Semiconductor memory device, method of controlling read preamble signal thereof, and data transmission system

A system, comprising a semiconductor memory device and a controller,nthe semiconductor memory device comprising:na system clock terminal configured to receive a system clock with a first frequency or a second frequency;a first external provided configured to receive a read command;a second external terminal provided to output a data strobe signal in response to the read command, the data strobe signal including a read preamble and a toggle transition following to the preamble;a third external terminal provided to output a read data in synchronization with the toggle transition of the data strobe signal; anda read preamble register that includes a bit area to store an information specifying a length of the read preamble of the data strobe signal, the controller comprising:a fourth external terminal provided to output the read command;a fifth external terminal provided to receive the read data;a sixth external terminal provided to receive the data strobe signal related to the read data; anda seventh external terminal provided to be capable of outputting the information specifying the length of the read preamble, the length of the read preamble including first and second lengths corresponding to the first and second frequencies, respectively, each of the first and second lengths being defined by a number of clock of the system clock.