A Circuit and Architecture Codesign Approach for a Hybrid CMOS–STTRAM Nonvolatile FPGA

Research efforts to develop a novel memory technology that combines the desired traits of nonvolatility, high endurance, high speed, and low power have resulted in the emergence of spin-torque transfer RAM (STTRAM) as a promising next-generation universal memory. Although industrial efforts have been made to design efficient embedded memory arrays using STTRAM, the prospect of developing a nonvolatile field-programmable gate array (FPGA) framework with STTRAM exploiting its high integration density remains largely unexplored. In this paper, we propose a novel CMOS-STTRAM hybrid FPGA framework, identify the key design challenges, and propose optimization techniques at circuit, architecture, and application mapping levels. We show that intrinsic properties of STTRAM that distinguish it from conventional static RAM (SRAM), such as asymmetric readout power, where a cell storing “0” has 5× less read power than a cell storing “1”, can be leveraged to skew lookup table contents for FPGA power reduction. We also argue that the proposed framework should operate on static voltage-sensing-based logic evaluation. We identify static power dissipation during logic evaluation and read noise margin as key design concerns and present an optimized resistor-divider design for voltage sensing to reduce static power and noise margin. Finally, we investigate the effectiveness of Shannon-decomposition-based supply gating to reduce static power. Simulation results show improvement of 44.39% in logic area and 22.28% in delay of a configurable logic block (CLB) and average improvement of 16.1% dynamic power over a conventional CMOS FPGA design for a set of benchmark circuits.