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A fully on-chip CMOS relaxation oscillator (ROSC) with a PVT variation compensation circuit is proposed in this paper. The circuit is based on a conventional ROSC and has a distinctive feature in the compensation circuit that compensates for comparator's non-idealities caused by offset voltage and delay time. We also developed a bias circuit consisting of positive and negative temperature coefficient resistors to obtain the temperature compensated clock frequency. Measurement results demonstrated that the circuit can generate a stable clock frequency of 6.66 kHz. The power dissipation was 940 nW. The measured line regulation and temperature coefficient were 0.98%/V and 56ppm/°C, respectively.