A processor, comprising a processing unit having an active state and a sleep state in which at least one of its sub-sections is inactive and a communication port adapted to receive signals from external units over a bus, which is configured to be not fully operative when the processor is in the sleep state. The processor additionally includes a bus monitoring unit configured to stall the bus responsive to identifying transmissions on the bus directed to the communication port, while the processing unit is in the sleep state and to indicate to the communication port that a transmission started while it was in the sleep state.