# A Review of Test Stimulus Compression Methods for Ultra-Large-Scale Integrated Circuits
## 4 Scan Chain Optimization-Based Compression Methods
### Broadcast Scan Structure  
The test stimulus compression method based on broadcast scan is another type of scan chain optimization, allowing the test stimuli to be shared simultaneously with multiple scan chains in the circuit under test. Compared to the linear decompression structure, a broadcast scan structure can effectively reduce the number of test vectors in certain cases but exhibit inferior compression capabilities for the overall data volume [28]. [PERSON] was the first to propose this broadcast-sharing scan chain optimization method [29]. This optimization method is simple in structure and efficient in compression, but when applying this method to test the circuit under test, the fault coverage is limited and cannot cover some rare faults. To improve the fault coverage of broadcast scan testing, researchers from the University of Illinois in the United States proposed the famous Illinois scan architecture [30], which includes a broadcast scan mode and a serial scan mode. Faults that cannot be detected in the broadcast scan mode need to be detected in the serial scan mode to increase the fault coverage of the testing method. The broadcast scan mode of the Illinois scan architecture is depicted in Figure 5. The original single scan chain can be divided into four scan chains of length L, and the test vectors are input to these four scan chains in a shared broadcast manner. In this mode, a single test channel can simultaneously drive four scan chains, and the responses from the scan chains are also transmitted to the Multiple Input Signature Register (MISR). Although the detection efficiency is high in the broadcast scan mode, there are some special non-common fault structures that cannot be detected by this mode. In such cases, it is necessary to switch to the serial scan mode and input additional test vectors to detect these non-common faults. The serial scan mode of the Illinois scan architecture involves connecting all the scan chains in series to form a single 4L serial scan chain. The test time in this mode is four times that of the broadcast scan mode. As the circuit scale of integrated circuits continues to expand, to ensure effective testing, the length of a single scan chain also increases. In this situation, the test time cost of the serial scan mode increases significantly, severely affecting the testing efficiency of the Illinois scan architecture. Therefore, it is necessary to switch between test modes in real-time through selectors between scan slices according to the testing requirements. The Illinois architecture relies on a fixed connection relationship between the scan chains and the scan inputs, and under this structural limitation, the test time cannot be further compressed. Therefore, researchers have improved the Illinois structure to form a reconfigurable Illinois scan structure, which allows the connection relationship between the scan chains and the original scan inputs to be reconfigured. In this structure, the fault coverage can be improved by reconstructing the connection relationship, reducing the need for mode switching and subsequently shortening the test time. According to the different stages of reconfiguring the connection relationship, the reconfigurable Illinois scan structure can be further categorized into static reconfiguration and dynamic reconfiguration. Static reconfiguration refers to the reconfiguration of the connection relationship during the switching process of test vectors, i.e., before the next test vector is inputted, the connection structure between the input terminal and the scan chains is changed through a multiplexer. In dynamic reconfiguration, the reconfiguration of the connection relationship occurs during the input process of the test vector into the scan chain. Compared to static reconfiguration, dynamic reconfiguration offers higher flexibility and is suitable for circuits with complex functions and structures, enabling the detection of more faults. However, its control circuitry is more complex to achieve accurate structural changes.