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With each iterative check, the valley search operation compares a result (e.g., number of activated memory cells or bit error rate) with a prior result for a previous check. If the candidate read level being checked/tested results in higher bit errors or more activated memory cells, the valley search operation may check a threshold voltage in an opposite direction than the direction that lead to the higher bit errors (activated memory cells). So, if the candidate read level #5 results in fewer activated memory cells than candidate read level #6, then the valley search operation may next test candidate read level #4. If the candidate read level #4 results in fewer activated memory cells than candidate read level #5, then the valley search operation may next test candidate read level #3. If the candidate read level #3 results in more activated memory cells than candidate read level #4, then the valley search operation may stop iterating and determine that candidate read level #4 is the "valley" between the Er memory state and A memory state. Consequently, the valley search operation may change a default read level from candidate read level #5 to candidate read level #4. Next, the valley search operation may follow this similar process for read level D 412, read level F 416, and read level K 426. For example, candidate read levels from read level window 604, read level window 606, and read level window 608 may be iteratively tested. Furthermore, of a valley search operation is done for all logical pages, eventually, all the read levels of the Vt window will be tested and potentially changed to obtain optimal read operation results. A valley search operation is an iterative trial and error method for determining how to adjust read levels.

Added by DJM 12 2021