In this paper, the susceptibility of an 8-bit microcontroller to electrostatic discharge (ESD) and electrically fast transients was tested by injecting currents through a capacitive probe into the microcontroller package pins. The reaction of the microcontroller to discharges with different rise times and polarities were investigated by measuring the voltage on the tested pins and by observing the microcontroller's clock output. Susceptibility varied significantly when injecting to one pin compared to another. Interestingly, the clock was more sensitive to currents injected into I/O pins than into pins directly related to the clock (e.g. EXTAL). Further work is underway to explain the causes of susceptibility inside the IC.