What is Scan in VLSI Design? Key Technique for Improved Testing

In VLSI (Very Large Scale Integration) design, scan is a technique used to facilitate testing and debugging of digital circuits by making it easier to observe and control the internal states of the design. It is especially useful for complex systems, such as microprocessors, ASICs, and other high-density circuits, ensuring better fault detection and improving the overall testability of the design.

How Does Scan Work in VLSI?

Scan involves converting sequential circuits into a form that behaves like combinational logic for easier testing. This is achieved by adding scan chains, which are series of flip-flops connected in a way that allows the shifting of data in and out of them.

• Scan Chain: A series of flip-flops that are connected in a linear sequence.
• Scan In/Out: Special input and output ports used for shifting data in and out of the scan chain.
• Scan Enable: A control signal that enables or disables the scan operation.
• Test Vectors: Predefined data sets used during testing to verify the circuit’s functionality.

Types of Scan Techniques in VLSI

1. Scan Path: Flip-flops are connected in a linear series to allow data to be shifted in and out for testing.
2. Scan Chain: A series of connected flip-flops, where data can be loaded or extracted through a control mechanism.
3. Full Scan: All flip-flops in the circuit are connected in scan chains, providing complete testability.
4. Partial Scan: Only a subset of flip-flops are included in the scan chain, balancing testability with circuit size and power consumption.

Benefits of Scan in VLSI Design

1. Improved Fault Detection: Scan allows for better observability of internal states, making it easier to detect faults like stuck-at faults and others.
2. Simplified Testing: Scan chains convert sequential elements into a form similar to combinational logic, making testing more efficient.
3. Faster Debugging: Engineers can quickly access the internal state of the circuit, speeding up the debugging process.
4. Automated Test Generation: Tools can automatically generate test vectors to verify functionality, reducing manual effort and ensuring comprehensive testing.
5. Design for Testability (DFT): Scan is a key component of DFT, which focuses on making the circuit easier to test, especially in complex integrated circuits like microprocessors and FPGAs.

Scan Operation Process in VLSI

1. Scan In: Test vectors are shifted into the scan chain using the Scan In port, with the scan enable signal turned on.
2. Functional Operation: The design runs with the test vectors applied, simulating normal operation.
3. Scan Out: Data from the flip-flops is shifted out through the Scan Out port, allowing observation of the internal states.
4. Comparison: The output data is compared with expected results to identify any discrepancies or faults.

Scan Logic Example

A typical scan setup involves connecting flip-flops in series to form a scan chain. The Scan In port loads test data, while the Scan Out port reads the internal state after the test. The Scan Enable control signal determines when the scan chain is active.

Scan Chain Diagram:

            +---------+
Clock ---> | AND Gate | ---> Clock to Flip-Flop
            +---------+
                  ^
                  |
          Control Signal (Enable)

Applications of Scan in VLSI Design

• Logic Testing: Scan is widely used in testing complex digital logic circuits, including processors and communication systems.
• Post-Silicon Debugging: Scan helps debug circuits post-manufacturing, ensuring the design works as intended.
• Manufacturing Test: Scan is used in manufacturing tests to verify that fabricated chips meet design specifications.
• Low-Power Design: By facilitating testability, scan chains can help identify and optimize low-power sections of a circuit.

Challenges with Scan in VLSI

1. Area Overhead: Adding scan chains increases the area of the circuit, which may impact chip size and performance.
2. Power Consumption: Scan chains can increase dynamic power consumption during testing.
3. Design Complexity: Implementing scan testing requires careful design modifications and timing analysis to ensure the functionality of the circuit is not disrupted.

Scan vs. Other Testing Techniques

While scan is an effective technique, it is often used in combination with other testing methods like Boundary Scan, Built-In Self-Test (BIST), and JTAG to enhance the overall test coverage of a design. These methods provide comprehensive strategies for detecting faults and ensuring the integrity of the VLSI design.

Conclusion: Importance of Scan in VLSI Design

In VLSI design, scan is a vital method that simplifies the testing and debugging of complex digital circuits. By converting sequential logic into a testable format, scan improves fault detection and helps ensure the proper functionality of a design. While it introduces some overhead in terms of area and power, the benefits in testability make it a crucial technique for modern VLSI systems, particularly in high-complexity chips such as processors and ASICs. Effective use of scan ensures that these designs meet industry standards for quality and reliability, making it a key component in Design for Testability (DFT).