Scan chain is a technique used in
design for testing Design for testing or design for testability (DFT) consists of IC design techniques that add testability features to a hardware product design. The added features make it easier to develop and apply manufacturing tests to the designed hardware. Th ...
. The objective is to make testing easier by providing a simple way to set and observe every
flip-flop in an
IC.The basic structure of scan include the following set of signals in order to control and observe the scan mechanism.
#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.
#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.
#
Clock signal
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.
A clock sign ...
which is used for controlling all the FFs in the chain during shift phase and the capture phase. An arbitrary pattern can be entered into the chain of flip-flops, and the state of every flip-flop can be read out.
In a full scan design,
automatic test pattern generation (ATPG) 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 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.
Controllability and observabil ...
and
observability of the internal signals in a
sequential circuit
In automata theory, sequential logic is a type of logic circuit whose output depends on the present value of its input signals and on the sequence of past inputs, the input history. This is in contrast to ''combinational logic'', whose output i ...
are in general much more difficult than those in a
combinational logic 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: the input to the scan chain is provided by on-board logic.
See also
*
Design for testing Design for testing or design for testability (DFT) consists of IC design techniques that add testability features to a hardware product design. The added features make it easier to develop and apply manufacturing tests to the designed hardware. Th ...
*
Automatic test pattern generation
*
Electronic design automation
Electronic design automation (EDA), also referred to as electronic computer-aided design (ECAD), is a category of software tools for designing Electronics, electronic systems such as integrated circuits and printed circuit boards. The tools wo ...
*
Integrated circuit design
Integrated circuit design, or IC design, is a sub-field of electronics engineering, encompassing the particular logic and circuit design techniques required to design integrated circuits, or ICs. ICs consist of miniaturized electronic component ...
*
Serial Peripheral Interface Bus
*
Iddq testing Iddq testing is a method for testing CMOS integrated circuits for the presence of manufacturing faults. It relies on measuring the supply current (Idd) in the quiescent state (when the circuit is not switching and inputs are held at static values). ...
External links
An example of a scan chain for stuck-at faults
{{DEFAULTSORT:Scan Chain
Electronic circuit verification