A Soft Processor Overlay with Tightly-coupled FPGA Accelerator

TL;DR

This paper introduces a customizable, tightly-coupled soft RISC-V processor overlay for FPGAs that enhances application acceleration and flexibility while maintaining high performance and portability across various FPGA platforms.

Contribution

It presents a portable, open-source soft RISC-V processor integrated with FPGA accelerators within an overlay framework, balancing resource use and performance.

Findings

Achieves up to 268.67MHz frequency on FPGA.

Provides performance comparable to hardware-only accelerators.

Offers increased run-time flexibility in FPGA applications.

Abstract

FPGA overlays are commonly implemented as coarse-grained reconfigurable architectures with a goal to improve designers' productivity through balancing flexibility and ease of configuration of the underlying fabric. To truly facilitate full application acceleration, it is often necessary to also include a highly efficient processor that integrates and collaborates with the accelerators while maintaining the benefits of being implemented within the same overlay framework. This paper presents an open-source soft processor that is designed to tightly-couple with FPGA accelerators as part of an overlay framework. RISC-V is chosen as the instruction set for its openness and portability, and the soft processor is designed as a 4-stage pipeline to balance resource consumption and performance when implemented on FPGAs. The processor is generically implemented so as to promote design portability and compatibility across different FPGA platforms. Experimental results show that integrated software-hardware applications using the proposed tightly-coupled architecture achieve comparable performance as hardware-only accelerators while the proposed architecture provides additional run-time flexibility. The processor has been synthesized to both low-end and high-performance FPGA families from different vendors, achieving the highest frequency of 268.67MHz and resource consumption comparable to existing RISC-V designs.