A micro-controller is connected to an external circuit via an address bus and a read control signal line. The external circuit includes an enable circuit, a decoder and a register group. The enable circuit produces an enable signal from the sixteenth bit of 16-bit information on the address bus and a control signal on the read control signal line. The decoder creates an address from the ninth to fifteenth bits of the 16-bit information. When the enable signal is valid, the register group writes a signal value of the first to eighth bits of the 16-bit information into a register specified by the address. Accordingly, the micro-controller can send the read control signal, the register address and the register data to the external circuit via the address bus. It is therefore possible to write data into the external circuit without using a write control signal line.