Apparatus and method for verifying reliability of data read from memory device through clock modulation, and memory system including the same

A memory system including: a memory device looping back a first clock to generate a second clock and outputting read data that are read from a memory cell region of the memory device in synchronization with the second clock; and a memory controller generating the first clock that includes a plurality of modulation sections by performing a modulation operation on a source clock according to a specific scheme, outputting the first clock to the memory device, and receiving the read data in response to the second clock. The read data includes a plurality of section data corresponding to the plurality of modulation sections included in the second clock, respectively, and the memory controller verifies reliability of each of the plurality of section data included in the read data by performing a demodulation operation on the second clock according to the specific scheme.