Scan Design

1. Definition: What is Scan Design?

Scan Design is a sophisticated methodology employed in Digital Circuit Design that enhances the testability of integrated circuits (ICs), particularly in Very Large Scale Integration (VLSI) systems. The primary purpose of Scan Design is to facilitate the detection of manufacturing defects and functional errors in digital circuits by transforming the internal flip-flops of a circuit into a serial scan chain. This technique allows for easier observation and control of the internal states of the circuit during testing.

The importance of Scan Design cannot be overstated, as it significantly reduces the complexity and cost associated with testing VLSI circuits. Traditional testing methods often require extensive test vectors and complex setups, which can be both time-consuming and expensive. In contrast, Scan Design enables the use of a smaller set of test vectors, thereby streamlining the test process. This is achieved by converting the circuit’s flip-flops into scan cells, which can be accessed serially through a dedicated scan input and output, allowing for effective state observation and manipulation.

The technical features of Scan Design include the incorporation of multiplexers and additional control signals that facilitate the transition between normal operating mode and test mode. During normal operation, the circuit functions as intended, while in test mode, the scan chains are activated, enabling the application of test patterns and the observation of output responses. This dual functionality is crucial for ensuring that the circuit operates correctly under both normal and test conditions.

Scan Design is particularly useful in scenarios where high fault coverage is required, such as in safety-critical applications. By enabling the detection of faults that may arise from manufacturing variances or design errors, Scan Design plays a vital role in ensuring the reliability and performance of digital systems. Furthermore, the integration of Scan Design techniques into the design process can lead to significant reductions in time-to-market, as the testing phase becomes more efficient and less resource-intensive.

2. Components and Operating Principles

The architecture of Scan Design consists of several key components, each playing an essential role in enabling effective testing of digital circuits. The primary components include scan flip-flops, scan chains, test access ports (TAP), and control logic. Understanding the interactions among these components is crucial for comprehending the overall functionality of Scan Design.

2.1 Scan Flip-Flops

Scan flip-flops are the fundamental building blocks of Scan Design. These specialized flip-flops are designed to operate in two modes: normal mode and scan mode. In normal mode, the flip-flops function as standard storage elements, capturing the input data on the clock edge. In scan mode, however, they can be connected in a series to form a scan chain, allowing for the serial input and output of test data.

The transition between these modes is controlled by dedicated control signals. For instance, when the scan enable signal is activated, the flip-flops enter scan mode, allowing test data to be shifted in or out through the scan chain. This ability to manipulate the internal states of the circuit is fundamental to achieving high fault coverage during testing.

2.2 Scan Chains

Scan chains are the interconnected series of scan flip-flops that facilitate the serial testing of a circuit’s internal states. The design of these chains is critical, as it determines the efficiency of the testing process. Typically, multiple scan chains are employed to cover different portions of the circuit, allowing for parallel testing and reducing the overall test time.

The implementation of scan chains involves careful mapping of flip-flops to ensure that they are connected in a manner that optimizes test coverage while minimizing the impact on the circuit’s performance. This mapping process can involve trade-offs between test efficiency and circuit area, as additional multiplexers and control logic may be required to facilitate the scan operations.

2.3 Test Access Port (TAP)

The Test Access Port (TAP) is an essential component that provides external access to the scan chains. It is often compliant with standards such as the Joint Test Action Group (JTAG) protocol, which defines a standardized method for accessing and controlling test circuitry within an IC. The TAP consists of several pins that allow for the input of test patterns and the observation of output responses.

The TAP interface is crucial for enabling the integration of Scan Design into automated test equipment (ATE), allowing for efficient testing of devices post-manufacturing. The use of TAP also facilitates boundary scan testing, where the interconnections between ICs on a printed circuit board (PCB) can be tested without requiring physical access to the pins.

2.4 Control Logic

Control logic orchestrates the operation of the scan flip-flops and scan chains. This logic is responsible for generating the necessary control signals to switch between normal and scan modes, as well as managing the shifting of test data through the scan chains. The design of this control logic can significantly impact the overall performance and complexity of the Scan Design implementation.

The control signals must be carefully timed to ensure that the circuit operates correctly during both normal and test modes. Timing analysis is essential to verify that the control logic does not introduce timing violations that could lead to incorrect operation or false test results.

Scan Design is often compared to other testing methodologies, such as Built-In Self-Test (BIST) and traditional functional testing. Each of these approaches has its advantages and disadvantages, making them suitable for different applications and design contexts.

3.1 Built-In Self-Test (BIST)

Built-In Self-Test (BIST) is a testing methodology that incorporates test generation and response analysis directly into the circuit itself. Unlike Scan Design, which relies on external test equipment, BIST allows the circuit to perform its own testing. This can be particularly advantageous in applications where access to the circuit is limited or where automated testing is required.

However, BIST can lead to increased area overhead due to the additional circuitry required for test generation and response analysis. In contrast, Scan Design typically incurs less area overhead since it primarily modifies existing flip-flops into scan cells. Additionally, Scan Design often achieves higher fault coverage than BIST, particularly for complex circuits where internal state observation is critical.

3.2 Traditional Functional Testing

Traditional functional testing involves applying a set of test vectors to the circuit’s inputs and observing the outputs. While this approach can be effective for simple circuits, it becomes increasingly challenging for complex VLSI designs. The primary limitations of traditional functional testing include the difficulty in achieving high fault coverage and the extensive time required to generate and apply the necessary test vectors.

In contrast, Scan Design significantly enhances testability by allowing the internal states of the circuit to be accessed and manipulated. This capability enables the application of fewer test vectors while still achieving comprehensive fault coverage. Moreover, the use of scan chains allows for the efficient observation of internal states, which is not feasible with traditional functional testing methods.

3.3 Summary of Comparisons

In summary, while BIST offers self-test capabilities and traditional functional testing is straightforward, Scan Design provides a balanced approach that maximizes fault coverage while minimizing area overhead and test complexity. Each of these methodologies has its place in the testing landscape, but Scan Design remains a preferred choice for many VLSI applications due to its effectiveness and efficiency in ensuring the reliability of integrated circuits.

4. References

IEEE (Institute of Electrical and Electronics Engineers)
ACM (Association for Computing Machinery)
JTAG (Joint Test Action Group)
Various semiconductor companies specializing in VLSI design and test solutions, such as Synopsys, Cadence, and Mentor Graphics.

5. One-line Summary

Scan Design is a critical testing methodology in Digital Circuit Design that enhances the testability of integrated circuits by converting flip-flops into serial scan chains, facilitating efficient fault detection and verification.