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This brief presents a one cycle startup relaxation oscillator (RO) suitable for resource-constrained wearable and implantable applications. For improving energy efficiency without relying on a low supply voltage, we propose a self-synchronized power gating technique to completely shut down almost all the analog building blocks except near the clock toggling moment. We further relax the system startup energy and size by introducing the ratiometric threshold-referenced technique and deadlock avoidance circuit to quickly pre-bias the internal RC network without using extra on-chip references. The proposed RO is designed and fabricated in standard 0.18- $\mu \text{m}$ CMOS, with four distinct implementations at 1kHz, 10kHz, 100kHz, and 1MHz. Measurement results show that all the ROs can achieve one-cycle startup operation. The 100kHz RO, which occupies an active area of only 0.041mm2, achieves a line sensitivity of ±1.14%/V from 1.7 to 1.9V while exhibiting an energy efficiency of 2.7nW/kHz. After trimming on-chip resistors with complementary temperature coefficients (TCs), the 1MHz RO attains a measured TC of 82ppm/°C from–25 to 100°C, as well as a noise-power figure-of-merit (FoM) of 149 dBc/Hz@100kHz.