Now let us consider s-a-1 in an internal net (output of G1), as shown in Figure 10. As the net output of G1 is stuck-at-1, we need to drive it to 0 to verify the presence/absence of the fault. So at least on input of G1 is to be made 0; at G1, I1=0 and I2 through I5 are 1. To propagate the effect of fault to O, all inputs of G2 though G5 are made 1. Now, if fault is present, then O is 1, else 0. So, I1=0, I2=1,….,I25=1, is a test pattern for the stuck-at-1 fault at net output of G1. It is interesting to note that same test pattern I1=0, I2=1,….,I25=1 tests both s-a-1 at net I1 and output of G1. In other words, in structural testing with stuck-at fault model, one test pattern can test more than one fault.

Figure 10. s-a-1 fault in net OG1 with input test pattern

Now let us enumerate the gains and price paid for structural testing with stuck-at fault model

• Gains
• No extra pin outs or DFT circuitry like 2-1 Multiplexers and shift resisters for controlling and observing internal nets
• Low test time as one test pattern can test multiple stuck-at faults
• Price
• Functionality is not tested, even for the units (gates and Flip-flops). However, testing history reveals that even with this price paid, quality of test solution is maintained.

To conclude, Table 2 compares Structural and Functional Testing

Table 2. Comparison of structural and functional testing

Functional testing	Structural Testing
Without fault models.	With fault models.
Manually generated design verification test patterns.	Automatic test pattern generation (ATPG).
Slow and labor intensive.	Efficient and automated.
Fault coverage not known	Fault Coverage is a quantified metric.
More Test Patterns	Less Test Patterns
Can be applied at the operating speed.	Difficult to be applied at the speed the design is expected to work.