One of the traditional issues in space missions is the reliability of the electronic components on board spacecraft. There are numerous techniques to deal with this, from shielding and rad-hard fabrication to ad-hoc fault-tolerant designs. Although many of these solutions have been extensively studied, the recent utilization of FPGAs as the target architecture for many electronic components has opened new possibilities, partly due to the distinct nature of these devices. In this study, we performed fault injection experiments to determine if a RISC-V soft processor implemented in an FPGA could be used as an onboard computer for space applications, and how the specific nature of FPGAs needs to be tackled differently from how ASICs have been traditionally handled. In particular, in this paper, the classic definition of the cross-section is revisited, putting into perspective the importance of the so-called "critical bits" in an FPGA design.Electronics 2020, 9, 175 2 of 12 physically shielding the devices to deflect radiation or fabricate them with so-called rad-hard processes are some alternatives. This type of approach usually implies hefty overheads in terms of cost, area, performance, and power consumption. Another approach consists in protecting the circuits utilizing design techniques, usually adding redundancy [4]. In this case, techniques range from classic schemes such as dual modular redundancy (DMR) or triple modular redundancy (TMR) to ad-hoc techniques that use behavioral or structural properties of the circuits to protect.In any case, the effects of radiation and the most appropriate technique to deal with them strongly depend on the architecture of the circuit. Traditionally, manufacturing an application-specific integrated circuit (ASIC) has been the most usual way of implementing electronic circuits, since they used to provide the best possible performance and power consumption. Errors produced by radiation on ASICs usually come in the shape of bit flips induced in the storage elements or by transients that propagate through the circuit, which can eventually be registered by a storage element. In both cases, errors can be modeled as propagation of logic values through combinational and/or sequential nets [5]. However, in recent times, field-programmable gate arrays (FPGAs) are steadily becoming the predominant architecture to implement digital circuits in space applications, especially those related to low-cost missions, such as small satellites. The advantage of FPGAs is that they offer a reduced cost, together with high flexibility in terms of reconfiguration capability. Besides, the performance of FPGAs has improved enormously, being appropriate for most kinds of applications. However, SRAM-based FPGA architectures (hereinafter referred to as FPGAs) are quite different from the ASIC ones. In these FPGAs, although the user logic is still vulnerable to radiation, the configuration memory is also vulnerable, and may sometimes be the predominant source of errors, mainly due to its size [6]. If this h...
