Abstract
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.
Highlights
T HE ACCURACY of synchronization in time plays a vital role in the modern society, such as the global positioning system (GPS), manufacturing, and stock markets
One critical starting point in creating low-cost and portable clocks is the invention of the quartz crystal oscillator by Walter Guyton Cady in 1921 [2]
A crystal oscillator (XO) is defined as an electric oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to generate an electrical signal of a constant frequency [3], [4]
Summary
T HE ACCURACY of synchronization in time plays a vital role in the modern society, such as the global positioning system (GPS), manufacturing, and stock markets. Because the available batteries and harvested energy are limited due to small form factors, power-hungry communication blocks (RX/TX) in these wireless nodes must be heavily duty cycled and synchronized to reduce average power while achieving reliable data transmission and reception, as shown in Fig. 2 [9]. This duty cycling and synchronization scheme is enabled by an alwayson sleep timer in each wireless node, and 32kHz XOs are a natural choice for these sleep timers owing to the properties discussed above.
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