Gate Level NBTI and Leakage Co-Optimization in Combinational Circuits with Input Vector Cycling

Negative Bias Temperature Instability (NBTI) effect occurs in a PMOS transistor when turned ON leading to threshold voltage degradation. As sub-threshold leakage is significant in nanoscale CMOS circuits, input vector control can be employed wherein a Minimum Leakage Vector (MLV) is applied to the circuit during idle periods. In such a case, the ON PMOS transistors on the critical path are subject to NBTI stress for prolonged periods. Transistors can recover from stress when turned OFF. Based on this observation, we propose a vector cycling based leakage/NBTI co-optimization: a pair of MLVs are identified such that when applied alternately, on the critical path, PMOS transistors activated by one vector are turned OFF by the other vector. We employ Simulated Annealing (SA) for stochastic search of the first vector followed by back tracking to identify the second vector. Experimental results for a subset of ISCAS85 benchmarks implemented in 45 nm technology demonstrate the feasibility of vector cycling approach. When compared to leakage-only optimization, NBTI-only optimization, and co-optimization, on average, vector cycling yields 11, 3, and 6 percent NBTI improvements with 18, -9, and 4 percent leakage overheads respectively. The average area and dynamic power overheads are 13.78 and 0.15 percent respectively.