A Sub-nW Single-Supply 32-kHz Sub-Harmonic Pulse Injection Crystal Oscillator

An extremely low-power 32-kHz crystal oscillator (XO) based on a sub-harmonic pulse injection technique is presented. By injecting current pulses into the crystal at the sub-harmonic of the oscillation frequency, the power consumption of the proposed XO reduces with an increase in the order of sub-harmonic ( ${N}$ ). For the generation of sub-harmonic pulses with accurate timing, a 50%-duty-cycle-locked loop and a sub-harmonic peak-detection delay-locked loop are employed. A clock divider is also utilized to generate the sub-harmonic clock, which is required for the real-time clock as well. Implemented in a 55-nm CMOS technology with a triple well, the proposed XO, including a clock divider, dissipates 0.74 nW from a single 0.3-V supply for $N = 16$ and the chip occupies only 0.019 mm2.