DATA PROCESSOR

PURPOSE:To set a low energy consumption state without increasing external terminals at a microcomputer for card in an ISO standard and further to completely return from the low energy consumption state to an original state. CONSTITUTION:An interruption detecting circuit 5 is added to an I/O port 3 of the microcomputer having the low energy consumption state so as to detect the start bit of a transmitting data signal as an interrupted input. In the low energy consumption state, this interruption detecting circuit 5 is activated and the I/O port 3 is turned to an inactive state. In a normal operation state, the I/O port 3 is activated and the interruption detection circuit 5 is turned to the inactive state. In the low energy consumption state, an I/O terminal is functioned as an interruption terminal, and the state is changed into the normal operation state only by applying the transmitting data signal and interrupting a CPU. In the normal operation state, the I/O terminal is functioned as the input/output terminal of the transmitting data signal.