Scan chain

Scan chain is a technique used in Design For Test. The objective is to make testing easier by providing a simple way to set and observe every flip-flop in an IC. A special signal called scan enable is added to a design. When this signal is asserted, every flip-flop in the design is connected into a long shift register, one input pin provides the data to this chain, and one output pin is connected to the output of the chain. Then using the chip's clock signal, an arbitrary pattern can be entered into the chain of flips flops, and/or the state of every flip flop can be read out.

In a full scan design, Automatic test pattern generation is particularly simple. No sequential pattern generation is required - combinatorial tests, which are much easier to generate, will suffice. If you have a combinatorial test, it can be easily applied.

Assert scan mode, and set up the desired inputs.
De-assert scan mode, and apply one clock. Now the results of the test are captured in the target flip-flops.
Re-assert scan mode, and see if the combinatorial test passed.

There are many variants:

Partial scan: Only some of the flip-flops are connected into chains.
Multiple scan chains: Two or more scan chains are built in parallel, to reduce the time to load and observe.
Test compression: the input to the scan chain is provided by on-board logic

See also

Design For Test
Automatic test pattern generation
Electronic design automation
Integrated circuit design
Serial Peripheral Interface Bus