A 1.5 nW, 32.768 kHz XTAL Oscillator Operational From a 0.3 V Supply

This paper presents an ultra-low power crystal (XTAL) oscillator circuit for generating a 32.768 kHz clock source for real-time clock generation. An inverting amplifier operational from 0.3 V $V_{DD}$ oscillates the XTAL resonator and achieves a power consumption of 2.1 nW. A duty-cycling technique powers down the XTAL amplifier without losing the oscillation and reduces the power consumption to 1.5 nW. The proposed circuit is implemented in 130 nm CMOS with an area of $0.0625\ \text{mm}^{2}$ and achieves a temperature stability of 1.85 ppm/°C.