A data input circuit includes a first data input unit, a second data input unit, and a clock unit. The first data input unit is configured to receive external data at rising edges of a data strobe signal and output the external data as first internal data in response to an internal clock. The second data input unit is configured to receive the external data at falling edges of the data strobe signal and output the external data as second internal data in response to the internal clock. The clock unit is configured to generate the internal clock using an external clock signal.