Scan chain is a technique used in Design For Test

Design For Test

Design for Test is a name for design techniques that add certain testability features to a microelectronic hardware product design. The premise of the added features is that they make it easier to develop and apply manufacturing tests for the designed hardware...

. The objective is to make testing easier by providing a simple way to set and observe every flip-flop

Flip-flop (electronics)

In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic...

 in an IC

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

.The basic structure of scan include the following set of signals in order to control and observe the scan mechanism.

1. Scan_in and scan_out define the input and output of a scan chain. In a full scan mode usually each input drives only one chain and scan out observe one as well .
2. A scan enable pin is a special signal that is added to a design. When this signal is asserted, every flip-flop in the design is connected into a long shift register
   Shift register
   In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, which has the output of any one but the last flip-flop connected to the "data" input of the next one in the chain, resulting in a circuit that shifts by one position the one-dimensional "bit array" stored in...
   
   .
3. Clock signal
   Clock signal
   In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...
   
    which is used for controlling all the FF in the chain during shift phase and the capture phase . An arbitrary pattern can be entered into the chain of flips flops, and the state of every flip flop can be read out.

In a full scan design, Automatic test pattern generation

Automatic test pattern generation

ATPG is an electronic design automation method/technology used to find an input sequence that, when applied to a digital circuit, enables automatic test equipment to distinguish between the correct circuit behavior and the faulty circuit...

 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.

In a chip that does not have a full scan design -- i.e., the chip has sequential circuits, such as memory elements that are not part of the scan chain, sequential pattern generation is required.
Test pattern generation for sequential circuits searches for a sequence of vectors to detect a particular fault through the space of all possible vector sequences.

Even a simple stuck-at fault requires a sequence of vectors for detection in a sequential circuit. Also, due to the presence of memory elements, the controllability

Controllability

Controllability is an important property of a control system, and the controllability property plays a crucial role in many control problems, such as stabilization of unstable systems by feedback, or optimal control....

 and observability

Observability

Observability, in control theory, is a measure for how well internal states of a system can be inferred by knowledge of its external outputs. The observability and controllability of a system are mathematical duals. The concept of observability was introduced by American-Hungarian scientist Rudolf E...

 of the internal signals in a sequential circuit are in general much more difficult than those in a combinational logic

Combinational logic

In digital circuit theory, combinational logic is a type of digital logic which is implemented by boolean circuits, where the output is a pure function of the present input only. This is in contrast to sequential logic, in which the output depends not only on the present input but also on the...

 circuit. These factors make the complexity of sequential ATPG much higher than that of combinational ATPG.

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
  Test compression
  Test Compression is a technique used to reduce the time and cost of testing integrated circuits. The first ICs were tested with test vectors created by hand. It proved very difficult to get good coverage of potential faults, so Design for testability based on scan and automatic test pattern...
  
  : the input to the scan chain is provided by on-board logic

See also

• Design For Test
  Design For Test
  Design for Test is a name for design techniques that add certain testability features to a microelectronic hardware product design. The premise of the added features is that they make it easier to develop and apply manufacturing tests for the designed hardware...
  
  
• Automatic test pattern generation
  Automatic test pattern generation
  ATPG is an electronic design automation method/technology used to find an input sequence that, when applied to a digital circuit, enables automatic test equipment to distinguish between the correct circuit behavior and the faulty circuit...
  
  
• Electronic design automation
  Electronic design automation
  Electronic design automation is a category of software tools for designing electronic systems such as printed circuit boards and integrated circuits...
  
  
• Integrated circuit design
  Integrated circuit design
  Integrated circuit design, or IC design, is a subset of electrical engineering and computer engineering, encompassing the particular logic and circuit design techniques required to design integrated circuits, or ICs...
  
  
• Serial Peripheral Interface Bus
  Serial Peripheral Interface Bus
  The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard named by Motorola that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame. Multiple slave devices are allowed with individual slave select ...