Now let us see the gains and costs paid by the DFT circuitry (shift registers) for structural testing
•  Gain:
.. .......•  Instead of 2 32 test patters for functional testing, only 2 3 patterns are enough for structural testing

.........•  The number of extra pins for structural testing is only 3. It many be noted that if shift register is not used, then extra pins required are 64 as carry output bit of the full adders are to be brought out for observation and carry in bit of the full adders are to be brought out for sending the required test signal (controllability).

•  Price

.........•  2-1 Multiplexers and registers are required for each internal net to controlled

.........•  Registers required for each internal net to observed

.........•  3 extra pin outs

From Section 2 and Section 3 we note that by the use of internal registers, the problem of huge number of extra pins could be solved but it added to requirement of huge size of shift registers (equal to number of internal nets). In a typical circuit there are tens of thousand of internal lines, making the on-chip register size and number of 2-1 multiplexers extremely high. So addition to cost of a chip by such a DFT is unacceptable.

So our next target for achieving an efficient structural testing is the one with less number of on-chip components and yet maintaining the quality of test solution. Structural testing with Fault Models is the answer to the requirement. In the next section we will study Fault Models and then see why DFT requirement is low when structural testing is done with fault models.

Before going to next section, to summarize, `` structural testing is functional testing at a level lower than the basic input-output functionality of the system ''. For the example of the bitwise ANDing circuit, unit for structural testing was gates and for the case of 32-bit adder it was full adders. In general, in the case of digital circuits, structural testing is `` functional testing at the level of gates and flip-flops ''. Henceforth in this course, basic unit of a circuit for structural testing would be logic gates and flip-flops.