Power supply underpowering and negative power supply glitches are commonly used for the purpose of injecting faults into secure circuits. The related fault injection mechanism has been extensively studied: it is based on setup time violations. Positive power supply glitches are also used to inject faults. However, an increase of the supply voltage is not consistent with a mechanism based on setup time violation. Besides, no research work has yet identified the corresponding mechanism. In this work, we report the use of an embedded delay-meter to monitor the core voltage of a programmable device exposed to power supply glitches. It permitted us to gain a further insight into the mechanism associated with power glitches and also to identify the injection mechanism of positive power supply glitches.