A semiconductor device having a circuit that selectively adjusts an impedance of an output buffer. A calibration operation can be performed automatically without issuing a calibration command from a controller. Because a calibration operation to a memory is performed in response to an auto refresh command having been issued for a predetermined number of times, a periodic calibration operation can be secured, and a read operation or a write operation is not requested from a controller during a calibration operation. A start-up circuit activates the calibration circuit when a refresh counter indicates a predetermined value, and prohibits a refresh operation in response to the auto refresh command when the calibration circuit is activated. A temperature detecting circuit may be used to change the frequency of performing a calibration operation.