A 339-nW 32.768-kHz DFLL-Based Reference Clock Generator with an Embedded Temperature Sensor

This paper proposes a 32.768-kHz reference clock generator with an embedded temperature sensor (TS). The kHz-clock generator consists of an RC-based relaxation oscillator that is calibrated with an MHz-range crystal oscillator (XO). The calibration is based on a digital frequency-locked loop (DFLL) and operates in a duty-cycled manner. The embedded TS extracts the temperature information by monitoring the DCO control codes, which is inherently available in the system. Fabricated in a 90-nm CMOS process, this chip consumes an average power of 339 nW at 0.2% duty cycle. The clock signal achieves a frequency stability of +25/-63 ppm over a temperature range of -40 to 85 °C. The embedded TS achieves 1°C temperature resolution; it provides additional value to the system without extra overhead.