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The authors describe a low-voltage, low-power, fast system wakeup circuit, with no requirement for an external component in the RC oscillator. The wakeup circuit includes a POR (power-on-reset) circuit to detect the status of the power supply (VCC), a constant Gm bias circuit to bias the ring RC-oscillator, and a clock control unit to ensure stable clock frequency output. The ring RC-oscillator with five-stage inverter cell was designed for 1.2 MHz output frequency, with VCC ranging from 1.1 V to 2.0 V. The test chip was fabricated using a 0.18 μm 1P3 M standard CMOS logical process, with 130 μm×240 μm active area. Measurement results showed that the system wakeup circuit had 3 μs wakening time and 9.2 μW power consumption for 1.2 V power supply. The oscillator may be operated below 1 V if the POR detecting voltage is set below 1 V. Thus, it is suitable for embedding in portable device systems.