Scan Chain Re-ordering
DFT Basics Article : #10
In my previous blog DFT Basics: Article #9, we had explored the problems encountered during domain mixing.
In this blog, we will explore about scan chain re-ordering and how it helps in avoiding congestion.
Let’s get started !
We will explore scan chain reordering and it’s need with the following example :
Since the Q output of Flop 1 connects to the D input of Flop 3 through combinational logic, the physical design (PD) tool [PnR] will place these two flops close to each other, as shown below
As seen from the above figure, if the original order of scan chain is followed, it results in zig-zag connectivity, routing congestion and wires will get shorted.
Therefore, DFT team can give flexibility to the PD team to re-order the scan chains.
The above figure illustrates the design after scan chain reordering.
To perform scan chain reordering, the physical design (PD) team requires a Scandef file. The Scandef file contains information about which cells can be reordered.