A memory system includes a logic circuit and a phase locked loop (PLL) circuit. The logic circuit determines a first frequency of a first clock using a first signal and generates a second signal for adjusting the first frequency of the first clock. The PLL circuit receives a second clock, and generates the first clock having the first frequency determined by the logic circuit, using the second clock and the second signal. When a second frequency of the second clock varies, the logic circuit determines the first frequency of the first clock such that the first frequency of the first clock generated by the PLL circuit is uniform, and operates based on the first clock having the first frequency adjusted by the second signal.