Abstract: The PLL includes a selection signal generator configured to output a selection signal varying in response to a first clock signal, and a first dividing circuit configured to divide an externally input reference clock signal by a division ratio and output a first division signal. The first dividing circuit selects one of a plurality of edges of the reference clock signal applied for at least one cycle of the first division signal in response to the selection signal, and synchronizes and generates the first division signal on the basis of the selected edge of the reference clock signal. A second dividing circuit is configured to receive an output clock signal, divide the output clock signal by a division ratio, and output a second division signal.

Abstract: An output data strobe signal generating method and a memory system that includes a plurality of semiconductor memory devices, and a memory controller for controlling the semiconductor memory devices, wherein the memory controller provides a command signal and a chip selecting signal to the semiconductor memory devices. One or more of the semiconductor memory devices may detect a read command and a dummy read command in response to the command signal and the chip selecting signal and generate one or more preamble signals based on a calculated preamble cycle number.