Unexcitability Analysis of SEUs Affecting the Routing Structure of SRAM-based FPGAs

Testing SEUs in the configuration memory of SRAM-based FPGAs is very costly due to their large configuration memory, therefore it is necessary to optimize the generation of test patterns. In particular, in order to reduce the effort required of automatic test pattern generators, it is useful to identify early the unexcitable faults, i.e., those faults that cannot be excited by any combination of input signals. In this paper, the unexcitability of SEUs affecting the configuration bits controlling the routing resources of SRAM-based FPGAs is considered. Since this part of the configuration memory contains the largest number of configuration bits, its testing is particularly onerous. Faults in the routing resources are modeled considering the actual electrical behavior of the affected interconnections, thus the resulting fault model is more accurate than the classical open/short model usually considered. This paper introduces a methodology to prove the unexcitability of these faults. The methodology has been implemented in a tool based on a formal specification language (SAL) and a model checker (SAL-SMC). Results from the application of the tool to some circuits from the ITC'99 benchmark are reported.