An ultra-low power oscillator is designed for wake-up timers that can be used in compact wireless sensors, for example. A constant charge subtraction scheme removes continuous comparator delay from the oscillation period, which is the source of temperature dependence in conventional RC relaxation oscillators. This relaxes comparator design constraints, enabling low power operation. In 0.18 μm CMOS, the oscillator consumes 5.8 nW at room temperature with temperature stability of 45 ppm/° C. (−10° C. to 90° C.) and 1%/V line sensitivity.