3.3 A 0.5V 6.14μW Trimming-Free Single-XO Dual-Output Frequency Reference with [5.1nJ, 120μs] XO Startup and [8.1nJ, 200μs] Successive-Approximation-Based RTC Calibration

For miniature IoT nodes, it is desirable to develop a multi-output frequency reference from one quartz crystal to favor footprint and cost reduction. There have been attempts to utilize the MHz-range crystal oscillator (XO) to calibrate the integrated oscillator [1, 2], or to generate clocks for multiple purposes [3, 4]. The clock system in [1] provides an on-demand 76.8MHz clock with XO startup time (t <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> ) reduction, and an always-on 32.768kHz real-time clock (RTC) using a machine-learning-based RC oscillator. Yet, it involves heavy training and poses a high sleep-mode power (48.2μW). The XO-assisted RTC in [2] exhibits a 0.26ppm/°C TC, but its counter-based calibration takes a long time (10ms) and consumes high energy (567nJ). Also, there is no technique to accelerate the XO startup.