Abstract:
A broadcaster, system, and method for reducing test data volume and test application time in an ATE (automatic test equipment) in a scan-based integrated circuit. The scan-based integrated circuit contains multiple scan chains, each scan chain comprising multiple scan cells coupled in series. The broadcaster is a combinational logic network coupled to an optional virtual scan controller and an optional scan connector. The virtual scan controller controls the operation of the broadcaster. The system transmits virtual scan patterns stored in the ATE and generates broadcast scan patterns through the broadcaster for testing manufacturing faults in the scan-based integrated circuit. The number of scan chains that can be supported by the ATE is significantly increased. Methods are further proposed to reorder scan cells in selected scan chains, to generate the broadcast scan patterns and virtual scan patterns, and to synthesize the broadcaster and a compactor in the scan-based integrated circuit.

Description:
RELATED APPLICATION DATA 
       [0001]    This application is a continuation of nonprovisional U.S. patent application Ser. No. 14/169,404, filed Jan. 31, 2014, which is a continuation of nonprovisional U.S. patent application Ser. No. 13/527,137, filed Jun. 19, 2012, which is a continuation of nonprovisional U.S. patent application Ser. No. 12/216,639, filed Jul. 9, 2008, which is a continuation of nonprovisional U.S. patent application Ser. No. 11/104,651, filed Apr. 13, 2005, which is a continuation-in-part of nonprovisional U.S. patent application Ser. No. 10/339,667, filed Jan. 10, 2003, which claims the benefit of U.S. Provisional Application Ser. No. 60/348,383, filed Jan. 16, 2002, each listed priority application being hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention generally relates to the field of logic design and test using design-for-test (DFT) techniques. Specifically, the present invention relates to the field of logic test and diagnosis for integrated circuits using scan or built-in self-test (BIST) techniques. 
       BACKGROUND 
       [0003]    As the complexity of integrated circuits increases, it becomes more and more important to achieve very high fault coverage while minimizing test cost. Although traditional scan-based methods have been quite successful in meeting these goals for sub-million gate designs during the past few decades, for recent scan-based designs larger than one-million gates, achieving this very high fault coverage at a reasonable price has become quite difficult. This is mainly due to the fact that it requires a significant amount of test-data storage volume to store scan patterns onto the automatic test equipment (ATE). In addition, this increase in test-data storage volume has resulted in a corresponding increase in the costs related to test-application time. Conventional approaches for solving this problem focus on either adding more memory onto the ATE or truncating part of the scan data patterns. These approaches fail to adequately solve the problem, since The former approach adds additional test cost so as not to compromise the circuit&#39;s fault coverage, while the latter sacrifices the circuit&#39;s fault coverage to save test cost. 
         [0004]    As an attempt to solve this problem, a number of prior art design-for-test (DFT) techniques have been proposed. These solutions focus on increasing the number of internal scan chains, in order to reduce test-data volume and hence test application time without increasing, and in some cases while decreasing or eliminating the number of scan-chains that are externally accessible. This removes package limitations on the number of internal scan chains that in some cases can even exceed the package pin count. 
         [0005]    An example of such a DFT technique is Built-In Self-Test (BIST). See U.S. Pat. No. 4,503,537 issued to McAnney (1985). BIST implements on-chip generation and application of pseudorandom scan patterns to the circuit under test eliminating all external access to the scan-chains, and hence removing any limitation on the number of internal scan-chains that can be used. BIST, however, does not guarantee very high fault coverage and must often be used together with scan ATPG (automatic test pattern generation) to cover any remaining hard-to-detect faults. 
         [0006]    Several different approaches for compressing test data before transmitting them to a circuit under test have been proposed. See the papers co-authored by Koenemann et al. (1991), Hellebrand et al. (1995), Rajski et al. (1998), Jas et al. (2000), Bayraktaroglu et al. (2001), and U.S. Pat. No. 6,327,687 issued to Rajski et al. (2001). These methods are based on the observation that test cubes (i.e., arrangements of scan data patterns stored within the scan chains of a circuit under test) often contain a large number of unspecified (don&#39;t care) positions. It is possible to encode such test cubes with a smaller number of bits and later decompress them on-chip using an LFSR (linear-feedback shift register) based decompression scheme. This scheme requires solving a set of linear equations every time a test cube is generated using scan ATPG. Since solving these linear equations depends on the number of unspecified bits within a test cube, these LFSR-based decompression schemes often have trouble compressing an ATPG pattern without having to break it up into several individual patterns before compression, and hence have trouble guaranteeing very high fault coverage without having to add too many additional scan patterns. 
         [0007]    A different DFT technique to reduce test data volume is based on broadcast scan. See the papers co-authored by Lee (1999) et al., Hamzaoglu et al. (1999), and Pandey et al. (2002). Broadcast scan schemes either directly connect multiple scan chains, called broadcast channels, to a single scan input or divide scan chains into different partitions and shift the same pattern into each partition through a single scan input. In these schemes, the connections between each and every scan input and its respective broadcast channels is done using either wires or buffers, without any logic gates, such as AND, OR, NAND, NOR, XOR, XNOR, MUX (multiplexer), or NOT (inverter) in between. Although it is possible to implement this scheme with practically no additional hardware overhead, it results in scan chains with very large correlation between different scan-chain data bits, resulting in input constraints that are too strong to achieve very high fault coverage. 
         [0008]    Accordingly, there is a need to develop an improved method and apparatus for guaranteeing very high fault coverage while minimizing test data volume and test application time. The method we propose in this invention is based on broadcast scan, and thus, there is no need to solve any linear equations as a separate step after scan ATPG. A broadcast scan reordering approach is also proposed to further improve the circuit&#39;s fault coverage. 
       SUMMARY 
       [0009]    Accordingly, a primary objective of this invention is to provide such an improved method and apparatus. The method we propose is based on broadcast scan, but adds a broadcaster circuit placed between the ATE (automatic test equipment) outputs and the scan chain inputs of the circuit under test. This broadcaster can be embedded on-chip or designed into the ATE. For the sake of simplicity, in this discussion we assume that the broadcaster is placed between the ATE and the integrated circuit under test without specifying where it is located physically. The following discussion applies regardless of where the broadcaster is embedded in an actual implementation. 
         [0010]    The method according to the present invention is used to generate a broadcast scan patterns that are applied to the scan cells (memory elements) of an integrated circuit design under test. This process involves converting the virtual scan patterns stored in an ATE into broadcast scan patterns that are applied to the package scan input pins of the integrated circuit using a broadcaster. This broadcaster maps the virtual scan patterns into their corresponding broadcast scan patterns that are used to test for various faults, such as stuck-at faults, delay faults, and bridging faults in an integrated circuit. The integrated circuits tested contains multiple scan chains each consisting of any number of scan cells coupled together that store the broadcast scan pattern. 
         [0011]    One important aspect of this invention is the design of the broadcaster circuitry. The broadcaster can be as simple as a network of combinational logic circuitry (combinational logic network) or can possibly comprise a virtual scan controller in addition to a network of combinational logic. (Please refer to  FIG. 4  and  FIG. 6  in DETAILED DESCRIPTION OF THE DRAWINGS for more descriptions). Adding a virtual scan controller allows the mapping performed by the broadcaster to vary depending on the internal state of the controller. The broadcaster can also be implemented using a programmable logic array. In this scheme, each ATE output is connected to a subset of the scan chain (or scan partition) inputs via the combinational logic network. Any remaining inputs of the combinational logic network are directly connected to the virtual scan controller outputs if available. During scan test, the virtual scan controller is first loaded with a predetermined value using boundary-scan or other external means. This is used to initially setup the function of the broadcaster. Later in the test, It is possible and often desirable to load in a different predetermined value into the virtual scan controller in order to change the function of the broadcaster, and this can be repeated any number of times. This allows the outputs of the broadcaster to implement different or all combinations of logic functions. Since the function of the broadcaster is a programmable function of the value stored in the virtual scan controller, there is no limitation to the number of mappings that can be implemented. This relaxes the strong input constraints of traditional broadcast scan and increases the ability to generate broadcast scan patterns to test more and possibly all testable faults. This is true since the value stored in the virtual scan controller determines the input constraints imposed on the generation of broadcast scan patterns. 
         [0012]    While a combinational logic network is the preferred implementation for the broadcaster due to its simplicity and low overhead, the broadcaster described in this invention can comprise a virtual scan controller and any combinational logic network. The virtual scan controller can be any general finite state machine, such as an LFSR (linear feedback shift register), as long as predetermined values can be loaded into all memory elements of the finite-state machine, such as D flip-flops or D latches, when desired. The combinational logic network can includes one or more logic gates, such as AND, OR, NAND, NOR, XOR, MUX, NOT gates, or any combination of the above. This combinational logic network increases the chance of generating broadcast scan patterns that test additional faults, such as pattern resistant faults when compared to traditional broadcast scan. 
         [0013]    Another aspect of this invention is the creation and generation of broadcast scan patterns that meets the input constraints imposed by the broadcaster. When a combinational logic network is used to implement the broadcaster, the input constraints imposed by the broadcaster allow only a subset of the scan cells to receive a predetermined logic value either equal or complementary to the ATE output, at any time. Unlike the prior-art broadcast scan schemes which only allow all-zero and all-one patterns to be applied to the broadcast channels, the present invention allows different combinations of logic values to appear at these channels at different times. The only thing needed to generate these test patterns is to enhance the currently available ATPG tools to implement these additional input constraints. Hence. the process of generating broadcast scan patterns will be to generate patterns using an initial set of input constraints and to analyze the coverage achieved. If the fault coverage achieved is unsatisfactory, a different set of input constraints is applied and a new set of vectors are generated. This process is repeated until predetermined limiting criteria are met. 
         [0014]    In order to reduce the number of input constraints needed to achieve very high fault coverage, the present invention may involve a broadcast scan chain reordering step before ATPG takes place. Our approach is to perform input-cone analysis from each cone output (scan cell input) tracing backwards to all cone inputs (scan cell outputs), and then to uses a maximal covering approach to reorder all cone inputs (scan cell outputs) so that only one constrained scan cell is located on a single broadcast channel during any shift clock cycle. These broadcast scan order constraints reduce, if not eliminate, the data dependency among broadcast channels associated with one ATE output. This gives the ATPG tool a better chance of generating broadcast scan patterns that achieve the target fault coverage without having to use a different set of input constraints. Please note that this applies only to integrated circuits that are still in the development phase, and hence broadcast scan reordering should be performed before the chip tapes out. 
         [0015]    Although this process does add some CPU time to the ATPG process, it is much simpler and less computationally intensive as having to solve sets of linear equations after ATPG. The one-step “broadcast ATPG” process makes it easier to generate broadcast scan patterns as compared to LFSR-based decompression schemes. In addition, it is possible to use maximum dynamic compaction, an essential part of combinational ATPG, to fill in as many as unspecified (don&#39;t-care) positions in an effort to detect the most possible faults using a single scan pattern. This is in sharp contrast to LFSR-based decompression schemes where unspecified (don&#39;t-care) positions are desirable in order to be able to solve the linear equations needed to obtain a compressed test pattern. This is the fundamental conflict and flaw in LFSR-based decompression schemes that require starting out with a set of ATPG vectors with little compaction in order to be able to generate a set of more compact vectors. This reduces the actual compaction achieved when compared to an initial set of compact ATPG vectors testing the same faults, and allows the virtual-scan controller-based broadcast-scan method described in the present invention to cover more faults per scan test pattern than any LSFR-based decompression scheme. 
     
    
     
       THE BRIEF DESCRIPTION OF DRAWINGS 
         [0016]    The above and other objects, advantages and features of the invention will become more apparent when considered with the following specification and accompanying drawings wherein: 
           [0017]      FIG. 1  shows a block diagram of a conventional system for testing scan-based integrated circuits using an automatic test equipment (ATE); 
           [0018]      FIG. 2  shows a block diagram of a broadcast scan test system, in accordance with the present invention, for testing scan-based integrated circuits using an ATE; 
           [0019]      FIG. 3  shows a prior art broadcaster design with only pure wires; 
           [0020]      FIG. 4  shows a block diagram of a broadcaster, in accordance with the present invention, consisting of a combinational logic network and an optional scan connector; 
           [0021]      FIG. 5A  shows a first embodiment of a broadcaster shown in  FIG. 4 , in accordance with the present invention, consisting of a combinational logic network; 
           [0022]      FIG. 5B  shows the inputs constraint imposed by the embodiment of a broadcaster shown in  FIG. 5A ; 
           [0023]      FIG. 5C  shows a second embodiment of a broadcaster shown in  FIG. 4 , in accordance with the present invention, consisting of a combinational logic network and a scan connector; 
           [0024]      FIG. 5D  shows the inputs constraint imposed by the embodiment of a broadcaster shown in  FIG. 5C ; 
           [0025]      FIG. 6  shows a block diagram of a broadcaster, in accordance with the present invention, consisting of a virtual scan controller, a combinational logic network, and an optional scan connector; 
           [0026]      FIG. 7  shows a first embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention; 
           [0027]      FIG. 8  shows a second embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention; 
           [0028]      FIG. 9  shows a third embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention; 
           [0029]      FIG. 10  shows a fourth embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention; 
           [0030]      FIG. 11  shows a fifth embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention; 
           [0031]      FIG. 12  shows a sixth embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention; 
           [0032]      FIG. 13  shows a block diagram of a compactor, in accordance with the present invention, consisting of a mask network and a XOR network or a multiple-input signature register (MISR); 
           [0033]      FIG. 14  shows a first embodiment of a compactor shown in  FIG. 13 , in accordance with the present invention; 
           [0034]      FIG. 15  shows a second embodiment of a compactor shown in  FIG. 13 , in accordance with the present invention; 
           [0035]      FIG. 16A  shows an embodiment of the method before reordering scan cells or changing the scan chain length for generating broadcast scan patterns to test more faults, in accordance with the present invention; 
           [0036]      FIG. 16B  shows an embodiment of the method after reordering scan cells for generating broadcast scan patterns to test more faults, in accordance with the present invention; 
           [0037]      FIG. 16C  shows an embodiment of the method after changing the scan chain length for generating broadcast scan patterns to test more faults, in accordance with the present invention; 
           [0038]      FIG. 17  shows a flow chart of the method for reordering scan cells for fault coverage improvement, in accordance with the present invention; 
           [0039]      FIG. 18  shows a flow chart of the method for generating broadcast scan patterns used in testing scan-based integrated circuits, in accordance with the present invention; 
           [0040]      FIG. 19  shows a flow chart of the method for synthesizing a broadcaster and a compactor to test a scan-based integrated circuit, in accordance with the present invention; and 
           [0041]      FIG. 20  shows an example system in which the broadcast scan test method, in accordance with the present invention, may be implemented. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    The following description is presently contemplated as the best mode of carrying out the present invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the principles of the invention. The scope of the invention should be determined by referring to the appended claims. 
         [0043]      FIG. 1  shows a block diagram of a conventional system for testing scan-based integrated circuits using an ATE. The system  101  includes a tester or external automatic test equipment (ATE)  102  and a circuit-under-test (CUT)  107 , which contains scan chains  109 . 
         [0044]    The ATE  102  applies a set of fully specified test patterns  103 , one by one, to the CUT  107  via scan chains  109  in scan mode from external scan input pins  111  as well as from external primary input pins  113 . The CUT is then run in normal mode using the applied test pattern as input, and the response to the test pattern is captured into the scan chains. The CUT is then put back into scan mode again and the test response is shifted out to the ATE via scan chains from external scan output pins  112  as well as from external primary output pins  114 . The shifted-out test response  104  is then compared by the comparator  105  with the corresponding expected test response  106  to determine if any fault exists in the CUT, and indicates the result by the pass/fail signal  115 . 
         [0045]    In the conventional system  101 , the number of scan chains  109  in the CUT  107  is identical to the number of the external scan input pins  111  or the number of the external scan output pins  112 . Since the number of external pins is limited in an integrated circuit, the number of scan chains in the conventional system is also limited. As a result, a large integrated circuit with a large number of scan cells (SC)  108  usually contains very long scan chains for scan test. This will result in unacceptably large test data volume and costly long test application time. 
         [0046]      FIG. 2  shows a block diagram of a broadcast scan test system, in accordance with the present invention, for testing scan-based integrated circuits using an ATE. The system  201  includes an ATE  202  and a circuit  207  that includes a broadcaster  208 , a CUT  209 , and a compactor  213 . The CUT contains scan chains  211 . 
         [0047]    The broadcaster  208  may contain only a combinational logic network as shown in  FIG. 4  or a virtual scan controller in addition to a combinational logic network as shown in  FIG. 6 . The broadcaster is used to map virtual scan patterns  203  to broadcast scan patterns, where the number of bits of a virtual scan pattern is usually smaller than that of a broadcast scan pattern. The mapping function of a broadcaster is fixed if it only contains a combinational logic network. However, the mapping function is variable if it also contains a virtual scan controller. In this case, the output values of the virtual scan controller can change the mapping function that the combinational logic network realizes, thus implementing different mapping relations from external scan input pins  215  to internal scan chain inputs  219 . The compactor  213  is a combinational logic network, such as an XOR network, designed to map the internal scan chain outputs  220  to external scan output pins  216 . Note that in practice, the number of external scan input or output pins is smaller than the number of internal scan chain inputs or outputs. 
         [0048]    Note that the element  213  can be replaced with an optional space compactor and a multiple-input signature registers (MISR). In this case, all test responses will be compressed into a single signature, which can be compared with a reference signature either in the circuit  207  or in the ATE  202  after all broadcast scan patterns have been applied. 
         [0049]    In addition, the compactor  213  usually contains a mask network used to block several output streams from coming into a XOR compaction network or a MISR. This is useful in fault diagnosis. 
         [0050]      FIG. 3  shows a prior art broadcaster design with only pure wires. This example broadcaster design  301  has two broadcast scan inputs  314  and  315 . The broadcast scan input  314  is connected directly to scan chains  303  to  307  while the broadcast scan input  315  is connected directly to scan chains  308  to  312 . Although the overhead of this pure-wire broadcast design is very low, the test pattern dependency among the scan chains fed by the same broadcast scan input is very high. From the point of view of automatic test pattern generation (ATPG), this pure-wire broadcast design puts a strong constraint on the inputs to scan chains. As a result, this scheme usually suffers from severe fault coverage loss. 
         [0051]      FIG. 4  shows a block diagram of a broadcaster, in accordance with the present invention, consisting of a combinational logic network and an optional scan connector. Virtual scan patterns are applied via broadcast scan inputs  407  of the broadcaster  401  to the combinational logic network  402 . The combinational logic network implements a fixed mapping function, which converts a virtual scan pattern into a broadcast scan pattern. The broadcast scan pattern is then applied to all scan chains  409  in the CUT  404 , through an optional scan connector  403 . 
         [0052]    The broadcaster  401  serves the purpose of providing test patterns to a large number of internal scan chains  406  through a small number of external broadcast scan input pins  407 . As a result, all scan cells SC  405  in the CUT  404  can be configured into a large number of shorter scan chains. This will help in reducing test data column and test application time. By properly designing the combinational logic network  402 , one can reduce the fault coverage loss caused by additional constraints imposed on the input pins of the scan chains. 
         [0053]      FIG. 5A  shows a first embodiment of a broadcaster shown in  FIG. 4 , in accordance with the present invention, consisting of a combinational logic network. In this example, a 3-bit virtual scan pattern is converted into an 8-bit broadcast scan pattern via the broadcaster  501 . 
         [0054]    The broadcaster  501  consists of a combinational logic network  502 , which contains two inverters  503  and  507 , one multiplexer  508 , one XOR gate  504 , one OR gate  505 , and one NOR gate  506 . Virtual scan patterns are applied via broadcast scan inputs X2  518  to X0  520 . The combinational logic network implements a fixed mapping function, which converts a virtual scan pattern into a broadcast scan pattern. The broadcast scan pattern is then applied to all scan chains  510  to  517  via Y7  521  to Y0  528  in the CUT  529 . 
         [0055]      FIG. 5B  shows the inputs constraint imposed by the embodiment of a broadcaster shown in  FIG. 5A . 
         [0056]    The broadcaster  501  in  FIG. 5A  has three broadcast scan inputs X2  518  to X0  520 . Thus, there are 8 input combinations for the broadcast scan inputs as listed under &lt;X2, X1, X0&gt; in the table  531 . These are all possible input value combinations to the combinational logic network  502  in  FIG. 5A . Therefore, as the outputs of the combinational logic network, there are 8 value combinations as listed under &lt;Y7, Y6, Y5, Y4, Y3, Y2, Y1, Y0&gt; in the table  531 . These are all possible logic value combinations that may appear at the inputs of the scan chains  510  to  517  in  FIG. 5A , and they are the input constraints in the process of ATPG. 
         [0057]      FIG. 5C  shows a second embodiment of a broadcaster shown in  FIG. 4 , in accordance with the present invention, consisting of a combinational logic network and a scan connector. In this example, a 3-bit virtual scan pattern is converted into an 8-bit broadcast scan pattern via the broadcaster  561 . 
         [0058]    The broadcaster  561  consists of a combinational logic network  562  and a scan connector  566 . The combinational logic network contains one inverter  565 , one XOR gate  563 , and one OR gate  564 . Virtual scan patterns are applied via broadcast scan inputs X2  581  to X0  583 . The combinational logic network implements a fixed mapping function, which converts a virtual scan pattern into a broadcast scan pattern. The broadcast scan pattern is then applied to all scan chains  573  to  580  through the scan connector  566 . The scan connector consists of one buffer  567 , one inverter  570 , one lock-up element LE  569 , and one spare cell SC  568 . Generally, two scan chains can be connected into one by using a buffer, an inverter, or a lock-up element in a scan connector. In addition, a spare cell can be added into an existing scan chain to change its length in order to reduce the dependency among different scan chains. This will help improve fault coverage. 
         [0059]      FIG. 5D  shows the inputs constraint imposed by the embodiment of a broadcaster shown in  FIG. 5C . 
         [0060]    The broadcaster  561  in  FIG. 5C  has three broadcast scan inputs X2  581  to X0  583 . Thus, there are 8 input combinations for the broadcast scan inputs as listed under &lt;X2, X1, X0&gt; in the table  591 . These are all possible input value combinations to the combinational logic network  562  in  FIG. 5C . Therefore, as the outputs of the combinational logic network, there are 8 value combinations as listed under &lt;Y4, Y3, Y2, Y1, Y0&gt; in the table  591 . These are the input constraints in the process of ATPG. 
         [0061]      FIG. 6  shows a block diagram of a broadcaster, in accordance with the present invention, consisting of a virtual scan controller, a combinational logic network, and an optional scan connector. 
         [0062]    The broadcaster  601  consists of a virtual scan controller  602 , a combinational logic network  603 , and an optional scan connector  604 . Virtual scan patterns are applied via two types of input pins: broadcast scan inputs  608  and virtual scan inputs  609 . The broadcast scan inputs are connected directly to the combinational logic network, while the virtual scan inputs are connected directly to the virtual scan controller. In addition, the virtual scan controller may have optional virtual scan outputs  613 . 
         [0063]    Note that the virtual scan controller  602  can be either a combinational circuit such as a decoder, or a sequential circuit such as a shift register. The logic values applied through virtual scan inputs  609  may or may not change in each clock cycle although logic values applied through broadcast scan inputs  608  change in each clock cycle. The purpose of applying virtual scan input values is to change and store a proper set-up value combination in the virtual scan controller. This set-up value combination is applied to the combinational logic network  603  through  610  in order to change the mapping function that the combinational logic network implements. Since one mapping function corresponds to one set of input constraints for ATPG, providing the capability of changing mapping functions results in more flexible input constraints for ATPG. As a result, fault coverage loss due to the broadcast scheme can be substantially reduced. 
         [0064]    Generally, the broadcaster  601  serves two purposes during test. One purpose is to provide test patterns to a large number of internal scan chains  607  through a small number of external broadcast scan input pins  608  and virtual scan input pins  609 . As a result, all scan cells SC  606  in a circuit can be configured into a large number of shorter scan chains. This will help in reducing test data volume and test application time. Another purpose is to increase the quality of broadcast scan patterns applied from the combinational logic network  603  to all scan chains in order to obtain higher fault coverage. This is achieved by changing the values loaded into the virtual scan controller. Because of this flexibility, the combinational logic network can realize different mapping functions rather than a fixed one. 
         [0065]      FIG. 7  shows a first embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention. The broadcaster  701  consists of a virtual scan controller  702  and a combinational logic network  705 . The virtual scan controller consists of two inverters  703  and  704 . The combinational logic network is composed of 8 XOR gates  706  to  713 . In this example, a 4-bit virtual scan pattern is converted into an 8-bit broadcast scan pattern via the broadcaster. 
         [0066]    Obviously, the outputs  730  and  731  of the virtual scan controller  702  must have complementary values. In addition, the outputs  732  and  733  of the virtual scan controller must also have complementary values. Suppose that the values applied to the two broadcast scan inputs  728  and  729  are V1 and V2, respectively. In this case, the values appearing at scan chain inputs  734  to  743  should be P1, ˜P1, P2, ˜P2, V1, V2, P3, ˜P3, P4, ˜P4, respectively. Here P1 and ˜P1 are complementary, P2 and ˜P2 are complementary, P3 and ˜P3 are complementary, P4 and ˜P4 are complementary. In addition, P1 and P2 are either the same as V1 or are the complement of V1 while P3 and P4 are either the same as V1 or are the complement of V2. This is the input constraint for ATPG. 
         [0067]      FIG. 8  shows a second embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention. The broadcaster  801  consists of a virtual scan controller  802  and a combinational logic network  804 . The virtual scan controller consists of a 2-to-4 decoder  803 . The combinational logic network is composed of 8 XOR gates  805  to  812 . In this example, a 4-bit virtual scan pattern is converted into an 8-bit broadcast scan pattern via the broadcaster. 
         [0068]    Obviously, there are four possible logic value combinations for the outputs  829  to  832  of the 2-to-4 decoder  803 . They are 1000, 0100, 0010, and 0001 for the outputs  829  to  832 , respectively. Suppose the output value combination of the 2-to-4 decoder is 1000. Also suppose that the logic values applied to the two broadcast scan inputs  827  and  828  are V1 and V2, respectively. In this case, the values appearing at scan chain inputs  833  to  842  should be ˜V1, V1, V1, V1, V1, V2, ˜V2, V2, V2, V2, respectively. Here V1 and ˜V1 are complementary, while V2 and ˜V2 are complementary. This is the input constraint for ATPG. Obviously, by changing the values of virtual scan inputs  825  and  826 , one can get different set of input constraints for ATPG. This will help in improving fault coverage. 
         [0069]      FIG. 9  shows a third embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention. 
         [0070]    The broadcaster  901  consists of a virtual scan controller  902  and a combinational logic network  911 . The virtual scan controller consists of an 8-stage shift register with memory elements  903  to  910 . There is one virtual scan input  932 , which is the input to the shift register. There is one optional virtual scan output  935 , which is the output of the shift register. Optionally, the virtual scan input and the virtual scan output can be connected to TDI and TDO in the boundary scan design, respectively. The combinational logic network is composed of 8 XOR gates  912  to  919 . There are two broadcast scan inputs,  933  and  934 . Test patterns applied via the input  933  are broadcasted to scan chains  922  to  926 ; while test patterns applied via the input  934  are broadcasted to scan chains  927  to  931 . 
         [0071]    The scan chains  926  and  927  are loaded directly from the broadcast scan input  933  and  934 , respectively, while the scan chains  922  to  925 , as well as the scan chains  928  to  931 , are loaded through XOR gates  912  to  915  and  916  to  919 , respectively. If the value of the memory element  903  is a logic 0, the scan chain  922  will get the identical values as those applied from the broadcast scan input  933 . If the value of the memory element  903  is a logic 1, the scan chain  922  will then get the complementary values to those applied from the broadcast scan input  933 . The same observation applies to the scan chains  923  to  925  as well as  928  to  931 . This means that, by applying a set of properly determined values to the shift register in the virtual scan controller  902 , it is possible to apply any of the 1024 combinations of logic values to the scan chains  922  to  931  in any shift cycle. As a result, any detectable fault in the CUT  920  can be detected by loading a set of properly determined logic values to the shift register and by applying a broadcast scan pattern through the inputs  933  and  934 . 
         [0072]    From the point of view of ATPG, which tries to generate broadcast scan patterns to drive all scan chains in order to test the CUT  920 , the broadcaster configuration determined by the values of the memory elements in the shift register of the virtual scan controller  902  represents an input constraint. Suppose that the values for the memory elements  903  to  910  are 0, 1, 0, 1, 0, 1, 0, 1, respectively. In this case, the ATPG for the CUT should satisfy such an input constraint that, in any shift cycle, the scan chains  922 ,  924 , and  926  have the identical value V, the scan chains  923  and  925  have the identical value ˜V that is the complement of V, the scan chains  927 ,  928 , and  930  have the identical value P, the scan chains  929  and  931  have the identical value ˜P that is the complement of P. 
         [0073]      FIG. 10  shows a fourth embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention. 
         [0074]    The broadcaster  1001  consists of a virtual scan controller  1002  and a combinational logic network  1006 . The virtual scan controller consists of a 3-stage shift register with memory elements  1003  to  1005 . There is one virtual scan input  1023 , which is the input to the shift register. There is one optional virtual scan output  1026 , which is the output of the shift register. Optionally, the virtual scan input and the virtual scan output can be connected to TDI and TDO in the boundary scan design, respectively. The combinational logic network is composed of 4 XOR gates  1007  to  1010 . There are two broadcast scan inputs,  1024  and  1025 . Test patterns applied via the input  1024  are broadcasted to scan chains  1013  to  1017 ; test patterns applied via the input  1025  are broadcasted to scan chains  1018  to  1022 . 
         [0075]    The major difference between the broadcaster  901  in  FIG. 9  and the broadcaster  1001  in  FIG. 10  is that test patterns are broadcasted directly to some scan chains instead of going through XOR gates in the broadcaster  1001 . The scan chains  1013 ,  1015 , and  1017  are driven directly from the broadcast scan input  1024 . This means that, in any shift cycle, scan chains  1013 ,  1015 , and  1017  will have the identical values. In addition, the scan chains  1018 ,  1020 , and  1022  are driven directly from the broadcast scan input  1025 . This means that, in any shift cycle, scan chains  1018 ,  1020 , and  1022  will have the identical values. As a result, by applying a set of properly determined values to the shift register in the virtual scan controller  1002 , it is only possible to apply any of the 64 combinations of logic values to the scan chains  1013  to  1022  in any shift cycle. That is, the broadcaster  1001  needs less hardware overhead at the expense of stronger constraints at the inputs to the scan chains. 
         [0076]      FIG. 11  shows a fifth embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention. 
         [0077]    The broadcaster  1101  consists of a virtual scan controller  1102  and a combinational logic network  1106 . The virtual scan controller consists of a 3-stage shift register with memory elements  1103  to  1105 . There is one virtual scan input  1127 , which is the input to the shift register. There is one optional virtual scan output  1130 , which is the output of the shift register. Optionally, the virtual scan input and the virtual scan output can be connected to TDI and TDO in the boundary scan design, respectively. The combinational logic network is composed of four XOR gate ( 1108 ,  1109 ,  1112 ,  1114 ), two inverters ( 1107 ,  1113 ), one AND gate ( 1110 ), and one OR gate ( 1111 ). There are two broadcast scan inputs,  1128  and  1129 . Test patterns applied via the input  1128  are broadcasted to scan chains  1117  to  1121 ; test patterns applied via the input  1129  are broadcasted to scan chains  1122  to  1126 . 
         [0078]    The broadcaster  1101  realizes more complex broadcast mapping relations from the broadcast scan inputs  1128  and  1129  to the inputs of the scan chains  1117  to  1126 . The general form of the mapping relations can be represented by &lt;VB, VC, V, VC, V*P, V+P, PC1, PB, PC2, P&gt; corresponding to the inputs of the scan chains  1117  to  1126 , respectively. Here, V and P are two logic values applied from the broadcast scan inputs  1128  and  1129  in any shift cycle, respectively. VB and PB are the complements of V and P, respectively. VC equals V or VB if the output value of the memory element  1103  is a logic 0 or 1, respectively. PC1 equals P or PB if the output value of the memory element  1104  is a logic 0 or 1, respectively; PC2 equals P or PB if the output value of the memory element  1105  is a logic 0 or 1, respectively. Obviously, the broadcast mapping relation can be changed by changing VC, PC1, and PC2 through loading different sets of logic values into the shift register in the virtual scan controller  1102 . As a result, less inter-dependent test stimuli can be applied to the CUT  1115  so that higher fault coverage can be reached. 
         [0079]    From the point of view of ATPG, which tries to generate broadcast scan patterns to drive all scan chains  1117  to  1126  in order to test the CUT  1115 , the broadcaster configuration determined by the values of the memory elements in the shift register of the virtual scan controller  1102  represents an input constraint whose general form is &lt;VB, VC, V, VC, V&amp;P, V+P, PC1, PB, PC2, P&gt;. This constrained ATPG can be performed if the original sequential CUT  1115  is transformed to a combinational circuit model reflecting the constraint after the values of the memory elements are determined. 
         [0080]      FIG. 12  shows a sixth embodiment of a broadcaster shown in  FIG. 6 , in accordance with the present invention. 
         [0081]    The broadcaster  1201  consists of a virtual scan controller  1202 , a combinational logic network  1203 , and a scan connector  1207 . The combinational logic network contains two inverters  1204  and  1206  in addition to one OR gate  1205 . Virtual scan patterns are applied via broadcast scan inputs  1226  and  1227  as well as a virtual scan input TDI  1224 . One output X2  1229  from the virtual scan controller is applied to the combinational logic network, making it able to implement different mapping functions. The output values  1232  to  1236  from the combinational logic network is then applied to all scan chains  1215  to  1223  through the scan connector  1207 . The scan connector consists of one buffer  1209 , one inverter  1212 , one lock-up element LE  1211 , one spare cell SC  1210 , and one multiplexer  1208 . Generally, two scan chains can be connected into one by using a buffer, an inverter, or a lock-up element in a scan connector. In addition, a spare cell can be added into an existing scan chain to reduce the dependency among different scan chains. This will help improve fault coverage. Furthermore, a multiplexer can be used to split a scan chain into two parts. As shown in  FIG. 12 , if the selection signal  1228  of the multiplexer  1208  is a logic 1, the scan chains  1215  and  1216  will get different input value streams. However, if the selection signal  1228  of the multiplexer  1208  is a logic 0, the scan chains  1215  and  1216  can be seen as one scan chain, and only one input value stream goes though them. Obviously, a scan connector can be used to adjust the length of scan chains in the CUT in order to shorten test time or improve fault coverage. 
         [0082]      FIG. 13  shows a block diagram of a compactor, in accordance with the present invention, consisting of a mask network and a XOR network or a MISR. 
         [0083]    The test responses on the outputs  1308  of the CUT corresponding to broadcast scan patterns applied on the inputs  1307  of the CUT pass through a compactor  1304 , which consists of a mask network  1305  and a XOR network or a MISR  1306 . MC  1311  is the signal used to control the mask network. It can be applied from an ATE or generated by a virtual scan controller. The mask network is used to mask some inputs to a XOR network or a MISR. This is useful in fault diagnosis. A XOR network is used to conduct space compaction, i.e. reducing the number of test response lines going out of the circuit. On the other hand, a MISR can be used to compress test responses in both space and time domains. That is, there is no need to check test results cycle by cycle when a MISR is used. On the contrary, it is only necessary to compare the signature obtained at the end of the whole test session. However, it should be noted that no unknown values (X&#39;s) are allowed to come into a MISR. This means stricter design rules should be followed. 
         [0084]      FIG. 14  shows a first embodiment of a compactor shown in  FIG. 13 , in accordance with the present invention. 
         [0085]    The test responses on the outputs  1441  to  1448  pass through a mask network  1412  and then a XOR network  1422 . The mask network consists of two groups of AND gates  1414  to  1417  and  1418  to  1421 , each group being controlled by the four outputs generated by a modified 2-to-4 decoder  1413 . In the diagnosis mode where the mode signal  1449  is a logic 1, this decoder maps logic values on MC1  1429  and MC2  1430  to one of the following combinations: 1000, 0100, 0010, and 0001. With any of these logic combination, it is clear that either group of AND gates will allow only one test response stream to pass to  1431  or  1432 . Obviously, this will help in fault diagnosis. In the test mode where the mode signal  1449  is a logic 0, this decoder will generate an all-1 logic combination. This will allow all test response streams pass to  1431  or  1432 . The XOR network  1422  consists of two groups of 4-to-1 XOR sub-networks, composed of XOR gates  1423  to  1425  and  1426  to  1428 , respectively. 
         [0086]      FIG. 15  shows a second embodiment of a compactor shown in  FIG. 13 , in accordance with the present invention. 
         [0087]    The test responses on the outputs  1540  to  1547  pass through a mask network  1512  and then a MISR  1525 . The mask network consists of two groups of AND gates  1517  to  1520  and  1521  to  1524 , each group being controlled by the four outputs of a shift register composed of memory elements  1513  to  1516 . In the diagnosis mode, this shift register can be loaded from TDI  1526  with one of the following combinations: 1000, 0100, 0010, and 0001. With any of these logic combination, it is clear that either group of AND gates will allow only one test response to pass stream to the MISR. Obviously, this will help in fault diagnosis. In the test mode, an all-1 logic combination will be loaded into the shift register. This will allow all test response streams pass to the MISR. The content of the MISR at the end of a test session can be shifted out from TDO  1529  for comparison with the expected signature. 
         [0088]      FIG. 16A  shows an embodiment of the method before reordering scan cells or changing the scan chain length for generating broadcast scan patterns to test more faults, in accordance with the present invention. A broadcaster  1601  has one broadcast scan input  1614 , which broadcasts logic values to three scan chains,  1606 ,  1608 , and  1611 . 
         [0089]    Since logic values are applied to the scan chain  1611  via an XOR gate  1604 , by properly loading the shift register in the virtual scan controller  1602 , it is possible, in any shift cycle, to apply any logic value which can be different from the one applied via scan chains  1606  and  1608 . However, scan chains  1606  and  1608  will be loaded with the same logic values in any shift cycle. As a result, the scan cells A3  1607  and B3  1610  will have the same logic value in any broadcast test patterns. Since the outputs from the scan cells A3  1607  and B3  1610  are connected to the same combinational logic block  1612 , it is possible that some faults in the combinational logic block cannot be detected due to this strong test pattern dependency. For example, in order to detect some faults in the combinational logic block, it may be necessary to have a logic 0 as the output of the scan cell A3  1607  and a logic 1 as the output of the scan cell B3  1610 . Obviously, these faults will not be detected. 
         [0090]      FIG. 16B  shows an embodiment of the method after reordering scan cells for generating broadcast scan patterns to test more faults, in accordance with the present invention. A broadcaster  1601  has one broadcast scan input  1614 , which broadcasts logic values to three scan chains,  1606 ,  1608 , and  1611 . 
         [0091]    The only difference between  FIG. 16A  and  FIG. 16B  is that, in the scan chain  1608 , the order of the scan cells B2  1609  and B3  1610  is changed. Now, although the outputs of the scan cells A3  1607  and B2  1609  have the same logic value in any shift cycle, the outputs of the scan cells A3  1607  and B3  1610  can have different logic values. As a result, this makes it possible to detect some faults that cannot be detected with the scan order shown in  FIG. 16A . 
         [0092]      FIG. 16C  shows an embodiment of the method after changing the scan chain length for generating broadcast scan patterns to test more faults, in accordance with the present invention. A broadcaster  1601  has one broadcast scan input  1614 , which broadcasts logic values to three scan chains,  1606 ,  1608 , and  1611 . 
         [0093]    The only difference between  FIG. 16A  and  FIG. 16C  is that, one spare scan cell B0  1617  is added to the scan chain  1608  through a multiplexer  1618 . It is clear that, if the selection signal  1619  is a logic 1, the spare scan cell will be added to the scan chain  1608 . As a result, although the outputs of the scan cells A3  1607  and B2  1609  have the same logic value in any shift cycle, the outputs of the scan cells A3  1607  and B3  1610  can have different logic values. As a result, this makes it possible to detect some faults that cannot be detected with the scan order shown in  FIG. 16A . 
         [0094]      FIG. 17  shows a flow chart of the method for reordering scan cells for fault coverage improvement, in accordance with the present invention. This method  1700  accepts the user-supplied HDL codes  1701  together with the chosen foundry library  1702 . The HDL codes represent a sequential circuit comprised of a broadcaster, a full-scan CUT, and a compactor as shown in  FIG. 2 . The HDL codes and the library are then complied into an internal sequential circuit model  1704 , which is then transformed into a combination circuit model  1706 . Then, based on the original scan order information  1709  and the scan order constraints  1710 , the input-cone analysis  1707  is conducted to identify scan cells whose order needs to be changed. Then, scan chain reordering  1708  is conducted. After that, the HDL test benches and tester programs  1711  are generated while all reports and errors are saved in the report files  1712 . 
         [0095]      FIG. 18  shows a flow chart of the method for generating broadcast scan patterns used in testing scan-based integrated circuits, in accordance with the present invention. This method  1800  accepts the user-supplied HDL codes  1801  together with the chosen foundry library  1802 . The HDL codes represent a sequential circuit comprised of a broadcaster, a full-scan CUT, and a compactor as shown in  FIG. 2 . The HDL codes and the library are then complied into an internal sequential circuit model  1804 , which is then transformed into a combination circuit model  1806 . Then, based on input constraints  1810 , combinational fault simulation  1807  is performed, if so required, for a number of random patterns and all detected faults are removed from the fault list. After that, combinational ATPG  1808  is performed to generate virtual scan patterns and all detected faults are removed from the fault list. If predetermined limiting criteria, such as a pre-selected fault coverage goal, are met, the HDL test benches and ATE test programs  1811  are generated while all reports and errors are saved in the report files  1812 . If the predetermined limiting criteria are not met, new input constraints  1810  will be used. For example, a new set of values can be loaded into the virtual scan controller to specify new input constraints. After that, optional random-pattern fault simulation  1807  and ATPG  1808  are performed. This iteration goes on until the predetermined limiting criteria are met. 
         [0096]      FIG. 19  shows a flow chart of the method for synthesizing a broadcaster and a compactor to test a scan-based integrated circuit, in accordance with the present invention. This method  1900  accepts the user-supplied HDL codes  1901  together with the chosen foundry library  1902 . The HDL codes represent a sequential circuit comprised of a broadcaster, a full-scan CUT, and a compactor as shown in  FIG. 2 . The HDL codes and the library are then complied into an internal sequential circuit model  1904 . Then, based on the broadcaster constraints  1908  and the compacter constraints  1909 , broadcaster synthesis  1905  and compactor synthesis  1906  are conducted, respectively. After that, based on the stitching constraints  1910 , stitching  1907  is conducted to integrate the broadcaster and the compactor to the original circuit. At the end, the synthesized HDL codes  1911  are generated while all reports and errors are saved in the report files  1912 . 
         [0097]      FIG. 20  shows an example system in which the broadcast scan test method, in accordance with the present invention, may be implemented. The system  2000  includes a processor  2002 , which operates together with a memory  2001  to run a set of the broadcast scan test design software. The processor  2002  may represent a central processing unit of a personal computer, workstation, mainframe computer or other suitable digital processing device. The memory  2001  can be an electronic memory or a magnetic or optical disk-based memory, or various combinations thereof. A designer interacts with the broadcast scan test design software run by processor  2002  to provide appropriate inputs via an input device  2003 , which may be a keyboard, disk drive or other suitable source of design information. The processor  2002  provides outputs to the designer via an output device  2004 , which may be a display, a printer, a disk drive or various combinations of these and other elements. 
         [0098]    Having thus described presently preferred embodiments of the present invention, it can now be appreciated that the objectives of the invention have been fully achieved. And it will be understood by those skilled in the art that many changes in construction &amp; circuitry, and widely differing embodiments &amp; applications of the invention will suggest themselves without departing from the spirit and scope of the present invention. The disclosures and the description herein are intended to be illustrative and are not in any sense limitation of the invention, more preferably defined in scope by the following claims.