Scan Chain Re-Ordering
A scan chain is a common testing concept used for testing a circuit. The scan chain approach reorders the flops in the
circuit such that the flops that are placed close to each other are placed closer in the chain.
Reordering flops in this manner makes it easy to access these flops for data scan through a serial shift register. This
approach helps applying parallel scan and improving overall chip performance during testing.
Once the initial placement of cells is available, PnR tools can reorder flops. The PnR tool must be aware of the flops in the
chain and be able to trace them to determine what it can reorder.
This can be specified to the PnR tool using the scandef files.
PnR tools supports following setting to control scan chain reordering:
Reordering based on timing
If you have multiple chains in design, you can tell PnR tools to automatically move flops from one chain to another.
If you have multiple chains in design, you can tell PnR tools to automatically move flops from one chain to another. Usually, this scenario will come in post-CTS, because CTS causes skew and the skew can cause one flip-flop to intercept another flip-flop before the hold has completely occurred, hold violations are more likely in this scenario.
Reordering scans consumes most of its resources to provide more scan chains. Hence enough routing resources must be reserved for both vertical and horizontal directions.