How do you use scan chain compression techniques to reduce test time and data volume in RTL design?

1 What is scan chain compression?

Scan chain compression is a method of reducing the number of scan cells and scan pins in a design by using compression logic. Scan cells are special flip-flops that can capture and shift the internal state of the design during test mode. Scan pins are the inputs and outputs of the scan chains that connect to the test equipment. Compression logic is a combination of encoders and decoders that can compress and decompress the test data.

2 How does scan chain compression work?

Scan chain compression works by encoding the test data at the scan inputs and decoding it at the scan outputs. The encoder can use different algorithms to compress the test data, such as run-length encoding, frequency encoding, or dictionary encoding. The decoder can use the same or a different algorithm to decompress the test data and compare it with the expected values. The encoder and decoder can be implemented either in hardware or in software.

3 What are the benefits of scan chain compression?

Scan chain compression can offer several advantages for your RTL design, such as decreasing the test time, test data volume, scan power consumption, and area overhead. This is achieved by reducing the number of scan cycles needed to apply and capture the test data, the number of bits needed to represent the test data, the switching activity of the scan cells and scan pins, and the number of scan cells and scan pins in the design.

4 What are the challenges of scan chain compression?

Scan chain compression can also present some difficulties for your RTL design, such as increasing design complexity by adding compression logic and control signals, test generation complexity by requiring special algorithms and tools to generate and manipulate compressed test data, and fault coverage complexity by necessitating additional analysis and techniques to guarantee that the compression logic does not reduce the fault detection capability of the test.

5 How do you implement scan chain compression in RTL design?

When implementing scan chain compression in RTL design, there are several methods to choose from. Standard compression cells provided by synthesis or test tool vendors are pre-designed and pre-verified for a specific algorithm, and can be quickly inserted into the scan chains. Alternatively, custom logic designed by yourself or a third-party provider can be tailored to your design’s characteristics and requirements. Finally, software-based compression logic running on a processor or dedicated hardware module can perform advanced, dynamic algorithms and adapt to different test scenarios and modes.

6 How do you verify scan chain compression in RTL design?

Verifying scan chain compression in RTL design is a critical step to ensure that your design is functional and testable. During this process, you must check the correctness and completeness of the compression logic, its compatibility with test data generation and analysis tools, the fault coverage and quality of the compressed test data, and the performance and trade-offs of the compression logic and test data. This includes factors such as test time, data volume, power consumption, and area overhead.