Ultra-Low Power 32kHz Crystal Oscillators: Fundamentals and Design Techniques

One of the challenges to the proliferation of Internet of Things is ultra-low power circuit design. Wireless nodes common in IoT applications use sleep timers to synchronize with each other and enable heavy duty cycling of power-hungry communication blocks to reduce average power. 32kHz crystal oscillators remain the most popular choice for sleep timers thanks to their frequency stability, simplicity, and low cost. Because sleep timers must be always on, their power consumption must be low compared to the average power of wireless nodes. Meantime, 32kHz crystal oscillators must operate reliably under process, voltage, and temperature variations and exhibit good long-term stability, which make circuit design challenging considering their ultra-low power operation. This paper reviews the state-of-the-art in ultra-low power 32kHz crystal oscillators. Fundamentals of crystal oscillators are introduced and analyzed from the perspective of power and frequency stability. Based on these fundamentals and analyses, existing design techniques of 32kHz crystal oscillators are discussed, highlighting the evolution of architectures in ultra-low power 32kHz crystal oscillators. Finally, research directions related to 32kHz crystal oscillators are introduced.