A microcontroller unit (MCU) having a functional state, a reset state, and one or more assertable fault sources is described. Each fault source has its own fault source assertion count and its own fault source assertion limit; the MCU is arranged to perform the following sequence of operations in a cyclic manner: if one or more of the fault sources are asserted, pass from the functional state to the reset state and increase the respective fault source assertion counts by one increment; if one or more of the fault source assertion counts exceeds the respective fault source assertion limit, disable the respective fault source; and pass from the reset state to the functional state. A method of operating an MCU is also disclosed.