generation of the real-time keeping and system-standby clock. Both functions are universally demanded by various systems such as cellular phones, smart wearable devices, GPS, etc. High frequency stability against environmental variations is necessary to meet system requirements. To increase the system stand-by time under limited battery capacity, an ultra-low power crystal oscillator (XO) is strongly demanded. The Pierce oscillator, which adopts the XTAL as an inductive device in the loop and manages to make the loop unstable, is a common architecture for XO and sub-μW power consumption is achieved through amplitude control [1]. To reduce the extra power consumption induced by the large load capacitors required by the Pierce oscillator, a differential oscillator is proposed in [2]. Both works need amplifiers that operate in the linear region and consume static power, making it difficult to further reduce power. A DLL-assisted XO in [3]
MediaTek, Hsinchu, Taiwan, A 32.768kHz crystal (XTAL) with its oscillation circuit is widely adopted for the