ECE Departmental Seminar - Recent advances in ultra-low-power crystal oscillators for real-time clocks

Ultra-low power (ULP) circuit design is a long-lasting circuit research topic that has gained much attention recently. Systems built with these ultra-low-power circuit blocks are promising solutions on the Internet of Things (IoT) sensing applications, as they can enable ubiquitous data acquisition at an extended battery life. Such a vision, once implemented, will enhance human beings’ ability to monitor the environment at minimum cost overhead, which can lead to more intelligent and automated decision-making.

This talk will focus on a specific ultra-low-power circuit building block, the 32.768kHz (32kHz for short) crystal oscillator. It is a typical solution for real-time clock generation in battery-powered IoT sensor nodes, which controls the timing of all the tasks performed at the node. Though the traditional Pierce oscillator circuit offers simple and elegant implementation, many recent works focus on the possibility to further lower the power consumption. One promising direction is to exploit the resonant tank’s time-varying nature to align energy injections at instants when energy efficiency is high, based on the famous impulse sensitivity function theory. However, implementations based on this are typically complicated in design, with a compromised robustness, and require large area. Recently, we discovered a simple and compact circuit design that allows self-adaptive conduction angle tuning inside a Pierce crystal oscillator to lower the power consumption, reclaim the robustness, with a tiny area. The prototype circuit, presented in ISSCC 2025, achieves a word-record-low 0.36 nW power consumption while occupying only 820 μm2 area in 28 nm CMOS technology.