This work presents the design and implementation of a P-SSHI-phi interface circuit for piezoelectric energy harvesting along with a calibration technique. The proposed design can operate in either energy-extraction mode or reverse-charging calibration mode. During calibration when the system is at rest, the reverse-charging method operates the interface in a backward manner and drives the transducer with an AC voltage source. The interface voltage and current are then recorded and further processed to derive the operation parameters that lead to the maximum power point. An interface circuit is implemented with a 0.25-μm HV CMOS process to validate the efficacy of the proposed calibration technique at both resonant and off-resonant frequencies. Measurement results with an in-house piezoelectric transducer at 3 different operating frequencies show that the energy harvester is able to extract at least 96.58% of the maximum power with the proposed calibration, out-performing the conventional VOC/2 method by at least 13.46%.