This article reports a 12-/13.56-MHz fast-and-energy-efficient startup crystal oscillator (XO) featuring a binary-search-assisted two-step injection technique for ultralow-power duty-cycled radios. Specifically, after the first injection, a calibration module refines the auxiliary oscillator’s frequency toward the crystal’s resonant frequency in a binary-search fashion with a sub-500-ppm error for the second injection. This calibration method eliminates the closed-loop phase-locked loop (PLL) and shortens the calibration period into 48 clock cycles. Prototyped in a 65-nm CMOS process, the XO occupies an active area of 0.134 mm2. The 13.56-MHz XO can achieve a startup time ( $t_s)$ of $45.8 \mu$ s with a $V_{\mathrm {DD}}$ of 0.7 V, while consuming 5.0 nJ of energy. $t_s$ varies by ±2.2% amid temperature variations (−40 °C to 85 °C). Its steady-state power consumption is $28.4 \mu$ W, with an output phase noise of −143.7 dBc/Hz at 1-kHz offset (FoM: 241.8 dBc/Hz), manifesting its state-of-the-art performance.