US Patent 10332613 Nonvolatile memory system with retention monitor

A nonvolatile memory system, a nonvolatile memory controller and a method for assuring retention are disclosed. The nonvolatile memory controller includes a retention monitor that stores test characteristics corresponding to a use case and determines, each time that a read of a codeword is performed, whether the number of errors in the codeword exceed a retention threshold. If the number of errors in the codeword exceed the retention threshold, the block containing the codeword is retired. The retention monitor performs retention tests during the operation of the memory controller and adjusts the retention threshold when the results of the retention tests indicate deviation from the test characteristics corresponding to a use case.