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Self-powered monitoring refers to a signal processing technique where the computational power is harvested directly from the signal being monitored. In this paper, we present the design and calibration of a CMOS event counter for long-term, self-powered mechanical usage monitoring. The counter exploits a log-linear response of the hot-electron injection process on a floating-gate transistor when biased in weak-inversion. By configuring an array of floating-gate injectors to respond to different amplitude levels of the input signal, a complete analog processor has been designed that implements a level counting algorithm, which is widely used in mechanical usage monitoring. Measured results from a fabricated prototype in a 0.5-??m CMOS process demonstrate that the processor can sense, store and compute over 105 usage cycles with an injection limit approaching one single electron per second and with a counting resolution of 5 bits. This paper also presents a calibration algorithm that is used for compensating the variations which arise due to device mismatch, power supply and temperature fluctuations. The maximum current rating of the fabricated analog processor has been measured to be less than 160 nA making it ideal for practical self-powered sensing applications.