In this work, we propose an X0 design with frequency -divided (4kHz), high energy-to-noise-ratio pulse-injection oscillation (HERO). The performance of the X0 is compared with state-of-the-art X0s in Fig. 3.3.6. By allowing the crystal to run freely for a longer time between injections, HERO achieves a 2ppb Allan deviation floor, which is 5x lower than the deviation floors among the state-ofthe-art nW X0s. Furthermore, the less-frequent injections significantly reduce the injection overhead, enabling lowest-reported power consumption (0.51nW) among prior -art oscillators shown in Fig. 3.3.6. An integrated phase extraction and delay circuit obtains accurate injection alignment, resulting in stable operation from -25°C to 125°C, the widest reported range among the nW X0s shown in the comparison table.