Methodologies for tolerating cell and interconnect faults in FPGAs

The very high levels of integration and submicron device sizes used in current and emerging VLSI technologies for FPGAs lead to higher occurrences of defects and operational faults. Thus, there is a critical need for fault tolerance and reconfiguration techniques for FPGAs to increase chip yields (with factory reconfiguration) and/or system reliability (with field reconfiguration). We first propose techniques utilizing the principle of node-covering to tolerate logic or cell faults in SRAM-based FPGAs. A routing discipline is developed that allows each cell to cover-to be able to replace-its neighbor in a row. Techniques are also proposed for tolerating wiring faults by means of replacement with spare portions. The replaceable portions can be individual segments, or else sets of segments, called "grids." Fault detection in the FPGAs is accomplished by separate testing, either at the factory or by the user. If reconfiguration around faulty cells and wiring is performed at the factory (with laser-burned fuses, for example), it is completely transparent to the user. In other words, user configuration data loaded into the SRAM remains the same, independent of whether the chip is defect-free or whether it has been reconfigured around defective cells or wiring-a major advantage for hardware vendors who design and sell FPGA-based logic (e.g., glue logic in microcontrollers, video cards, DSP cards) in production-scale quantities. Compared to other techniques for fault tolerance in FPGAs, our methods are shown to provide significantly greater yield improvement, and a 35 percent non-FT chip yield for a 16 x 16 FPGA is more than doubled.