scan register

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 for testing.

The most common design is a multiplexed flip-flop:

	      ___	  ____
 normal in --|	 \	 |    |
	     |mux |------|D  Q|---- normal/scan
 scan in ----|___/    	 |    |     output
	       |	 |flip|
 test mode ----+         |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).