Design for Testability (DFT) and Scan Chain Concepts Quiz Quiz

Assess your understanding of design for testability (DFT) and scan chain concepts, exploring key techniques for enhancing digital circuit testability and identifying common pitfalls in scan-based testing. This quiz covers principles such as scan insertion, stuck-at faults, and the role of test structures in integrated circuit quality.

1. Purpose of Scan Chains

   What is the primary reason for inserting scan chains into a digital integrated circuit during design for testability (DFT)?

   1. To simplify the process of testing internal flip-flops
   2. To reduce the silicon area of the chip
   3. To automatically correct logic errors at runtime
   4. To increase the operating speed of the circuit

   Explanation: Scan chains are primarily inserted to make internal flip-flops accessible for testing by shifting test data in and out, which simplifies test processes. Scan chains do not increase circuit speed; in fact, they can add overhead. They typically increase, rather than reduce, silicon area. Scan structures do not correct logic errors at runtime; they only help in their detection during testing.

2. Role of Test Vectors

   When using scan-based testing, what is the role of 'test vectors' applied to the scan inputs?

   1. Test vectors detect and localize faults within the circuit
   2. Test vectors set the clock frequency for scan chains
   3. Test vectors provide power-up initialization values
   4. Test vectors encode configuration data for production

   Explanation: Test vectors are specifically created patterns applied to scan inputs to detect and sometimes localize faults in the circuit by observing output responses. They are not meant for storing or encoding configuration data. While initialization vectors could set states at power-up, these are not the purpose of scan test vectors. Test vectors have no role in setting the clock frequency for scan chains.

3. Stuck-At Fault Coverage

   Which statement best describes the relationship between scan design and stuck-at fault coverage in digital logic circuits?

   1. Scan design has no impact on stuck-at fault testability
   2. Scan design decreases stuck-at fault coverage due to added circuitry
   3. Scan design increases stuck-at fault coverage by making internal states observable and controllable
   4. Scan design is only useful for detecting delay faults, not stuck-at faults

   Explanation: By making flip-flops scanable, scan design makes it possible to set up and observe internal circuit states, thereby improving stuck-at fault coverage. The added circuitry does not inherently decrease coverage; instead, it facilitates more thorough testing. Saying scan design has no impact is incorrect, as it is implemented specifically to enhance fault detection. While scan designs also help test delay faults, their primary benefit is for stuck-at faults.

4. Typical Scan Chain Structure

   Which of the following is the typical structure of a scan chain implemented in a sequential digital circuit?

   1. A series connection of flip-flops where inputs and outputs form a shift register during test mode
   2. A parallel array of combinational gates feeding directly into outputs
   3. A set of flip-flops permanently wired in reset state
   4. A ring oscillator formed by looping logic gates

   Explanation: A scan chain is typically implemented as a series of flip-flops connected to act as a shift register when in test mode, allowing data to be shifted in and out. The parallel arrangement of gates is unrelated to scan chain concept. Ring oscillators are used for different purposes like measuring delays, not scan testing. Flip-flops wired in permanent reset cannot provide the controllability and observability required for testing.

5. Common Issue in Scan Chain Implementation

   During scan chain testing, what is a likely cause of the situation where test data cannot propagate through all flip-flops in the scan chain?

   1. A low clock frequency during scan operation
   2. A broken or missing connection between two scan flip-flops
   3. Combinational logic hazards adjacent to the scan chain
   4. Excessive flip-flop setup time

   Explanation: A broken or missing connection between adjacent scan flip-flops directly interrupts the path required to shift test data, causing test data not to propagate. While setup time violations may impact correct functioning, they usually result in timing errors rather than complete propagation failure. Hazards in adjacent logic do not block scan shift paths. A low clock frequency usually improves, not impedes, scan data shifting.