Nonvolatile memory device and method of operating the same

A program method using a nonvolatile memory device, comprising:

• encoding external input data such that a difference between a number of data ‘0’ and a number of data ‘1’ in the input data is minimized;
• generating a DSV indicating the difference between the number of data ‘0’ and the number of data ‘1’ in the encoded input data;
• and
• storing the encoded data in a main cell unit of a page of a memory cell array and the DSV in a spare cell unit of the page.

A read method using a nonvolatile memory device, comprising:

• in a page of a memory cell array, using a main cell unit to store data encoded to minimize a difference between a number of data ‘0’ and a number of data ‘1’ in the encoded data and using a spare cell unit to store a digital sum value (DSV) indicating the difference between the number of data ‘0’ and the number of data ‘1’ in the encoded data;
• performing a read operation on the page using a first reference voltage;
• generating a DSV from the stored data of the main cell unit;
• setting a read voltage by comparing the stored DSV of the spare cell unit and the DSV generated from the stored data of the main cell unit;
• and
• performing another read operation on the page using the set read voltage when the set read voltage is different from the first reference voltage.