circular self-test path

Circular Self-Test Path: A Compact and Efficient BIST Technique

Introduction:

Built-in self-test (BIST) techniques are crucial for enhancing the testability of digital circuits, especially in complex systems where testing with external equipment is impractical or costly. The Circular Self-Test Path (CSTP) is a powerful BIST technique that combines test pattern generation and result compaction into a single, compact structure.

How CSTP Works:

The core of CSTP lies in the clever arrangement of flip-flops within the circuit during testing. The flip-flops are interconnected to form a circular register, where each flip-flop output is XORed with a specific circuit signal and then fed into the input of the subsequent flip-flop. This circular structure enables the following functionalities:

Test Pattern Generation: The initial state of the circular register provides a seed for the test sequence. By clocking the register, a pseudorandom test pattern sequence is generated and applied to the circuit under test.
Test Result Compaction: As the test patterns propagate through the circuit, the outputs are XORed with specific circuit signals and fed back into the circular register. This effectively compacts the test results into a signature.

Advantages of CSTP:

Compactness: CSTP requires minimal additional hardware, making it suitable for integration into existing circuits.
Efficiency: The single circular structure eliminates the need for separate test pattern generators and result compaction units, reducing complexity and cost.
Pseudorandom Testing: CSTP generates pseudorandom test patterns, offering high fault coverage for a wide range of circuit defects.

Implementation:

Register Design: The flip-flops in the circular register can be designed using standard D flip-flops or other suitable types.
Signal Selection: The specific circuit signals XORed with the flip-flop outputs should be carefully chosen to ensure adequate fault coverage.
Signature Analysis: After completing the test sequence, the final state of the circular register represents the signature. This signature can be compared to a pre-computed fault-free signature for diagnosis.

Applications:

CSTP finds applications in a wide range of digital circuits, including:

Memory Testing: CSTP can effectively test memory circuits by generating random address and data patterns.
Logic Circuit Testing: CSTP can be used to test complex logic circuits with high fault coverage.
Microprocessor Testing: CSTP can be employed for testing the internal logic of microprocessors and embedded systems.

Conclusion:

The Circular Self-Test Path is a valuable BIST technique that offers an efficient and compact solution for testing digital circuits. By combining test pattern generation and result compaction into a single structure, CSTP enables cost-effective and comprehensive fault detection, contributing to the reliability and robustness of modern electronic systems.

Test Your Knowledge

Circular Self-Test Path Quiz

Instructions: Choose the best answer for each question.

1. What is the primary function of the circular register in CSTP?

a) Storing the test patterns for the circuit under test b) Generating pseudorandom test patterns c) Compacting test results into a signature d) Both b) and c)

Answer

d) Both b) and c)

2. Which of the following is NOT an advantage of CSTP?

a) Compactness b) High fault coverage c) Requires separate test pattern generators d) Efficiency

Answer

c) Requires separate test pattern generators

3. How are test results compacted in CSTP?

a) By storing all output values in a separate memory unit b) By XORing the outputs with specific circuit signals and feeding them back into the circular register c) By comparing the outputs with pre-computed fault-free values d) By using a dedicated result compaction unit

Answer

b) By XORing the outputs with specific circuit signals and feeding them back into the circular register

4. What type of testing does CSTP typically perform?

a) Functional testing b) Boundary scan testing c) Pseudorandom testing d) Deterministic testing

Answer

c) Pseudorandom testing

5. Which of the following is NOT a potential application of CSTP?

a) Memory testing b) Logic circuit testing c) Analog circuit testing d) Microprocessor testing

Answer

c) Analog circuit testing

Exercise

Design a simple CSTP structure for a 4-bit adder circuit.

Instructions:

Draw a schematic diagram of a 4-bit adder.
Include 4 D flip-flops to create a circular register.
Connect the output of each flip-flop to the input of the next flip-flop.
XOR the output of each flip-flop with a specific input or output signal from the adder (consider the carry-out signal and individual sum bits).
Explain how this CSTP structure generates test patterns and compacts test results.

Exercice Correction

A possible solution for this exercise would include: 1. A 4-bit adder circuit built using full adders. 2. A circular register composed of 4 D flip-flops. 3. Each flip-flop output would be XORed with the corresponding sum bit (S0, S1, S2, S3) before feeding into the next flip-flop. 4. The output of the last flip-flop could be XORed with the carry-out signal (Cout) to further enhance fault coverage. This structure generates pseudorandom test patterns by shifting the initial state of the register, effectively providing different combinations of input values to the adder. The XOR operations effectively compact the results by combining the output bits and carry-out into a signature. The final state of the register represents the signature, which can be compared to a pre-computed fault-free signature to diagnose any fault in the adder circuit.

Books

"Digital Testing and Testable Design" by M.L. Bushnell and V.D. Agrawal: This comprehensive textbook covers various BIST techniques, including CSTP, and provides detailed explanations and examples.
"Built-In Self-Test for VLSI Circuits" by S.P. Bennetts: This book focuses on BIST methodologies and discusses different techniques, including CSTP, with practical implementation considerations.
"Test Engineering: Theory and Practice" by P.P. Chakrabarti: This book covers the fundamental principles of testing and discusses various techniques, including CSTP, in the context of real-world applications.

Articles

"Circular Self-Test Path (CSTP): A Compact and Efficient BIST Technique" by A.K. Jain and V.D. Agrawal: This article provides a detailed description of CSTP, its advantages, and applications.
"A Survey of Built-In Self-Test Techniques for Digital Circuits" by K.K. Saluja: This survey paper reviews different BIST techniques, including CSTP, and compares their effectiveness and complexity.
"Design and Analysis of Built-In Self-Test Structures for Digital Circuits" by S.P. Bennetts: This article focuses on the design and analysis of BIST structures, including CSTP, with emphasis on fault coverage and testability.

Online Resources

IEEE Xplore Digital Library: Use keywords "Circular Self-Test Path," "BIST," "Built-In Self-Test," and "Test Pattern Generation" to find relevant research papers and conference proceedings.
Google Scholar: Similar to IEEE Xplore, use the same keywords to search for scholarly articles and publications.
ResearchGate: This platform allows you to search for researchers and their publications related to BIST techniques, including CSTP.

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