Hello,
Fast sequential and full sequential is help when there are so many non scan cells.
So first we have to check that how many non scan cells are there in between 2 consecutive scan flops. It there are so many consecutive flops with in between non scan flops, than we can go to increase the sequential engine.
AU faults means ATPG untestable, we have define some constraints, engine. Due to these constraints, engine, ATPG tool is not able to get coverage over there. By chainging the engine, we are removing 1 constraints (i.e. only 1 capture cycle), We can reduce the count of AU faults.
As earlier Vivek mentioned,
To reduce count of AU faults - Go for Fast or Full sequential engine
To reduce count of ND faults - Increase abort limits
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Thanks & Regards,
Maulin Sheth