Questions and Answers

 •  What are the problems of structural testing without fault models, if the units are as atomic as gates and as large as arithmetic block like 32-bit adder and 64-multiplier

Answer:

If structural testing is done without fault models and the units are small (like gates), then there is lot of internal nets (interconnecting the gates) to be controlled or observed. One 2-1 Multiplexer and 1 external pin is required for all internal nets (which are inputs to gates) which are to be controlled and one 1 external pin is required for all internal nets (which are outputs of gates) which are to be observed.

On the other hand if the units are large blocks like 32-bit adder and 64-multiplier, then the numbers of internal nets are only the interconnections between these blocks. The number of nets required to interconnect large blocks are many times smaller than those required to interconnect gates. So multiplexers and extra pins are less compared to the case when gate is treated as an uni.

Structural testing is functional testing at unit level (if done without fault models). Functional testing requires exponential number of test patterns with respect to the number of inputs of the units. In case of gates, inputs are generally four or less, however, in case of large blocks inputs may be as large as 128 (32 for the adder and 64 for the multiplier, in this example). So test patters for structural testing with large blocks as units is many fold more in number than the case when the unit is a gate.

•  How many silicon level defects result in nets getting stuck? Explain relevance of stuck-fault model from that perspective

Answer

Very rarely, silicon level defects result in nets getting stuck. However, from history of testing of fabricated chips, it is observed that if one verifies the absence of all stuck-at faults in the nets of a circuit, then with more than 99.9% probability it may be stated that there is no defect.