Piezoelectric materials are versatile. They were made into various successful engineering products, such as sensors, actuators, and generators. However, almost all of the previous designs were found only capable of one specific function among those three. From a more fundamental physical point of view, a piezoelectric element, as a transducer, provides a bidirectional channel for energy conversion between mechanical and electrical domains. It makes no bias to any of those functions. This paper analyzes the obstacle against developing multi-functional piezoelectric devices, clears that barrier, and further proposes a switched-mode interface circuit towards an unprecedented multi-functional piezoelectric design. It was developed based on a buck-boost switched-mode circuit. By properly controlling the switch actions, this interface circuit can realize all functions of dynamics sensing, energy harvesting (generation), and vibration excitation (actuation) in a time-sharing manner. The dynamics sensing function is realized according to the time ratio between piezoelectric capacitor discharging (through an inductor) and the inductor’s freewheeling. The discharging period is set to be as short as possible to avoid much interference to the stored charge of the piezoelectric capacitor, which is related to the dynamic displacement. The energy harvesting function is carried out by discharging the piezoelectric capacitor through an inductor (more drastically than that in sensing mode). The extracted energy in the inductor then freewheels into a storage capacitor. For the vibration excitation function, the control logic reverses. It first pumps energy from the storage capacitor to the inductor, then freewheels the energy to the piezoelectric capacitor. The actual control scheme is more complicated because the piezoelectric voltage is an ac output. The multi-functional design should comprehensively consider the discharging and freewheeling requirements under positive and negative polarities. Under these working principles, a prototyped multi-functional piezoelectric interface circuit is implemented and tested. The three functions are successfully demonstrated. By making a comprehensive collaboration between piezoelectric transducers and power electronics, this design starts a new chapter for the applications of piezoelectric materials toward multi-functional engineering designs.