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It's called scan chain reordering. It is an optimisation to ensure the scan chain is connected in the most efficient way - based upon the placement of the flip-flops.
Re: why scan chain is deleted before placement and reconnect
jbeniston said:
It's called scan chain reordering. It is an optimisation to ensure the scan chain is connected in the most efficient way - based upon the placement of the flip-flops.
Re: why scan chain is deleted before placement and reconnect
At initial stage , we dont have the placement information. So we just stich the flops register by register. But after placement it might be possible (and generally in pratice happens) that the two flops stiched at initial stage of a different block sits far or too far from each other when the placement is done. So if we keep the scan chanin stiched same as a initial stage, very long and wearied and inefficient scan path results(i.e. one flop at this end of chip and next flop in chain is on ohter corener of chip). So before we do placement, remove the scan chain stiching, and once you done with the placement we re-stich the whole scan chain. (called scan chain re-ordering as well). So tool will systematically stich the scan flops.
Hope this help.!!
Re: why scan chain is deleted before placement and reconnect
No, the scan chain is still required before placement because the ATPG test coverage tools require it to generate their test vectors. Test coverage is usually determined before waiting for final placement. The bit order of these vectors also have to be reordered if the scan chain is reordered.
Re: why scan chain is deleted before placement and reconnect
you simply don't want the placement tool to mess up the placement with scan cells & give incorrect congestion figure, normally you will optimize the scan chain once the placement is frozen/good timing results with placement.
Re: why scan chain is deleted before placement and reconnect
Hi,
In intial routing the scan chains are routed in an alphanumeric manner. In placement the routing is done considering the routing and clock optimization and other factors...
Re: why scan chain is deleted before placement and reconnect
Based on timing and congestion the tool optimally places standard cells. While doing so, if scan chains are detached, it can break the chain ordering (which is done by a scan insertion tool like DFT compiler from Synopsys) and can reorder to optimize it.... it maintains the number of flops in a chain.
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