Designing Low-Power RISC-V Multicore Processors With a Shared Lightweight Floating Point Unit for IoT Endnodes

The increasing interest in RISC-V from both academia and industry has motivated the development and release of a number of free, open-source cores based on the RISC-V instruction set architecture. Specifically, the use of lightweight RISC-V cores in processors tailored for IoT endnode devices is on the rise. As the range and complexity of these applications grow, there is an increasing demand for multicore processors that can handle floating-point operations. This poses a significant challenge because most lightweight RISC-V cores are integer cores lacking a floating-point unit (FPU). This limitation makes it difficult to design processors optimized for applications that require floating-point operations concurrently with integer operations. While it is inefficient to have a dedicated FPU per core in a multicore processor (because it would give rise to unnecessary power consumption), it is crucial to find a solution that balances performance and energy efficiency. To address this challenge, we propose to utilize an external lightweight FPU that can be added to any RISC-V integer core, along with a low-power multicore architecture that shares the said FPU. We have applied this concept to design a RISC-V processor that integrates these technologies, implemented it on an FPGA device, and completed the fabrication of a System-on-Chip for functional verification. Our experiments, which involved testing various applications on different processor prototypes, demonstrated significant energy savings of up to 79.6% in a quad-core processor prototype, highlighting the potential energy efficiency of our proposed technology.

I. Introduction

The rising interest in RISC-V has sparked the development of numerous free and open-source cores that utilize this instruction set [2], [3], [4], [5], [6], [7]. Particularly, in industries sensitive to development costs—such as those producing devices for IoT, wearable, and embedded systems—the adoption of open, lightweight RISC-V cores in specialized processors is on the rise [8], [9], [10], [11], [12], [13], [14]. This increasing demand for lightweight cores, combined with the increasing availability of open cores that can meet diverse and specific requirements, is creating a positive feedback loop, with each trend reinforcing the other. This feedback loop broadens the scope of application for these cores and points to the potential for future growth and advancement within the RISC-V ecosystem.