An input/output (I/O) buffer circuit includes an I/O unit, a first register and a second register. The I/O unit selectively transmits digital signals and analog signals according to a first enable signal, and selectively receives signals and outputs signals at an I/O terminal according to a second enable signal. The first register latches a first control signal received before power is turned off, and outputs the first enable signal corresponding to the first control signal to the I/O unit when power is turned on. The second register latches a second control signal received before power is turned off, and outputs the second enable signal corresponding to the second control signal to the I/O unit when power is turned on.