Design for Test (DFT) techniques - Verilog Tutorial

Design for Test (DFT) techniques play a crucial role in ensuring the manufacturability and testability of complex digital designs. These techniques are specifically employed to facilitate testing during the manufacturing process and improve fault coverage. In this tutorial, we will explore various DFT techniques using Verilog, provide examples, explain the steps in detail, and offer best practices for incorporating DFT features into your designs.

Example: Scan Chain for Testability

One of the fundamental DFT techniques is the use of a scan chain. A scan chain is a series of flip-flops connected in a chain to enable easy shifting of test patterns and observed responses. Here's an example of a Verilog module with a scan chain:


    // Original Verilog module without scan chain
    module dft_module (
        input wire clk,
        input wire reset,
        input wire [7:0] data_in,
        output wire [7:0] data_out
    );
        reg [7:0] internal_data;

        always @(posedge clk) begin
            if (!reset)
                internal_data <= 8'h00;
            else
                internal_data <= data_in + 8'h01; // Some computation
        end

        assign data_out = internal_data;
    endmodule

In this design, there is no scan chain, making it difficult to apply test patterns and observe the responses during manufacturing testing. Let's add a scan chain for testability:


    // Verilog module with scan chain for testability
    module dft_optimized_module (
        input wire clk,
        input wire reset,
        input wire scan_en,
        input wire [7:0] data_in,
        output wire [7:0] data_out
    );
        reg [7:0] internal_data;
        reg [7:0] scan_chain;

        always @(posedge clk) begin
            if (!reset)
                internal_data <= 8'h00;
            else
                internal_data <= data_in + 8'h01; // Some computation

            if (scan_en)
                scan_chain <= internal_data; // Capture internal_data into scan_chain
        end

        always @(posedge clk) begin
            if (scan_en)
                internal_data <= scan_chain; // Update internal_data from scan_chain
        end

        assign data_out = internal_data;
    endmodule

Steps for Incorporating DFT Techniques in Verilog

To include DFT techniques in your Verilog designs, follow these steps:

Identify Critical Signals: Determine signals that need to be observable during testing for detecting faults.
Insert Scan Chains: Add scan chains to capture and update internal states during testing.
Implement Boundary Scan (JTAG): Include boundary scan cells for easy access to I/O pins during testing.
Use Compression Techniques: Apply data compression techniques to reduce the size of test data and decrease testing time.
Insert Test Access Mechanisms: Incorporate test access mechanisms like Test Access Ports (TAP) for standardized test interfacing.
Integrate Built-In Self-Test (BIST): Include BIST controllers for self-testing circuit blocks or memories.
Use Memory Built-In Self-Test (MBIST): Implement MBIST controllers for memory testability and repair.
Implement Error Correction Codes (ECC): Integrate ECC codes to detect and correct errors during testing.
Perform Fault Simulation: Conduct fault simulation to ensure effective fault detection and test coverage.
Verify ATPG Patterns: Verify Automatic Test Pattern Generation (ATPG) patterns for accuracy and efficiency.

Common Mistakes with DFT Techniques in Verilog

Applying unnecessary scan chains, leading to increased area and complexity.
Overlooking critical signals during DFT implementation.
Insufficient fault simulation, resulting in lower test coverage.

Frequently Asked Questions

Q: What is the purpose of a scan chain in DFT?
A: The scan chain allows easy shifting of test patterns and observed responses for testing internal states of the design.
Q: How does boundary scan (JTAG) help in testing?
A: Boundary scan provides a standardized interface for testing I/O pins and allows easy access for external testing equipment.
Q: What are the advantages of using BIST and MBIST?
A: BIST and MBIST allow self-testing of circuit blocks and memory, reducing the need for external test equipment and minimizing test time.
Q: What are the different compression techniques used in DFT?
A: Some compression techniques include Test Response Compression (TRC) and Test Data Compression (TDC).
Q: How can I verify the effectiveness of DFT techniques?
A: Fault simulation and test coverage analysis are used to verify the effectiveness of DFT techniques.

Summary

Design for Test (DFT) techniques in Verilog are essential for ensuring the testability and manufacturability of complex digital designs. By incorporating scan chains, boundary scan, compression techniques, BIST, MBIST, and ECC, designers can achieve high fault coverage and efficient testing. Avoiding common mistakes and performing thorough verification are critical in successful DFT implementation. These DFT techniques enable robust testing of FPGA and ASIC designs, leading to reliable and high-quality electronic products.