Optimized Two-Step Store Control for MTJ-Based Nonvolatile Flip-Flops to Minimize Store Energy Under Process and Temperature Variations

Introducing a magnetic tunneling junction (MTJ) into a flip-flop enables nonvolatile power gating (PG) but large store energy to MTJ is a critical concern. We propose an optimized two-step store (TSS) control to first perform a short store with an optimal time for all nonvolatile flip-flops (NVFFs) and then perform a long store only at the failed ones for reducing the store energy. As the key technologies to realize this, we present a verify-and-retryable NVFF (VR-NVFF) circuit enabling the TSS control and an analytical expression for the optimal short-store time (T-short_opt) minimizing the store energy. To examine the effectiveness of the optimized TSS control and the validity of analytically derived T-short_opt, we implemented the TSS control on a coarse-grained reconfigurable array (CGRA)-based accelerator chip and fabricated it in a 40-nm CMOS/MTJ hybrid process technology. Results demonstrated that analytical T-short_ opt showed a good agreement with the measured value (within 8% difference) under process and temperature variations. The TSS control with Tshort_ opt reduced the store energy to 0.32x of that of the conventional long-store-only technique. The break-even time (BET), which is the minimum power-gating time to get the gain in energy savings, was shortened to 0.51-0.7x by the TSS control, achieving the BET of 50-923 mu s in the range of 0 degrees C-80 degrees C.