A data processing unit includes a first power supply consisting of a stable power supply, a central unit operating on the firsts power supply, a second power supply, a peripheral unit operating on the second power supply, an interface block for transferring an external signal to the peripheral unit, and a power-down detector operating on the first power supply and connected to the second power supply, for detecting a power down of the second power supply and for generating a signal informing of the power down when it occurs. This power down signal can prevent supply of the external input signal to the central unit. The data processing unit solves problems of conventional data processing units including separate central unit and peripheral power supplies in that an unexpected power down of the peripheral use power supply can cause malfunctions of the central unit because of, e.g., an undesired interrupt signal.