14.1. Design for Testability

Summary

This video explores the critical concept of Design for Testability (DFT) in chip design, which involves adding extra hardware to ensure a finished chip is testable. It discusses the challenges of controllability and observability, particularly when parts of the chip are not easily accessible. Through the use of multiplexers and systematic approaches, DFT enables efficient testing and diagnosis of internal issues without exceeding the chip’s pin count. The video emphasizes the importance of DFT in ensuring that chips function correctly before being shipped out, making it an integral part of the design process.

Takeaways

• 😀 Design for Testability (DFT) refers to adding extra hardware to a chip to make testing easier and more efficient.
• 😀 DFT is critical for ensuring a chip functions properly, even though it may seem peripheral to other design processes.
• 😀 Testing involves applying input patterns to a chip and comparing the resulting output to a gold standard to verify functionality.
• 😀 Mismatches between observed output and expected output indicate a malfunction in the chip, requiring further diagnosis.
• 😀 Diagnosis helps identify the specific submodule causing the malfunction in a chip and can point to specific issues like timing violations.
• 😀 One major challenge in chip testing is limited access to internal signals, as only the input and output pins are externally accessible.
• 😀 Submodules deep within a chip’s pipeline might not be directly controllable or observable, complicating the testing process.
• 😀 Multiplexers can be used to bring internal nodes to external pins, enabling controllability and observability during testing without altering normal operation.
• 😀 Design for Testability provides a systematic method to enhance access to internal nodes without significantly increasing the number of external pins.
• 😀 In complex chips, efficient and systematic testing methods are necessary to isolate issues in deeply nested submodules or components.
• 😀 DFT hardware can be integrated into the chip design to ensure that testing is manageable, systematic, and effective, regardless of the chip's complexity.

Q & A

What is the primary subject discussed in this module?

The primary subject is Design for Testability (DFT), which focuses on adding hardware to a chip to enable efficient testing and ensure that the chip functions properly.

How does Design for Testability (DFT) differ from other testing methods?

DFT is specifically concerned with adding additional hardware within the chip itself to make it easier to test. This contrasts with other testing methods that may rely more on software or external tools without modifying the chip's internal structure.

Why is Design for Testability (DFT) crucial in chip design?

DFT is crucial because it ensures that chips are functional and reliable before being shipped to customers. Without proper testing, the chip may have defects that affect its performance.

What is a test in the context of chip design?

A test in chip design is a pattern applied to the inputs of a chip to check if the output matches the expected results (the gold standard), ensuring the chip is functioning properly.

What is the role of the gold standard in testing?

The gold standard is the correct output that is expected from the chip. The observation (actual output) is compared to the gold standard to determine if the chip is working correctly.

What happens if the observation does not match the gold standard?

If the observation does not match the gold standard, it indicates that there is a problem with the chip. The testing process helps identify this mismatch, but additional diagnosis is required to determine the specific part of the chip that is malfunctioning.

What is diagnosis in the context of testing?

Diagnosis is the process of identifying which part of the chip has failed. It involves analyzing the test results to pinpoint the problematic sub-module or block within the chip.

What issue does the example of sub-module 5 illustrate?

The example of sub-module 5 highlights a common testing problem: when internal signals of a chip cannot be directly controlled or observed, making it difficult to test the sub-module effectively.

What are the two primary problems when testing internal modules of a chip?

The two primary problems are controllability (the inability to apply inputs to internal signals) and observability (the inability to observe internal signals at the output), which make testing and debugging more difficult.

How can a multiplexer help solve the testing issues in the example?

A multiplexer can help by allowing the input to an internal module (e.g., module 5) to be controlled in test mode, without interfering with normal operation. It enables testing through the same input pin used for normal operation, thus saving on pin count and improving testing flexibility.