Spin-in of RISC-V Processors in Space Embedded Systems

Abstract

The usage of terrestrial processors in space applications is not straightforward, as processors in space face unique challenges due to the effects of the space environment, like ionizing radiation causing Single Event Effects (SEEs). In the nineties, the European Space Agency chose the Scalable Processor ARChitecture (SPARC) Instruction Set Architectures (ISA) for its processors, as it was the only solution available at that time providing both openness and available software support in terrestrial applications. Currently, a large part of the worldwide space community is using SPARC-based radiation-hardened (rad-hard) or radiation-tolerant (rad-tol) LEON processors in ongoing and planned missions, although SPARC processors virtually disappeared from terrestrial applications. Rad-hard and rad-tol processors for space applications typically lag more than a decade behind their commercial counterparts in terms of performance and the gap is widening every year. This is mainly due to the use of Rad-Hard-By-Design (RHBD) cells and older technology nodes. The larger vulnerability to SEEs of complex microarchitectures is not the only reason why simple microarchitectures with low parallelism are still the vast majority of processors employed in space. As a matter of fact, most of the tasks executed by processors in space data systems are non-compute-intensive workloads. The reason is that they are mainly employed for non-demanding control and housekeeping operations. Therefore, enabling demanding tasks, such as the execution of Artificial Intelligence (AI) algorithms in space embedded systems, requires a large leap in spacegrade processors, especially because space data systems in satellites are typically powerconstrained. Recently, RISC-V, a novel free and open ISA, has risen in popularity in terrestrial applications, drawing the attention of several universities and companies. Given the similarity between SPARC and RISC-V, this dissertation starts by analyzing the advantages of using RISC-V in space applications. The openness of RISC-V already enabled a vast field of research activities for terrestrial applications, with many tools and models at different level of abstraction already available. Therefore, the space industry can spin-in developments from academia and industry, focusing efforts mainly on improvements concerning specific needs in space applications and without wasting efforts on other activities. In order to fully exploit modularity, the need of defining the types of processors required in space application was identified in this dissertation. The modularity of RISC-V was employed to identify several applications in space data systems and RISC-V processor profiles to address them. They were defined in this work by the ISA subset, Instruction-Level Parallelism (ILP), Data-Level Parallelism (DLP), Processor- Level Parallelism (PLP), reference implementation and expected performance. The processors profiles defined range from microcontrollers to general-purpose implementations to high-performance processors for AI. Finally, a roadmap to bring RISC-V IP cores for terrestrial applications to space level was defined, identifying the steps and models required. After the thorough analysis of the state-of-the-art of RISC-V processors was completed, two different sets of activities were identified…