What are Scan Chains?What does it contain(combo logic or FF)

[gmailbond]

scan chain

What are Scan Chains?What does it contain(combo logic or FF)?
Why do we detach Scan Chains before proceeding to Placements (though the area for Scan Chains are taken into consideration in Floorplanning)and attach again after CTS?

 

[shelby]

what are scan chains

Every (scanable) flop has two additional inputs for scan (scan_in and scan_enable). The scan_in is muxed inside the flop with the D input and controlled by scan_enable. During synthesis, flop outputs are also connected directly to the scan_in input of a different flop creating a long chain. The first flop in the chain is connected to a primary input and the last flop in the chain is connected to an output. When scan_enable is set high, you can the shift in values through the scan chain in order to set every flop in a known state.

You can then perform a scan test like this...
1) set scan_enable high and shift in values for every flop through the chain.
2) set scan_enable low and give the design one clock. This will allow flops to capture values through their functional paths (D pins on the flops).
3) set scan_enable high and shift out all values. Then check to see if values are as expected.

During placement the scan chain can end up zip-zagging around the whole chip, so we re-order the flops in the chain based on their relative placement.