In one embodiment, an apparatus comprises: an MRAM (magnetic random access memory) cell array comprising a plurality of MRAM cells including a calibration cell and a plurality of data cells; a reference MRAM cell controlled by a control signal; and a sensing-amplifier/latch; wherein: said plurality of data cells are used for storing user data; the calibration cell is used for a calibration purpose; the reference MRAM cell serves as a reference for comparison with a MRAM cell selected within the MRAM cell array; the sensing-amplifier/latch outputs a logical signal based on comparing a resistance of the MRAM cell selected within the MRAM cell array and a resistance of the reference MRAM cell; and the control signal is established in a calibration process by comparing a resistance of the calibration cell with the resistance of the reference MRAM cell.