Testing and inspection

When compressing scan test outputs, why must signals whose value (0 or 1) is undetermined in simulation (unknowns, X) be removed?

  1. AIf unknowns mix into the compressed result, it cannot be compared with expected values and faults cannot be detected
  2. BUnknowns make the chip heat up
  3. CUnknowns stop the clock
Show answer and explanation

Answer: A. If unknowns mix into the compressed result, it cannot be compared with expected values and faults cannot be detected

Unknowns (X) from uninitialized memory, analog circuits and the like, once mixed with other bits in the compactor, make the whole compressed value unknown so it cannot be compared with correct expected values. Mechanisms such as X-masking are therefore added to remove the influence of X from the outputs.

  • A. Correct. Unknowns mixed in prevent fault detection.
  • B. Unknowns do not cause heating.
  • C. Unknowns do not stop the clock.
Practice this topic Back to topic

More Testing and inspection questions

  1. In testing high-speed serial interfaces, which method loops the chip's transmitter output back to its receiver so the chip itself verifies communication?
  2. What is the technique called that dynamically changes test items or limits per chip or per lot based on intermediate results and statistics, such as from wafer test?
  3. In AD converter testing, what is the main purpose of inputting a slowly varying sine wave or similar and counting how often each output code appears (a histogram)?
  4. For a chip that failed scan testing, what is the process called that logically narrows down candidate fault locations from which outputs mismatched on which patterns?
  5. Which technique models defects such as transistor shorts and opens inside standard cells based on each cell's circuit and generates test patterns for them?