A semiconductor device includes a selection circuit and a phase detector. The selection circuit, in response to a first selection signal output from a controller, outputs as a timing signal a first clock signal output from the controller or an output signal of a PLL using the first clock signal as a first input. The phase detector generates a voltage signal indicating a phase difference between a second clock signal output from the controller and the timing signal output from the selection circuit. The semiconductor device further includes a data port, a memory core storing data, and a serializer, in response to the timing signal output from the selection circuit, serializing data output from the memory core and outputting serialized data to the controller via the data port. The controller generates the first selection signal based on at least one of the voltage signal and the serialized data.