A 0.36nW, 820μm², 32kHz Conduction-Angle-Adaptive Crystal Oscillator in 28nm CMOS for Real-Time Clock Applications

32.768kHz (32kHz) crystal oscillators (XOs) are widely used in real-time clocks (RTC) embedded in various electronic systems. Their performance is critical in battery-powered Internet-of-Things (IoT) sensor nodes when serving as wake-up timers. Extending the battery life requires an ultra-low-power (ULP) XO, as it must stay always-on. It also requires good frequency stability; otherwise, the system has to extend its communication guard times for synchronization, in which power-hungry transceivers need to stay active, leading to extra waste in power [1]. These demands have motivated much recent research to seek alternatives to the classic inverter-based Pierce oscillator, which is simple in design but typically consumes 10 to 100nW of power [2-5]. Pulse-injection XOs (PIXOs) are popular due to their sub-nW power consumption. Their power reduction comes from using a tiny conduction angle in the form of short pulses for Class-C-like operations. However, positioning current injections to the optimum timing typically requires generating an accurate delay, making pulse generators area-consuming [6-10]. Moreover, these methods cannot initiate the oscillation, as the zero-crossing instants, which are the starting times of the delay, are only available once an oscillation is present. These PIXOs, hence, need to embed separate start-up circuits to build up the initial oscillation, necessitating extra control logic and facing robustness challenges under accidental but strong interferences. A compact and easy-to-use ULP XO remains in demand from the cost-sensitive IoT market.