A microcontroller operable in a high power mode and a low power unit (LPU) run mode includes primary and LPU domains, primary and LPU mode controllers, and primary and LPU clock generator modules. The primary domain includes a first set of circuits and a first set of cores. The LPU domain includes second and third sets of circuits, a second set of cores, and a switching module. In the high power mode, the switching module connects the first and second sets of cores to at least one of the first, second and third sets of circuits, while in the LPU run mode, the switching module isolates the LPU domain from the primary domain and activates a small microcontroller system (SMS) that includes the LPU domain, the LPU mode controller and the LPU clock generator module. The SMS has further low power modes within the LPU run mode.