A ± 20-ppm -50°C-105°C 1-µA 32.768-kHz Clock Generator with a System-HFXO-Assisted Background Calibration

Abstract

A kHz-range clock is required in many applications. For example, an IoT device is in the sleep mode most of the time and often needs a kHz clock for the timer or time-stamp purposes [1]. For compact device size, implementing a kHz clock using a low-frequency crystal oscillator (LFXO) is not preferred because an extra kHz crystal (Xtal) is required [2]. Alternatively, the kHz clock can be generated by dividing a high-frequency XO (HFXO) signal through dividers. (An MHz-range HFXO is usually available to serve as the system clock for computation and communication purposes in an SOC.) However, the division approach requires the HFXO and dividers remain active even in the sleep mode, which consumes large power [3]. Some works exploit on-chip oscillators to produce a kHz clock. Such oscillators are PVT sensitive and prone to inferior frequency stability [4], [5]. MEMS-based clock generator achieves excellent performance [6]. However, this is at the cost of complex temperature trimming and an additional MEMS resonator.