Kansal, Rohan (2013-08). A Pure STT-MRAM Design for High-bandwidth Low-power On-chip Interconnects. Master's Thesis.
Network-on-Chip (NoC) is a de facto inter-core communication infrastructure for future Chip Multiprocessors (CMPs). NoC should be designed to provide both low latency and high bandwidth considering limited on-chip power and area budgets. The use of a high density and low leakage memory, Spin-Torque Transfer Magnetic RAM (STT-MRAM), in NoC routers has been proposed as it increases network throughput by providing more buffer capacities with the same die footprint. However, the inevitable use of SRAM to hide the long write latencies of STT-MRAM sacrifices buffer area and also wastes significant leakage and dynamic power in migrating flits between the disparate memories. In this thesis, the first NoC router designs that use only STT-MRAM is proposed. This allows for a much larger buffer space with the least power consumptions. To overcome the multi-cycle writes, a multi-banked STT-MRAM buffer is employed, which is a logically divided virtual channel where every incoming flit is seamlessly pipelined to each bank alternately every clock cycle simple latches inside the router links. Our STT-MRAM has aggressively reduced retention time, resulting in a significant reduction in latency and power overheads of write operations. We observe flit losses in our STT-MRAM buffer, and propose cost-efficient dynamic buffer refresh schemes to minimize unnecessary refreshes with minimum hardware overheads. Simulation results show that our STT-MRAM NoC router enhances the throughput by 21.6% and achieves 61% savings in dynamic power and 18% savings in total router power, respectively compared to a conventional SRAM based NoC router of same area.