During software development, a multichip module is used which encloses a target chip and a development chip in one package. A CPU of the development chip fetches instructions from a flash memory in the chip to execute them, and accesses RAM and peripheral circuits in the chip. For motor control circuits that exist only in the target chip, the CPU accesses them via serial communication. When the one-chip microcomputer operates alone, its CPU switches a switching circuit to a JTAG interface side to actuate a motor control circuit via internal serial communication.