Statistical Approach for the Design of Refresh-Free eDRAM with Retention Timing Constraint

Abstract: In digital integrated circuits, memories are often a limiter for main performance, power and area. Over the past decade, integrated memories have gained dominance in terms of area and power cost. In applications like machine learning or image processing the area share can be above 80% consuming more than 50% of the total power budget. On-chip memories in CMOS technologies can be categorized as static (SRAM) and dynamic (embedded DRAM). SRAM has been the main choice due to its high-access rates and static data retention feature. eDRAM offers higher area gain over SRAM counterpart. However, dynamic memories require periodic, power-hungry refresh operations. This operations increase the design complexity and have a high energy cost. Furthermore, they lower the access rates due to memory restriction during refresh periods, which makes it less desirable in SoC context. A wide range of computation intensive applications in today's digital systems need to buffer intermediate data for a short fraction of time. Multiple Input Multiple Output (MIMO) communication and convolutional image processing are two cases that use memories for storing data during short time periods. These are appropriate applications for the use of eDRAM without refresh operations. The goal of this study is to evaluate and develop an eDRAM memory compiler. This compiler evaluates the design of an eDRAM cell and considers the effect of manufacturing process deviations. These variations are based on statistics, and for their analysis, a new statistical approach 100 times faster than the generally used analysis method is developed. The output of the compiler is a macro layout according to the eDRAM cell and results from the statistical design evaluation. The final result is a tool that allows the automatic generation of eDRAM as a function of user requirements.