US Patent 7378887 Semiconductor integrated circuit with power-on state stabilization

The present invention is directed to assure an initial state of a circuit until a power supply voltage is stabilized at the time of power-on and to prevent an output circuit of an external input/output buffer circuit from performing erroneous operation at the time of setting a predetermined register value or the like to an initial value. A power supply detecting circuit outputs a power supply voltage detection signal indicating that a power supply voltage supplied from the outside enters a predetermined state. A power on reset circuit receives the power supply voltage detection signal, instructs an initial setting operation of the internal circuit at a predetermined timing and, in response to completion of the initial setting operation of the internal circuit, changes an external input/output buffer circuit from a high impedance state to an operable state. Consequently, when the external input/output buffer circuit becomes operable, the initial setting of the internal circuit has already completed.