This paper reports a regulation-free sub-0.5-V crystal oscillator (XO). The XO specifically designed for Bluetooth low-energy (BLE) radios aims for direct-powering by the harvested energy. To secure its performance against process, voltage, and temperature (PVT) variations, while reducing its startup time and energy, we propose a dual-mode g m scheme and a scalable self-reference chirp injection (SSCI) technique. The former employs an inductive multistage gm to mitigate the crystal's stray capacitance during the startup, but a single-stage gm in the steady state to preserve the phase noise (PN). For the latter (SSCI), we generate a scalable chirping sequence to kickstart the XO, avoiding trimming of the auxiliary oscillator. The XO fabricated in 65-nm CMOS is measured with two common crystals (16/24 MHz) over a 0.3-to-0.5-V supply. At 24 MHz and 0.35 V, the startup time and energy of the XO are 400 μs and 14.2 nJ, respectively, while showing a steady-state power of 31.8 μW and a PN of -134 dBc/Hz at 1-kHz offset. The frequency stability is 14.1 ppm against temperature (-40 °C-90 °C) and 17.9 ppm against voltage (0.3-0.5 V), both conform to the BLE standard (±50 ppm) with adequate margin.