Experimental study on SEU mitigation in a commercial-off-the-shelf SRAM FPGA for the LHC Phase-2 upgrade

Future upgrades to the Large Hadron Collider will lead to an increase in the levels of background radiation experienced by the detector electronics. Within the muon detectors of the Compact Muon Solenoid (CMS) experiment, a series of new components will be installed, including the GE2/1 and ME0 detectors. Inside their data acquisition chain is the subject of this thesis, the OptoHybrid. It is a board centred around an SRAM-based Field-Programmable Gate Array (FPGA), a type of programmable electronics which is highly susceptible to radiation-induced soft errors called single event upsets (SEU). These are non-damaging but they will modify the internal configuration of the FPGA, and cause errors in its output and internal signals. The FPGA model used in the OptoHybrid is the 28nm Xilinx Artix-7, which is not radiation-proof by nature. It must therefore be qualified for use in the CMS environment. This thesis studies the possible systems which can be implemented to mitigate the effects of SEUs on the OptoHybrid. The selected system is a combination of configuration memory scrubbing using an Error Correction Code built into the Xilinx FPGA, and Triple Modular Redundancy. There are two main limitations to this system, the first is that the scrubber cannot repair memory frames which contain more than one erroneous bit. The second is that Triple Modular Redundancy breaks if more than one of the triplicated module is compromised. This can occur when SEUs accumulate, or when a single SEU flips multiple bits in the configuration memory. Because of these limitations, a reliability model based on Markov chains is proposed to predict the mean time to failure of the system. The second contribution of this thesis is a study into the impact of radiation on the FPGA through accelerated radiation testing at the UCLouvain’s cyclotron facility. The main sources of SEUs in the CMS environment are neutrons. The UCLouvain’s proton line is used as a neutron analogue as both have a similar SEU cross-section. The experimental protocols for two tests are presented, the first provides proton SEU cross-section data, and statistical data on the proportion of multiple cell upsets (MCU) for a range of energies present in CMS, and for a range of particle angles of incidence. The second studies the efficiency of the mitigation system, specifically it tests various TMR implementation, and measures parameters for the Markov chain model on the experimental firmware, which does not use modules from the OptoHybrid firmware. Previous work on the OptoHybrid of the GE1/1 detector, which used a 40nm Xilinx Virtex-6 FPGA, found a linear relation between proton cross-section and energy in the 30 to 62 MeV range, and a drop at 20 MeV. In the Artix-7 test, the proton cross-section is found to be lower but uniform in the 28.9 to 62 MeV range. Furthermore, the MCU proportion is found to rise both with energy and angle of incidence, as does the subset proportion of non-repairable MCUs. The mitigation system tests found that increasing the logical granularity of TMR yields significant improvement in error rates, while simulations with the Markov chain model indicate that the rate of non-repairable errors has a major impact on reliability. These results are only valid for the experimental firmware, therefore methods are proposed for measuring the Markov model parameters on the OptoHybrid firmware in the future.