Scan Chain Reordering

shahal said:

Here is a DFT question:

How would you reorder a scan chain so that there is no Hold violations?

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cheelgo said:

in general, we don't do reorder after synthesis.

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shahal said:

Let me rephrase... There is supposedly a way where you can re order a scan chain and not have to worry about any hold violations.. I am trying to figure out the concept behind it.. There is a snug paper on it, unfortunatly I do not have access to solvenet any more.

Added after 13 minutes:

cheelgo said:

in general, we don't do reorder after synthesis.

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I think this depends on the tool right? It depends on where you are doing your synthesis, you definatly have to do some sort of scan chan re ordering if you have congestion.. and that is after Synthesis in your physical layout environment.

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dr_dft said:

Theoretically speaking, you can figure out the clock insertion delays to every flip-flop, sort the clock insertion delays from the longest to the shortest, and reordered the scan chains accordingly. This will guarantee that there is no hold time violations, assuming that the hold time requirement on the SI input of your flip-flops is 0ns or less.
Practically, this is not the best way, since it will still lead to long clock routes.
The best thing to do is to look at the clock trees, and do a bottom up approach by stitching all flip-flops from the same clock branch first. The theory is that FFs in the same branch must be physically close to each other, and have minimum skew, meeting both scan reordering criteria.

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