Apparatus and method for two-step read of resistive random access memory

A method comprising:

• receiving a command to read a first codeword from a memory array comprising a plurality of groups of Magnetoresistive Random Access Memory (MRAM) cell and a plurality of codewords that collectively comprise a corresponding first predetermined read reference value, each of the codewords comprising a corresponding second predetermined read reference value wherein the first predetermined read reference value is selected based on a lowest number of bit errors among the plurality of groups of MRAM when read with a plurality of candidate read reference values;
• reading the first codeword using the first predetermined read reference value to provide first read data;
• while reading the first codeword, retrieving from a lookup table the second predetermined read reference value corresponding to the first codeword, wherein the second predetermined read reference value is one of a plurality of second predetermined read reference values, each of the plurality of second predetermined read reference values corresponding to each candidate read reference value with codeword having a number of failed bits above a predetermined maximum number of failed bits;
• decoding the first read data using an error correction code algorithm;
• determining that the error correction code algorithm was unable to correctly decode the first read data;
• and
• reading the first codeword using the retrieved second predetermined read reference value to provide second read data.

A system comprising:

• a control circuit configured to:
• receive a command from a host device to read a first groups of Magnetoresistive Random Access Memory (MRAM) cells in a memory array comprising a plurality of groups of memory cells;
• perform a first read of the first group of MRAM cells using a first predetermined read reference value wherein the first predetermined read reference value is selected based on a lowest number of bit errors among the plurality of groups of MRAM when read with a plurality of candidate read reference values;
• retrieve a second predetermined read reference value from a lookup table, wherein the second predetermined read reference value is one of a plurality of second predetermined read reference values, each of the plurality of second predetermined read reference values corresponding to each candidate read reference value with a group of MRAM memory cells having a number of failed bits above a predetermined maximum number of failed bits;
• and
• selectively perform a second read of the first group of MRAM cells using the second predetermined read reference value only if data read during the first read failed to correctly decode,
• wherein:
• the retrieve step is performed in parallel with the first read step;
• the first predetermined read reference value minimizes a failed bit count across all of the groups of MRAM cells;
• and
• the second predetermined read reference minimizes a failed bit count of the first group of MRAM cells.