to-noise-ratio pulse injection at subharmonic frequency. A T/4-delay clock slicer is proposed to convert the sinusoidal crystal waveform into an output clock of 32 kHz and to introduce a delay of T/4, providing proper timing for energy injections. The output clock feeds frequency dividers and generates pulses to activate the proposed all-NMOS differential driver at 4 kHz. It enables two injections in eight periods at the peak and valley of the crystal oscillation, with the crystal running freely between injections. This configuration achieves a 2-ppb Allan deviation floor. The less frequent injections reduce the injection overhead, enabling the lowest reported power consumption of published nW XOs (0.51 nW). At 0.45 V, the proposed XO operates across a temperature range of $- 25\,\,^{\circ }\text{C}$ to 125 °C, the widest reported range for nW XOs. This design is fabricated in the 40-nm CMOS and occupies 0.02 mm ² .