Memory system and method having selective ECC during low power refresh

A method of reducing the power consumed by a memory device, comprising:

• refreshing memory cells in the memory device at a first rate when the memory device is active;
• refreshing at least some of the memory cells in the memory device at a second rate when the memory device is inactive, the second rate being substantially slower than the first rate;
• prior to transitioning from the first rate to the second rate, transitioning to an ECC protection mode by:
• determining which memory cells are storing data that should be protected from data retention errors;
• reading data from the memory cells determined to be storing data that should be protected;
• generating ECC syndromes corresponding to the read data;
• and
• storing the generated syndromes;
• and
• transitioning from the second rate to the first rate and thereafter continuing to refresh the memory cells that are storing data that should be protected at the second rate without using the ECC syndromes to determine if any of the data stored in the memory cells are in error until a request is received to read data from the memory cells that are storing data that should be protected;
• when a request is received to read data from at least some of the memory cells that are storing data that should be protected:
• reading the data that are the subject of the read request from the memory cells that are storing the read data;
• reading the stored ECC syndromes corresponding to the read data;
• using the syndromes to determine if any of the read data are in error;
• correcting any read data found to be in error to provide corrected data;
• storing the corrected data in the memory cells;
• and
• outputting the corrected data from the memory device.