50 VLSI Test Principles and Architectures where i = 0, 1, ±±± , n − 1. On the other hand, the probability-based observability of an input l at stage i on an output s k —O( l , s k ), where k>i —depends on the propagation of the carry output from stage i to the output s k . This calculation is left as a problem at the end of this chapter. In general, RTL testability analysis can sometimes lead to more accurate results than gate-level testability analysis. The reason is that the number of reconvergent fanouts in an RTL model is usually much less than that in a gate-level model. RTL testability analysis is also more time efficient than gate-level testability analysis because an RTL model is much simpler than an equivalent gate-level model; how-ever, the practical application of RTL testability analysis for testability enhancement in complex RTL designs remains a challenging research topic. 2.3
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