Estimation of level-thresholds for memory cells

A method for detecting codewords each having N qary symbols which are stored in respective q-level memory cells, the method comprising:

• reading a plurality of the memory cells to obtain respective read signal components, the memory cells storing respective qary symbols of N-symbol codewords of a predefined set of valid codewords;
• processing the read signal components in dependence on signal level to produce a signal level vector, comprising a series of elements, indicative of the distribution of read signal components in order of signal level;
• scanning the signal level vector with a sliding window of length greater than the spacing of successive window positions in the scan;
• at each window position, calculating a metric dependent on the elements of the signal level vector in the window;
• detecting a codeword corresponding to each said read signal in dependence on the or each level-threshold so determined;
• and
• determining a level-threshold for successive memory cell levels in dependence on variation of said metric over the scan.

An apparatus for level-threshold determination on readout of q-level memory cells, the apparatus comprising:

• a vector generator for receiving a plurality of read signal components, obtained by reading respective memory cells, and processing the read signal components in dependence on signal level to produce a signal level vector, comprising a series of elements, indicative of the distribution of read signal components in order of signal level;
• a sliding-window scanner for scanning the signal level vector with a sliding window of length greater than the spacing of successive window positions in the scan, and, at each window position, calculating a metric dependent on the elements of the signal level vector in the window;
• and
• a threshold identifier for determining a level-threshold for successive memory cell levels in dependence on variation of said metric over the scan.