(circuit design) A digital logic circuit which can act either as a flip-flop or as a serial shift register and which is used to form a scan path.

The most common design is a multiplexed flip-flop:

              ___          ____
 normal in --|         \         |    |
             |          |------|D  Q|---- normal/scan output
 scan in ----|___/ mux         |    |
               |         |    |
 test mode ----+    +----|>   | flip-flop
                    |         |____|
 clk ---------------+

The addition of a multiplexor (mux) to each flip-flop's input allows operation in either normal or test mode. The output of each flip-flop goes to the normal functional logic as well as to the scan input of the next multiplexor in the scan path.

The other common design is level-sensitive scan design (LSSD).