According to the system of the present invention, data (DQ) signals are outputted/received between a controller 100 and a memory 200 based on a data strobe signal sent out from the controller 100. The data strobe signal is independently and completely separated from a clock signal. The data strobe signal has a frequency different from a clock signal. Therefore, the memory 200 is not required to generate a read data strobe signal from the clock signal nor to send the read data strobe signal in synchronization with the clock signal.