DfT  techniques enable comprehensive testing of the chip, enhancing the test quality. Unlike combinational circuits, in which a set of input combinations should be exercised to archive maximum fault coverage, sequential circuits, in addition, need to be traversed through all possible states. Thus, a sequence of test vectors may be required to detect any fault in a sequential circuit. However, having access to the primary inputs and outputs of the chip may be insufficient to cover all the states of the design, which can reduce the fault coverage, and, thus, the test quality. DfT modifies the design by adding hardware to enhance the test quality and minimize the test cost without affecting the functionality of the circuit itself. Testability, which represents the level of difficulty of testing internal signals in the design, is measured by controllability and observability of each signal line, where controllability measures the difficulty of setting a signal line to the required value, while observability measures the difficulty of propagating the logic value of a signal line to the output. DfT improves observability and controllability by providing access to the internal nodes of the design, which enhances the testability at the cost of limited hardware and performance overhead.
Many DfT techniques have been proposed to address the testing challenges. Structural DfT techniques, such as scan, partial scan, and boundary scan, are applicable to any circuit. Scan provides full access to the flip flops, turning them into scan cells, through the scan input/output pins so that the state of the design can be updated via shift-in operations. Partial scan provides full access to a selected subset of flip flops, providing a trade-off between area/performance overhead and testability. Boundary scan enables the test of the interconnect of logic using scan cells directly connected to the primary inputs and the outputs of the logical block. Next, we will describe in detail each one of these DfT approaches.