Patent Document

This is a continuation-in-part of patent application Ser. No. 09/768,121 filed Jan. 23, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and system for compressing test data used in the testing of logic products such as integrated circuit chips and the like, to economize on the amount of computer memory, disk storage and time required to test such products. 
     In the testing of devices utilized in computers, such as combinational logic circuit chips and memory arrays, tester devices are used. To test, for example, a combinational logic circuit, a tester applies stimuli to inputs of the chip, and resulting output responses are observed in order to detect faults in an expected performance of the logic circuit. The detected faults may be used to debug the circuit design. 
     One method of applying test stimuli to a logic circuit includes placing “scan latches” before and after the combinational logic circuits to be tested. In this method, test data is clocked or scanned from a data input pin to a chain of input latches. The test data typically comprises 0s and 1s which may be arranged as a plurality of vectors. The test vector data comprises “care” bits and “non-care” bits. Care bits are bits which are set by test-generating software to target specific or “focal” faults in the logic being tested. Non-care bits are not targeted toward any particular focal fault. A chain of input latches loaded with test vector data may be referred to as a “scan chain.” 
     According to the method, application of the test vectors to the combinational logic does not begin until all of the input latches are filled using scan chains. Once all the input latches are filled, the test vectors are released and applied to the combinational logic. The combinational logic processes the test vectors and captures the results in latches, and from there the results are scanned to a test output pin where faults may be detected. 
     Advances in computer technology have permitted significant increases in the density with which logic circuits can be packed onto a chip, and accordingly, the number of functions and operations that a chip can perform has increased significantly. Attendant to the increased density and number of logic circuits on a chip, there is an increase in the number, complexity and size of test vectors which need to be applied by a tester to fully verify the chip&#39;s operation. Along with the increased volume of test vectors are associated costs in computer resources used in generating and storing the test vectors, and in tester memory and time needed to download test data to the tester. 
     In view of the foregoing, a method and system for efficiently handling test vector data which addresses the noted concerns is needed. 
     SUMMARY OF THE INVENTION 
     A method and system according to the present invention provides for highly compressible test vectors to be formed, allowing substantial economies in computer resources, tester memory requirements and test processing time. 
     According to the new invention, the test response output data are compressed into a much more compact data representation by using error-detecting data compression means. In an embodiment, the response data are compressed into signatures using Cyclic Redundancy Check (CRC) codes. The CRC-based signatures can be generated by hardware that is located in the product under test, inside the tester, or between the product under test and the tester, or by software in the tester. The amount of expected signature data that needs to be stored on the tester for comparison is much smaller than the total amount of test output response data produced by the product under test. 
     Furthermore, according to the invention, filling the non-care bits in the test input stimulus vector data set with repetitive, repeated, or other algorithmically generated value sequences forms a highly compressible test vector data set. A further reduction in the input stimulus data can be achieved by allowing a substantial portion of the care bits also to be set to repetitive, repeated, or other algorithmically generated value sequences. The proposed combination of data compression techniques for both, test input stimulus data and test output response data, results in significantly better overall data reduction than the use of either method by itself. 
     In an embodiment, the highly compressible test input stimulus vector data set comprises a differential test vector data set. An “exclusive OR” (XOR) operation may be performed between the original care bits of the test vector data and a background vector data set, to generate the differential vector data set. In the differential data set, a substantial portion of bits corresponding to the original care bits may be set to a value of 0 (zero). Bit positions in the differential vector data set corresponding to the non-care bits of the original test vector data are filled with 0s. The background vector data is constructed in a way that permits easy algorithmic generation of the fill data values from a small amount of initialization data using software residing in the tester and operating while the test is performed, hardware that is located in the product under test, inside the tester, or between the tester and the product under test and is operating while the test is performed, or by software used for preparing the tester program and associated data loaded into the tester prior to testing. 
     The repetitive values of 0 generated in the differential vector data set increase the compressibility of the differential vector data set. The differential vector data set may be compressed to economize on memory and storage requirements in stages of a testing process, and subsequently decompressed to recover the original care bits of the test vector data set when it is to be loaded into input latches. 
     A method and system according to the present invention provides for highly compressible test vectors to be formed, allowing substantial economies in computer resources, tester memory requirements and test processing time. 
     According to the invention, a highly compressible test vector data set is formed by filling the non-care bits in the test vector data set with repetitive or repeated values. A substantial portion of the care bits may also be set to repetitive values. 
     In an embodiment, the highly compressible test vector data set comprises a differential test vector data set. An “exclusive OR” (XOR) operation may be performed between the original care bits of the test vector data and a background vector data set, to generate the differential vector data set. In the differential data set, a substantial portion of bits corresponding to the original care bits may be set to a value of 0 (zero). Bit positions in the differential vector data set corresponding to the non-care bits of the original test vector data are filled with 0s. 
     The repetitive values of 0 generated in the differential vector data set increase the compressibility of the differential vector data set. The differential vector data set may be compressed to economize on memory and storage requirements in stages of a testing process, and subsequently decompressed to recover the original care bits of the test vector data set when it is to be loaded into input latches in a tester. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the hardware structure used for testing, with a response data compression device that accumulates the response data from the scan chains; 
     FIG. 2 illustrates a process flow for creating highly compressible test vectors from original test vector data by filling bit positions with repeated values; 
     FIG. 3 shows a process flow for creating a compressed test vector data set; 
     FIG. 4 illustrates a process for recovering original test vector data from a compressed test vector data set; 
     FIGS. 5A-5C show an example of a test vector data set, a randomly-generated background vector data set, and the differential vector data set resulting from XORing the test vectors and background vectors; 
     FIG. 6 illustrates a testing system for testing logic products according to the present invention, illustrating stages at which original test vector data may be reconstructed to load input latches to test logic products; 
     FIG. 7A shows an example of input latches in a logic product loaded with test vectors; 
     FIG. 7B shows an example of the test vector data shown in FIG. 7A after processing according to a “repeat last care bit” step of the invention; and 
     FIG. 8 shows a general purpose computer system for executing software according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Previous methods for reducing the volume of test data stored on Automatic Test Equipment (ATE) for testing scan-based designs have focused on reducing the number of tests by means of merging tests for as many individual target faults as possible into a single scan test. The sophistication of the algorithms used for test merging have matured to a point where further gains are limited. Also, stimulus data compaction on the ATE using constant or repeat-count values for filling in don&#39;t care bit values have been practiced in the past. 
     FIG. 1 is a block diagram that illustrates the overall configuration used during test. A tester is connected to multiple scan chains  10  in the product under test. Test input stimuli are serially shifted into the scan chains in the product under test. After the loading is completed, the test can commence and test response data are captured into the scan chains in the product under test. The captured responses are serially shifted out into a reponse data compression device  30 , such as a MISR (Multiple-Input Signature Register), in a serial fashion. The response data compression device  30  accumulates the serial response data streams into an error detecting code word called “signature”. The tester  50  does not participate in the accumulation or generation of the signature. Hence, the tester interface is available to shift new input stimuli into the scan chains in the product under concurrently with shifting the current set of captured responses out into the response data compression means. The width (number of bits) of the signature word accumulated by the response data compression device generally is equivalent to the number of scan chains. Each scan chain in complex microlectronic porducts may contain several thousands of scan cell bits. Hence, the total number of bits in the signature is by several orders of magnitude less than the total number of reponse bits. At the end of the combined serial load/unload operation, the signature is transferred to the tester and compared with a pre-calculated expected signature word. Only the expected signature word needs to be stored in the tester buffer memory, meaning that the amount of response-related data that is stored on the tester is rather insignificant compared to the traditional method of storing the full set of expected responses for all scan cell bits. Roughly speaking, the amount of data normally stored on the tester without compression (like a MISR) is about 50% input stimulus data and 50% output stimulus data. By removing the 50% output stimulus data via compression, the tester data are now dominated by the input stimulus data, and any reduction in the amount of stimulus data now affects the majority of the data volume—at lease 95%. 
     The input stimulus data for scan based testing arrangements like the one illustrated in FIG. 1 typically are derived by Automatic Test Pattern Generation (ATPG) software from a computer-readable model of the product under test. It should be appreciated for the context of the proposed improved test apparatus and method that ATPG software algorithms allow for substantial flexibility in how a large portion of the stimulus data content is constructed. The ATPG software algorithm options can be intelligently selected in such a way that said software algorithms are compatible with software and/or hardware algorithms available in the tester. The respective data content then can be recreated on the tester by the software/hardware algorithm from a minimal amount of initialization data rather than having to be stored in a bit-for-bit fashion in the tester buffer memory. This freedom in selecting the algorithms used in ATPG is described below which indicates how the amount of input stimulus data can be reduced by matching the software algorithm used in ATPG with software/hardware algorithms used on the tester. The total tester data reduction possible is limited by the remaining amount of compressed input stimulus data plus the amount of uncompressed expected reponse data. It has been found that by combining the advantage of reduced input stimulus data achieved with the additional significant data reduction may be achieved by the use of response data compression device  30  as illustrated in FIG.  1 . 
     FIG. 2 is a flow chart illustrating the creation of a highly compressible test vector data set according to the invention. Original test vector data, for example, a distribution of 0s and 1s as shown in block  101 , may be generated by known means such as Automatic Test Pattern Generation (ATPG) software  100 . The ATPG software utilizes a representation of the combinational logic being tested to generate care bits which target focal faults in the logic circuitry. The care bits may be 0s or 1s. In block  101 , the care bits are circled and the non-care bits are indicated by dashes (-). The non-care bits may be 1s or 0s. Each row of block  101  represents a test vector. 
     Typically, ATPG-generated test vectors for large (1+million gate) circuits, on average, contain less than 1 care bit for every 100 scan chain bits. Thus, care bits are very sparsely distributed throughout the test vectors. The present invention takes advantage of this characteristic of the test vectors by filling the non-care bit positions with repetitive values to increase the compressibility of the test vectors. The non-care bit positions of the test vectors are filled with all 0s, as shown in block  102 . A substantial portion of the original care bits may also be set to 0, and their original values subsequently recovered. The test vector data, with non-care bit positions filled as described and having a substantial number of the original care bits set to 0, is consequently highly compressible due to being dominated by 0s. The test vector data may then be compressed and subsequently decompressed for loading into input latches of a logic product, as described in greater detail in the following. 
     Referring now to FIG. 3, according to an embodiment of the invention, original test vector data is generated as shown in block  200 , and background vectors are also generated as shown in block  201 . The background vectors may be all 0s, all 1s, or a random distribution of 0s and 1s. A known software algorithm and seed may be used to generate the background vectors as shown in Block  202 . A “seed” refers to a data string of a comparatively limited size which is used in combination with the algorithm to generate the background vector data set. The background vector data set, when XORed with the care bits of the original test vector data, will typically produce a differential vector data set in which a substantial portion of bits corresponding to the care bits of the original test vector data are set to 0. Independently of the XOR operation, the non-care bits of the differential vector data set are also set to 0, thereby forming a highly compressible test vector data set from the original test vector data. The original test vector care bits, with the non-care bits having values determined by the background vector data, may be recovered from a compressed differential vector data set as described in greater detail below. 
     As shown in Block  203 , the logical operation “exclusive OR” (XOR) is performed between the original test vectors and the background vectors to create a differential vector data set. Only the care bits of the test vectors are XORed, with corresponding bits of the background vectors. Accordingly, if a background vector data bit corresponding to a care bit has a value of 0, the care bit in the resulting differential vector will be unchanged from its value in the original test vector. On the other hand, if a background vector data bit corresponding to a care bit has a value of 1, the care bit in the resulting differential vector will be flipped or reversed from its value in the original test vector. Independently of the XOR operation, the non-care bits in the differential vector data are set to 0 by, for example, a software algorithm. 
     If the background vectors generated contain a random fill pattern, (a random distribution of both 1s and 0s), it is typical that half of the care bits in the test vectors generated by the ATPG software will tend to match the corresponding values in the randomly generated background vector. Accordingly, an XOR of the background vectors and the test vectors will result in a differential vector data set in which approximately half of the bits corresponding to the care bits of original test data are set to 0. Thus, after the non-care bit positions are filled with 0s, the differential vector data set will typically be dominated by over 99% 0s, making it highly compressible. 
     As shown in block  204 , a data set header identifying the algorithm and seed used to generate the background vectors may be created, and attached to the differential vector data set as shown in block  205 . The differential vectors with attached header may then be compressed using known compression means, as shown in Block  206 . 
     FIG. 3 is a flow chart illustrating the recovery of the original test vector data care bits from the differential data set. As shown in Block  300 , the compressed differential vector data set plus header is decompressed. The header data attached is extracted, and the algorithm and seed used to generate the background vector data set are recovered, as shown in blocks  301  and  302 . Also, the differential vector data set is extracted from the decompressed data as shown in Block  303 . 
     With the algorithm and seed recovered, it is possible to reconstruct the background vector data set as shown in Block  305 . As shown in Blocks  304  and  306 , an XOR between the differential data set and the reconstructed background data set results in a reconstructed test vector data set which reproduces the original test vector care bits, with the non-care bit positions being filled with the corresponding values from the background vector data. 
     FIGS. 5A-5C illustrate the XOR operation to create the differential vectors. FIG. 5A shows an example of a test vector data set such as might be generated by ATPG software, prior to compression according to the present invention. The test vector data set includes care bits at row  1 , columns  2  and  4 , row  2 , columns  3  and  5 , and row  3 , columns  4  and  6 . Non-care bits are illustrated by dashes. FIG. 5B shows an example of a background vector data set such as might be generated by a random fill algorithm. 
     FIG. 5C shows a differential vector data set resulting from performing an XOR operation between the test vector data set illustrated in FIG.  5 A and the background vector data set shown in FIG. 5B, for only the care bits and corresponding background vector bits. It may be noted that in the differential vector data set shown in FIG. 5C, the bit position at row  3 , column  6 , corresponding to the care bit at row  3 , column  6  of the test vector data set shown in FIG. 5A, has been changed from a 1 to a 0. As discussed above, it has been observed that typical ATPG-generated test vectors on average contain less than 1 care bit for everyone 100 scan chain bits, and half of the care bits will tend to match the values in a randomly generated background vector. Accordingly, the XOR operation will tend to set half of the care bits to 0 in the corresponding differential vector data set, yielding a high degree of compressibility. 
     A further advantage is offered by using random fill background vector data sets. When the background vector data set is reconstructed and XORed with the decompressed differential vector data set, the resulting data set will include the care bits of the original test vector data set, with the non-care bits being randomly filled by the background vector data set. When this resulting data set is subsequently loaded into the input latches of a logic product to conduct a test, there is the possibility that the randomly distributed background fill data will detect faults in addition to those detected by the care bits, which are targeted at focal faults. This may be referred to as “fortuitous” fault detection. 
     FIG. 6 shows a test system including major components of a tester  501  for implementing the invention. An I/O bus  503  allows data flow to a central processing unit (CPU)  500  of, for example, a personal computer connected to the tester. The CPU is connected to a memory buffer  504  of the tester by a second bus  509 . Via pin connections  508 , the buffer  504  fans out data to individual chip test pins on logic products  502  to load input latches therein with test vectors. 
     The tester further comprises a sequencer  505  for determining the sequence of patterns for applying the test vectors loaded into the input latches, timing logic  506  for determining the rate at which to run the logic product, and DC subsystems  507  for applying the appropriate voltage levels to the logic product. 
     Typically, a compressed differential vector data set plus header  510  is created by a computer system external to the tester. The computer system which generates the test data may be at a different physical site from the tester, which is typically located in a manufacturing environment. The computer system executes ATPG software to generate original test vector data, and executes software for performing the algorithm to generate background vectors and XOR the background vectors with the care bits of the original test vectors. The compression of the resulting differential vectors plus attached header also takes place externally to the tester. Certain of the foregoing operations, such as the compression, may be performed in hardware. Compression of the differential vector plus header data provides for economizing on computer resources at the site generating the test data, including memory, disk storage or portable storage resources such as diskettes, CD-ROMs, tapes and the like. The compressed test data may also be transmitted over connections of a computer network in less time as compared to the original test data, and downloaded to a tester more efficiently, from either a network or a portable storage source At the tester, the compressed differential vector data set plus header  510  may be input to CPU  500  via I/O bus  503 . Points A, B and C in FIG. 6 represent points where decompression of the differential vector data set plus header may be done. Decompression could be performed by software at point A, or by hardware at points B (tester hardware) and C (hardware built in to the logic product). As discussed above in connection with FIG. 3, the decompression would be followed by the steps of extracting the differential vector data set, and extracting the header to recover the algorithm and seed required to reconstruct the background vectors. The XOR operation between the differential vector data set and reconstructed background vector data set to recover the original test vector care bits plus background fill bits could be performed at the same point that which decompression was performed, or at any point further downstream from the decompress operation. For best results in terms of tester memory buffer conservation, the decompression would take place at point C, in hardware embodied in the logic product itself 
     In view of the above, it may be appreciated that the compressibility of the differential vector allows computer resources including memory, storage and data transmission time, to be conserved at processing stages preceding the loading of scan chains into the input latches of a logic product being tested. When processing has reached a point where the input latches are to be loaded, the original test vector care bits plus background fill are recreated from the compressed differential vector plus header, and loaded into the input latches. 
     An alternative “repeat last fill” embodiment of the present invention, as described hereinafter, generates repetitive or redundant test vectors and utilizes a “repeat” capability of the tester to economize on memory and storage requirements. Typically, commercially available testers have the capability to repeat data such as the strings of 1s and 0s forming test vectors, and load the repeated strings into specified input latches of a logic product. 
     FIG. 7A shows an example of input or scan latches in a logic product loaded with test vector data. The test vector data is arranged between scan-in inputs  600  and scan-out outputs  601 , and may be viewed as a matrix comprising rows and columns of test vector data. As described earlier, the test vector data is loaded via the scan-in inputs prior to being released via the scan-out outputs to combinational logic in a logic product for detecting faults therein. In the example shown in FIG. 7A, each of rows  1 - 15  represents a nine-bit test vector. The circled bits are care bits. 
     It is noted that the vectors at rows  11  and  12  of FIG. 7A are identical. The repeat capability of testers can be used to take advantage of such duplication occurring in test vector data to reduce the amount of data which must be stored. As can be seen by the row designations on the right side of FIG. 7A, a single test vector  11  may be used to fill both row positions  11  and  12 . 
     FIG. 7B shows an example of the input latches of FIG. 7A wherein the test vectors have been processed according to the invention. Considering the rows of test vectors in ascending order, each care bit has been replicated in the rows above it in the corresponding column, until another care bit is encountered with a different value. If a care bit with a different value is encountered, the non-care bit positions are filled with that different care bit value in the ascending row positions in the corresponding column. 
     As a specific example, consider column  2  of FIG.  7 B. At row  1 , there is a care bit having a value of 0. Accordingly, according to the invention the non-care bit at column  2 , row  2  is assigned a value of 0. Then, at row  3 , a care bit having a value of 1 is encountered. Accordingly, the non-care bits at column  2 , rows  4 - 15  are assigned the value of 1, since no other care bit having a different value is present in the column. 
     As another specific example, consider column  4 . There is a care bit in row  3 , column  4  having a value of 0. Accordingly, the non-care bits in rows  4 - 7  of column  4  are assigned the value 0. At row  8  of column  4 , a care bit having a value of 1 is encountered, and accordingly, the non-care bits in rows  9 - 15  of column  4  are assigned the value of 1. 
     It may observed from FIG. 7B that only five distinct test vectors need to be stored. The input latches of the logic product may be loaded using the repeat capabilities of the tester to reproduce each distinct test vector as often as needed. 
     Redundant vectors as described above could be created by a software algorithm; for example, software which interfaced with ATPG software executed on a computer system upstream from the tester. The algorithm would take as input the original test vectors, locate the care bits, and repeat them as described above. For better data compaction, the redundant vectors could be further processed to reduce them to a minimum set of distinct vectors. Such a minimum set, represented by element  511  in FIG. 5, would then be transmitted to the tester, along with information specifying how often and where in the input latches the vectors were to be repeated. 
     The sparseness of the care bits is a factor in generating redundant test vectors by using the technique of repeating the last care bit. Because care bits are sparsely distributed, the probability favors the phenomenon that repeating the care bits in the non-care bit position as described above will generate duplicate test vectors. 
     As discussed above, random filling of the non-care bit positions in the test vectors occasions the phenomenon of “fortuitous” fault detection while testing targeted or focal faults with the care bits. Fortuitous fault detection allows numerous faults to be detected in fewer tests than would otherwise be the case. 
     However, there is a point of diminishing returns where using random fill no longer offers the best test economy. As faults continue to be detected, the number of non-care latches continues to increase to a point where there are very few care latches. There are also many fewer faults remaining to be detected. 
     Thus, at a point in the testing process, it has been observed that better test economy can be achieved by using random-fill test vectors during a first period of testing, followed by a second or subsequent period of using the “repeat last fill” technique of generating a reduced or minimum set of vectors and repeating them, as described above. 
     Accordingly, the present invention further includes applying a first testing technique to logic products during a first testing period, the first testing technique comprising loading input latches of a tester with test vectors comprising a random distribution of bits. The first testing period is followed by a second testing period of applying a second testing technique comprising loading input latches by repeating test vectors of a minimum set of test vectors obtained by repeating a last care bit in neighboring non-care bit positions as described above. 
     Of a full or complete testing period defined as one during which the number of faults detected has been reduced to an acceptable level, one proportion of the first period to the second period that is contemplated is approximately 95% to 5%; i.e., 95% of the faults with random-fill testing followed by approximately 5% with repeat-last-fill testing. The first 95% of the faults are the easier faults and the first period will typically require 10% or less of the test patterns. The second period will contain the large majority of test patterns and use the repeat-last-fill testing method. 
     In conceivable embodiments, the invention is implemented in computer-executable instructions which when executed perform method steps of the invention. The instructions are embodied in a program module or modules stored on computer-usable media. When executed by a processor, the instructions would accept as input ATPG-generated test vector data. The instructions would generate the background vector data using the algorithm and seed as discussed above in connection with FIG.  3 . Subsequently, the instructions would XOR the care bits of the test vector data with the background vector data to create the highly-compressible differential vector data, and create and attach the header as discussed. Compression of the differential vector data, while preferably taking place in hardware for greater speed, could also be implemented in software. 
     Recovery of the original test vector data care bits plus background fill data as discussed above in connection with FIG. 4 could also be performed by computer-executable instructions according to the invention, at, for example, point A in FIG. 6, as noted above. 
     Computer-executable instructions for generating redundant test vectors by repeating a last-encountered care bit as described in connection with FIGS. 7A and 7B, and reducing the redundant test vectors to a distinct set, are also encompassed by the invention. 
     FIG. 8 shows a high-level representation of a general purpose computer system for implementing an embodiment of the invention in the form of computer-executable instructions, such as might be realized by a variety of known and commercially available hardware and software elements. The system comprises a memory  700  including a ROM and RAM, processor  710  and user interface  711  comprising a video display  712 , keyboard  713  and mouse  714 . Elements may communicate via system bus  706 . The system may further be connected to a network  717  via a network medium  718  and network  715 . 
     A computer program or a collection of programs comprising computer-executable instructions for performing method steps according to the present invention may be stored and transported on computer-usable media such as diskette  701 , CD-ROM  702 , magnetic tape  703  and fixed disk  704 . To perform the steps of the method, the instructions may be retrieved from the computer-usable media  701 - 704  using their respective drives  705 - 708  into memory  700  and executed by a processor  710 . The method disclosed hereinabove may find specific implementations in a variety of programming structures and data forms, which are considered to be within the abilities of a programmer of ordinary skill in the art after having reviewed the specification. 
     The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention as such, or other, embodiments and with the various modifications required by the particular applications or uses or the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.

Technology Category: g