Non-destructive variability tolerant differential read for non-volatile logic

In Magnetoresistive RAMs (MRAMs), Magnetic Tunnel Junctions (MTJs) are used to store bits in memory and the bits are read by comparing the MTJ resistance to a reference value. But in non-volatile logic, MTJs are used to store as well as compute by coupling with one another. This results in a `1´ and `0´ lying side by side in logic. Motivated by this inherent property of logic, in this paper we have first developed a differential read for the logic where we have compared a bit with its complement. A differential read provides higher sense margin than any reference reading and eliminates the requirement of reference resistance. Under current technology, the variations in MTJ are sufficient to degrade the sense margin of the circuit to a point when error-free reading becomes a challenge. To make the read more robust, in this paper we have further modified the differential read to design a novel variability tolerant read. Apart from being more immune to MTJ variations, the circuit provides better sense margin than differential read. In this work we have also investigated the factors that influence the sense margin in variability tolerant differential read circuit. Some interesting results from our study include the increased sensitivity of the sense margin to MTJ state `0´ resistance than state `1´ resistance and improvement in variability tolerance with increased Tunnel Magnetoresistance (TMR) of MTJ. The study also includes supportive simulation results.