An 80 mV 0.54 nW, 32 kHz Crystal Oscillator Using Capacitive Positive Feedback in 180 nm CMOS for Real-Time Clock Applications

This paper presents an ultra-low voltage (ULV) and ultra-low-power (ULP) 32 kHz crystal oscillator (XO) designed in 180 nm CMOS technology for real-time clock (RTC) applications in energy-harvesting Internet of Things (IoT) systems. The proposed XO features a capacitively-coupled positive feedback transconductance <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(g_{m})$</tex> cell with 4 transistors and 2 capacitors, enabling operation at an ultra-low supply voltage of 80 mV while consuming only 0.54 nW of power at 37° C. The other operating mode incorporates on-chip regulation circuits with a higher voltage of 240 mV, allowing this work to operate in a wider range of temperatures <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(-20^{\circ} \mathrm{C} \sim 85^{\circ} \mathrm{C}\right)$</tex>. Compared to prior work, this work offers the lowest power consumption at 80 mV, advancing the feasibility of autonomous, long-term RTC solutions.