A 20.9-nW 32.768-kHz Real-Time Clock with 2.2-ppm Temperature Stability for Outdoor Wireless IoT Application

This paper presents an ultra-low power quartz crystal temperature-compensated real-time clock. By using a pulse injection oscillator (PIO) instead of a conventional Pierce oscillator, a multi-phase output is provided to improve digital compensation accuracy while reducing power consumption. The temperature compensation is achieved by dynamically selecting the multi-phase output of the PIO based on the lookup table data to offset the temperature drift, which improves the instantaneous compensation accuracy compared to the traditional digital frequency division-based method. An on-chip ring oscillator controlled by a proportional to absolute temperature current source using switched capacitor is implemented to obtain temperature information by counting its output frequency. Simulation based on the TSMC 65 nm CMOS process shows that the circuit has an absolute long-term compensation accuracy of 2.2 ppm and a total power consumption of 20.9 nW at typical operating conditions of 25°C and 0.6 V supply.