How would you describe scan chains?

Question Analysis

The question is asking for an explanation of "scan chains," which is a concept related to digital circuit design and testing. This is a technical question that requires an understanding of how scan chains function within integrated circuits (ICs) to facilitate testing and debugging. The candidate should focus on explaining the purpose, functionality, and benefits of scan chains in the context of design for testability (DFT).

Answer

Scan chains are an essential component of design for testability in digital circuits, particularly in the domain of integrated circuit (IC) design. They are used to enhance the testability of complex digital systems by providing a way to set and observe the state of all flip-flops in a circuit. Here’s how they work:

• Purpose: The primary purpose of scan chains is to facilitate testing and debugging of digital circuits. They allow for efficient testing of the logic paths and storage elements within an IC by converting sequential circuits into scan-compatible circuits.

• Functionality:

  • In a scan chain, flip-flops within a circuit are connected in a serial fashion to form a shift register.
  • During test mode, the normal functional connections of the flip-flops are overridden, and test data can be shifted in and out through the scan chain.
  • This process allows for the observation of circuit states and makes it possible to isolate faults by systematically testing each part of the circuit.

• Benefits:

  • Improved Testability: Scan chains make it easier to test complex circuits by simplifying the process of accessing internal states.
  • Fault Isolation: They help in identifying and diagnosing faults within the circuit by providing a controlled means of inputting and observing data.
  • Time Efficiency: By allowing for systematic and automated testing, scan chains significantly reduce the time required for testing compared to manual methods.

Overall, scan chains are a critical feature for ensuring the reliability and functionality of modern digital systems by enabling effective testing and diagnostics.