A Single-XO Dual-Output Frequency Reference Featuring Adaptive RTC Calibration Achieving 0.63 ppm/°C Temperature Coefficient From –40 °C to 125 °C

Abstract

This brief presents a trimming-free single-crystal dual-output (SXDO) frequency reference featuring adaptive binary-search (ABS)-based real-time clock (RTC) calibration. The on-demand 16-MHz crystal oscillator (XO) can serve as a frequency reference to calibrate the always-on 1-MHz on-chip RTC. To suppress the inaccurate calibration in the wake of the oscillator’s jitter, we propose the ABS-based calibration to adjust the accumulation cycles depending on the frequency difference between the XO and RTC to enhance the comparison accuracy. Further, we reuse the accurately calibrated RTC signal as a reference to generate a 16-MHz pulse train and inject it into the crystal to expedite the XO startup. We implemented the proposed SXDO frequency reference in a 65-nm CMOS process. The RTC achieves a temperature coefficient of 0.63 ppm/°C across -40 to $125~^{\circ }$ C. The calibration takes $720~\mu $ s while consuming 40.3 nJ of energy. The XO, benefitting from the RTC-assisted constant injection with improved accuracy, can exert injection of $60~\mu $ s and attain a startup time of $145~\mu $ s, with a startup energy of 9.9 nJ.