32.768 kHz Real-Time Oscillator in a General-Purpose Microcontroller Environment Whatever the Quartz Crystal

This paper presents a low-power 32.768 kHz multi-crystal oscillator implemented in an advanced 18 nm CMOS technology, 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. Experimental results demonstrate a current consumption below 200 nA under typical conditions (i.e., 3 V, 25°C), with reliable start-up and stable oscillations using various crystals that cover the full range of crystal market characteristics. The oscillator supports any crystals with equivalent series resistance (ESR) values from 20 kΩ and 100 kΩ and load capacitance between 4 pF and 12.5 pF without amplifier configuration. The design is based on a two-stage amplifier architecture that effectively reduces power consumption while improving frequency stability and predictability. 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 simulated and measured oscillation impedance behavior.