Resistive random access memory (rram) device, write verify method and reverse write verify method thereof

A reverse write verify method of a resistance random access memory (RRAM) device, comprising:

• comparing a first resistance value sensed from a RRAM cell with a plurality of reference resistance values to obtain a comparison value;
• performing a reset operation on the RRAM cell by applying a first reset pulse to change the first resistance value to a second resistance value;
• comparing the second resistance value with the comparison value to determine whether to continue the reset operation on the RRAM cell;
• determining that the second resistance value is not greater than the comparison value, stopping the reset operation and setting the set flag for the RRAM cell;
• performing a set operation on the RRAM cell that has a set flag after reset operation on another RRAM cell is completed;
• re-performing a reset operation on the RRAM cell that has the set flag after the set operation on the RRAM cell is completed;
• and
• determining whether the first resistance value is smaller than a first reference resistance value and greater than a second reference resistance value,
• when the first resistance value is smaller than the first reference resistance value and greater than the second reference resistance value, keeping a local counter of the RRAM cell at a counting value corresponding to the first reference resistance value, and setting the first reference resistance value to the comparison value.

A resistive random access memory (RRAM) device, comprising:

• a plurality RRAM cells;
• a comparator, configured to compare a first resistance value sensed from a RRAM cell among the plurality of RRAM cells with a plurality of reference resistance values to obtain a comparison value;
• a local counter, coupled to the comparator and configured to keep a counting value corresponding to the comparison value;
• a memory controller, configured to perform a set operation on the RRAM cell to change the first resistance value to a second resistance value, compare the second resistance value with the comparison value to determine whether to continue the set operation on the RRAM cell, determining the second resistance value is not smaller than the comparison value, stopping the set operation on the RRAM cell and setting the reset flag for the RRAM cell, perform a set operation on the RRAM cell that has a set flag after reset operation another RRAM cell is completed and re-perform a reset operation on the RRAM cell that has the set flag after the set operation on the RRAM cell is completed,
• determining whether the first resistance value is smaller than a first reference resistance value and greater than a second reference resistance value, when the first resistance value is smaller than a first reference resistance value and greater than a second reference resistance value, keeping a local counter of the RRAM cell at a counting value corresponding to the second reference resistance value, and setting the second reference resistance value to the comparison value.