Analyzing the Impact of Joint Optimization of Cell Size, Redundancy, and ECC on Low-Voltage SRAM Array Total Area

The increasing power consumption of processors has made power reduction a first-order priority in processor design. Voltage scaling is one of the most powerful power-reduction techniques introduced to date, but is limited to some minimum voltage V_DDMIN. Below V_DDMIN on-chip SRAM cells cannot all operate reliably due to increased process variability with technology scaling. The use of larger SRAM cells, which are less sensitive to process variability, allows a reduction in V_DDMIN. However, since the large-scale memory structures such as last-level caches (LLCs) often determine the V_DDMIN of processors, these structures cannot afford to use large SRAM cells due to the resulting increase in die area. In this paper we first propose a joint optimization of LLC cell size, the number of redundant cells, and the strength of error-correction coding (ECC) to minimize total SRAM area while meeting yield and V_DDMIN targets. The joint use of redundant cells and ECC enables the use of smaller cell sizes while maintaining design targets. Smaller cell sizes more than make up for the extra cells required by redundancy and ECC. In 32-nm technology our joint approach yields a 27% reduction in total SRAM area (including the extra cells) when targeting 90% yield and 600 mV V_DDMIN. Second, we demonstrate that the ECC used to repair defective cells can be combined with a simple architectural technique, which can also fix particle-induced soft errors, without increasing ECC strength or processor runtime.