Abstract
Dynamic voltage scaling (DVS) and fast storage are required for mobile chips to achieve low-power and reliable power-off procedures. Many DVS mobile chips use SRAM for fast/low-VDDmin access and embedded Flash for data-backup. However, the 2-macro approach requires long store time due to serial data transfer. The nonvola-tile-SRAM (nvSRAM) provides a better solution for mobile chips, thanks to theirs fast bit-to-bit parallel data storage. This work demonstrates the first fabricated macro-level RRAM-based nonvolatile SRAM (nvSRAM) that use a new 8T2R (Rnv8T) cell to achieve fast and low-energy NVM storage and low VDDmin read/write operations. The Rnv8T cell uses two fast-write low-current RRAM devices, 3D stacked over the 8T, to achieve low store energy with a compact cell area (1.6x that of a 6T cell). A 2T RRAM-switch provides both RRAM control and write-assist functions. This write assist feature enables Rnv8T cell to use read favored transistor sizing against read/write failure at a lower VDD. The fabricated 16Kb Rnv8T macro achieves the lowest store energy and R/W VDDmin (0.45V) than other nvSRAM and “SRAM+NVM” solutions.