# A Review of Test Stimulus Compression Methods for Ultra-Large-Scale Integrated Circuits
## 2 Overview of Test Stimulus Compression Techniques
### Comparative Analysis of Test Stimulus Compression Techniques  
Encoding-based test compression methods leverage the correlation among the fixed bits in test vectors to achieve compression of the test set. Since these methods compress the test set itself, they impose no constraints on Automatic Test Pattern Generation (ATPG) and can be widely applied to various integrated circuits without requiring knowledge of the structure of the CUT. However, the efficiency of encoding-based compression is limited by the number of don't-care bits in the test vectors; when there are many don't-care bits, the compression efficiency is reduced.  
The linear decompression structure based on combinational logic requires minimal control logic, making it easy to design and implement in hardware. This method utilizes don't-care bits in the test vectors to generate free variables for each scan slice during each clock cycle. The compression efficiency is limited when there are a large number of fixed bits in the scan slice. The linear decompression structure based on sequential logic can effectively overcome this limitation by selectively using free variables from the previous cycle when compressing the current cycle's scan slice. Thereby, it can enhance the flexibility of the compression encoding and increase the likelihood of successful compression.  
Broadcast scan-based test compression technology broadcasts the same test data to multiple scan cells within the CUT. Compared to the linear decompression structure method, it has a shorter test time but can encode fewer test vectors and has lower encoding flexibility. This issue can be mitigated by constraining ATPG through static or dynamic reconstruction. Static reconstruction requires less control information and has relatively lower encoding flexibility, while dynamic reconstruction requires more control information and offers relatively higher encoding flexibility.  
Among the two stimulus compression methods based on scan chain structure optimization, the compression method based on linear decompression structure requires solving a set of equations corresponding to the circuit relationships, while the broadcast scan-based compression method involves broadcasting test vectors to multiple scan chains. The application of both methods presupposes that the test personnel are familiar with the structure of the CUT. However, in general, third-party IP core suppliers do not provide internal structural information. Under these circumstances, test vector generation and fault simulation tools cannot be used normally, and the correctness and comprehensiveness of the analysis results cannot be guaranteed. In such cases, the only viable option is the encoding-based compression method.  
However, after years of development, this technology has matured, and there is limited room for improvement in compression efficiency, which is typically lower than that of the other two methods. To further enhance the efficiency of encoding compression, researchers have proposed preprocessing the original test set, leading to methods that enhance encoding compression efficiency. These methods can effectively improve encoding efficiency and are usually used in conjunction with encoding-based compression methods.  
A comparative analysis of the three test stimulus compression techniques is presented from the perspectives of compression principle, technical advantages, limitations, application scenarios, and compression objects, summarized in Table 2.