YosysHQ - Building alternative FPGA toolchains

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The design and verification of FPGA-based systems for space applications rely on complex toolchains that translate high-level logic into hardware implementations. Typically, only a single synthesis, placement, and routing tool is available for a given FPGA, making it difficult to assess how design choices and toolchain behavior interact. The availability of multiple toolchains allows cross-checking of design functionality, and an open-source tool can provide more avenues to inspect and control the implementation process. To enable this, we have developed an alternative toolchain for the NanoXplore NG-Ultra, the only large, radiation-hardened FPGA made in Europe. As an activity under ESA's OSIP Ideas platform and with help from NanoXplore, we have extended nextpnr to support placement and routing for NG-Ultra. While not a goal of the project, we have also contributed an alternative synthesis flow based on Yosys, which allowed us to target individual aspects of the nextpnr flow when synthesizing test cases during development. The result is a prototype of an almost fully open-source flow, with only the bitstream generation and board programming steps still requiring vendor tools. This presentation will discuss the technical challenges encountered in developing an alternate open source toolchain while trying not to replicate the vendor's approach. The FPGA's large size required scaling improvements to the generic architecture-independent parts of nextpnr, while its nontraditional architecture—particularly its routing network—demanded close attention to placement strategies to ensure successful routing. This is a result of both complex routing constraints requiring careful implementation, and an overall limited amount of routing resources relative to logic elements in the NG-Ultra architecture.

Additionally, we will present our validation approach to confirm the correctness of our implementation, and benchmarking results comparing nextpnr against the vendor tool…