A Static Analysis for the Minimization of Voters in Fault-Tolerant Circuits

We present a formal approach to minimize the number of voters in triple-modular redundant (TMR) sequential circuits. Our technique actually works on a single copy of the TMR circuit and considers a large class of fault mo dels of the form “at most 1 Single-Event Upset (SEU) or Single-Event Transient (SET) every k clock cycles”. Verification-based voter minimization guarantees that the resulting TMR circuit (i) is fault tolerant to the soft-errors defined by the fault model and (ii) is functionally equivalent to the initial TMR circuit. Our approach operates at the logic level and takes into account the input and output interface specifications of the circuit. Its implementation makes use of graph traversal algorithms, fixed-point iterations, and binary decision diagrams (BDD). Experimental results on the ITC’99 benchmark suite indicate that our method significantly decreases the number of inserted voters, yielding a hardware reduction of up to 55% and a clock frequency increase of up to 35% compared to full TMR. As our experiments show, if the SEU fault-model is replaced with the stricter fault-model of SET, it has a minor impact on the number of removed voters. On the other hand, BDD-based modelling of SET effects represents a more complex task than the modelling of an SEU as a bit-flip. We propose solutions for this task and explain the nature of encountered problems. We address scalability issues arising from formal verification with approximations and assess their efficiency and precision.