One question for edge merge for DFT

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liwei039

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when scan chain inserting, set edge merge enable.
Why the tool groups all trailing edge clock scan cells first and then groups all the positive edge clock scan cells?
what is the exactly reason?

Thanks.
 

To prevent the hold violation...To know how, just do waveform on paper for understanding all consequences.
 

For a scan chain to work properly, flops which receive the clock later should be placed first in the chain or else a lock-up lach should be inserted to avoid hold violation. For a 0-1-0 transition, NEGEDGE flop (TE) followed by POSEDGE flop (LE)case, does not need a lock-up latch since NEG flop receives the clock later and POS flop gets earlier. Hope this helps.
 

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