The following op-amp circuit is a type of relaxation oscillator that produces a square-wave output. The operation of this circuit relies on the RC time constant for the charging and discharging of the capacitor. During circuit start-up, the voltage at the capacitor is at ground. Because the capacitor connects to the inverting terminal of the op-amp, as the negative terminal of the op-amp gets to or slightly below ground, the op-amp output goes high. From that point, the capacitor voltage rises up as a function of the RC time constant. As the voltage rises to the non-inverting terminal, the op-amp output goes low and the RC circuit discharges with the RC time constant. The waveform below demonstrates the capacitor voltage as well as the op-amp's square wave output.
Square Wave Oscillators