A semiconductor device includes a phase difference detection circuit and an internal circuit. The phase difference detection circuit generates first and second phase difference detection signals by comparing a phase of a phase detection clock signal, generated from a command/address signal in synchronization with a clock signal, with phases of a division clock signal and an internal division clock signal that are generated by dividing a frequency of a data clock signal according to an operation mode. The internal circuit recognizes the phases of the division clock signal and the internal division clock signal according to a logic level combination of the first and second phase difference detection signals.