Testing and inspection
In addition to tests that compare logic outputs with expected values, what is an advantage of performing an IDDQ test?
- AIt can also find defects such as shorts between wires (bridges) that increase the quiescent current while the logic outputs happen to remain correct
- BIt can find defects in which signal propagation delays are too long at the actual operating frequency
- CIt can find every defect without preparing any test patterns
Show answer and explanation
Answer: A. It can also find defects such as shorts between wires (bridges) that increase the quiescent current while the logic outputs happen to remain correct
In a healthy CMOS circuit at rest, either the pMOS or the nMOS is off, so only leakage current flows from supply to ground. A bridge or gate-oxide defect increases the quiescent current when opposite logic values are set on its two sides, but if the logic value happens to stay correct, a test that compares outputs sees a good chip. The IDDQ test measures this current to find such defects, and it can sometimes also catch latent defects that may fail in use. However, scaling has increased the leakage current of good chips, making it harder to distinguish the increase caused by a defect.
- A. Correct. It finds defects that do not appear at the outputs by measuring current.
- B. That is the role of at-speed (delay fault) tests. The IDDQ test measures current in the quiescent state.
- C. The IDDQ test also needs patterns that set the circuit's internal nodes to various states.