Using planar embedded dram in memory intensive signal processing circuits: Case studies on LDPC decoding and motion estimation

This paper studies the feasibility and potential of using planar embedded DRAM (eDRAM), which is completely compatible with CMOS logic process, to improve circuit implementation efficiency of memory-hungry signal processing algorithms. In spite of its apparent cell area efficiency advantage over SRAM, planar eDRAM is not being widely used in practice, mainly due to its very short retention time (e.g., few μs and even a few hundreds ns). In this work, we contend that short retention time may not necessarily be a fundamental issue for implementing signal processing algorithms because they typically handle streaming data, which exhibits regular and predictable data access patterns, and has a large algorithm/architecture design space. This study elaborates on the rationale and application of using a planar eDRAM in memory-hungry signal processing circuit implementations, and discusses the possible algorithm and architecture design strategies to better embrace the use of planar eDRAM. For the purpose of demonstration, we use low-density parity-check (LDPC) code decoding and motion estimation in video encoding as test vehicles. Beyond a straightforward SRAM replacement, we propose an interleaved read/write page-mode DRAM operation to reduce planar eDRAM energy consumption by leveraging LDPC code decoding data access pattern, and we investigate the potential of using planar eDRAM to enable a higher degree of image data reuse in motion estimation by proposing a folded scan structure to further improve its effectiveness. We carried out detailed planar eDRAM SPICE simulations at 45 nm node to obtain its characteristics, based on which we quantitatively evaluate the effectiveness of using planar eDRAM in these two case studies.