Energy harvesting is a method that extracts electrical energy from the environment. This paper presents an integrated circuit in 0.35-mu m CMOS that harvests energy from mechanical vibration using a piezoelectric transducer. The circuit applies a bias-flip rectifier to improve the efficiency of the energy extraction. The paper focuses on the design of the key element of the bias-flip rectifier, the zero-crossing detector. It detects the zero crossing of the input current from the piezoelectric transducer and generates the control signals for the bias-flip rectifier. Post-layout simulations show a very low power consumption and high efficiency of the harvester.