| 摘要 |
<p>PURPOSE:To read the state of an ROM, provided in a chip, externally without providing a dedicated terminal, by connecting a two-terminal element for every bit of the ROM and a switching circuit, which turns on at a too low voltage for the chip to operate only when a clock is inputted. CONSTITUTION:With regard to an ROM in a chip (semiconductor device) which is never read during the normal operation of the chip, a power source Vcc, a two-terminal element for every bit of the ROM, and a switching circuit A are connected in series. Namely, when the power source Vcc has a voltage in normal operation, a transistor (TR) Q2 turns on and a TRQ1 turns off, so that the ROM is isolated. To read the state of the ROM, the voltage of the power source Vcc is made too low for the chip to operate and a clock CLK is applied to turn on the TRQ1, discriminating it by the presence of its current. Thus, the state of the ROM is read externally without providing a dedicated terminal, so terminal pins of the chip are not used for that.</p> |
