A Low-Power 32.768 kHz Multi-Crystal Oscillator for General-Purpose Microcontrollers

This paper presents a low-power 32.768 kHz multi-crystal oscillator implemented in advanced 18 nm CMOS technology and integrated into a general-purpose microcontroller (MCU) environment. The oscillator operates over a wide supply voltage range from 1.6 V to 3.6 V and across a temperature range from −40∘C to 130∘C, meeting industrial requirements. The experimental results demonstrate a current consumption below 200 nA under typical conditions (i.e., 3 V and 25∘C), with reliable start-up and stable oscillations using various crystals that cover a broad range of crystal market characteristics. The oscillator supports crystals with equivalent series resistance (ESR) values up to 100kΩ and datasheet load capacitances ranging from 4pF to 12.5pF without amplifier reconfiguration. The design is based on a two-stage amplifier architecture that effectively reduces power consumption while improving frequency stability and predictability. The simulations show that the proposed architecture offers greater robustness than the classical Pierce architecture regarding oscillation frequency variations caused by potential crystal ESR fluctuations. These findings are validated by measurements, which exhibit a high correlation between the simulated and measured oscillation impedance behavior.