Piezoelectric transducers provide a bidirectional channel for converting energy from mechanical form to electricalor in the opposite direction. Various applications were developed according to either of these two energy-flowprocesses. For example, energy harvesters take advantage of the energy flow from mechanical to electricaldomains, while vibration exciters take advantage of that from electrical to mechanical domains. Few of theformer designs have integrated the bidirectional energy-flow processes in a single device. Nevertheless, suchintegration can be very useful in some application scenarios. The key obstacle results from the interface circuitdesign, rather than the transducer. This paper makes a breakthrough by proposing a bidirectional energyconversion circuit (BECC) solution for the time-dividing energy harvesting and vibration exciting purposes. Thecircuit topology is derived from the synchronized triple bias-flip circuit, which was formerly used for the energyharvesting enhancement. The control logic for energy harvesting and vibration exciting modes are discussedin details. Two piezoelectric designs are studied for investigating the potential applications of BECC. For thelinear piezoelectric structure, the BECC can be used to provide vibration excitation and then reclaim a part ofthe injected energy. The proposed BECC can be also used to realize the controllable high-energy orbit exciterin the nonlinear piezoelectric energy harvesting systems. It is the first time to realize a compact and integratedself-excitable nonlinear energy harvester by using a single interface circuit. Simulations and experiments arecarried out for validating the performance of the BECC towards future engineering designs.