One-time PROM microcomputer

An OTP microcomputer comprising:

• a PROM having a terminal for receiving a voltage from a power supply Vpp that allows a program to be written therein;
• a mode-inputting means for selectively inputting a mode-setting signal which selects and designates one mode from a microcomputer mode in which the OTP microcomputer is operated as a normal microcomputer and an OTP mode in which said PROM is written in with a program, said inputting means comprising:
• a MOS transistor to be used as a resistance, and
• an input terminal connected to both said terminal of said PROM and a logic circuit of the microcomputer and connected with the drain of said MOS transistor;
• and
• a voltage supplying means supplying voltages in accordance with said input mode-setting signal of said mode-inputting means, wherein, at the time of the OTP mode, the voltage from said power supply Vpp for allowing a program to be written in is supplied to the well and source of said MOS transistor while, at the time of the microcomputer mode, a voltage from a device power supply Vcc (.vertline.Vpp.vertline.>.vertline.Vcc.vertline.) provided inside circuits of the microcomputer is supplied to the well and source of said MOS transistor.