Abstract:
A circuit arrangement for controlling the masking of test and diagnosis data with X values of an electronic circuit with N scan paths, wherein the test data are provided on insertion into the N scan paths by a decompressor with m inputs and N outputs (m&lt;N) and wherein the masked test data are compacted by a compactor with N data inputs and n data outputs and m&lt;N applies is provided.

Description:
The present disclosure relates to technologies in the field of test response compaction. 
     BACKGROUND 
     Scan testing transforms the test of a sequential circuit into a combinatorial test problem. In conjunction with automated test pattern generation (ATPG) software, this allows to handle the ever increasing complexity of digital designs. However, test data volume continues to grow with the design&#39;s complexity. Moreover, new process technologies and materials which allow smaller feature sizes require more comprehensive tests covering a range of different fault models [1]. Today, test cost constitutes a significant part of the production cost, typically in the range of 10-20% [2]. 
     To keep test time and automated test equipment&#39;s (ATE) memory requirements in check, test input stimuli as well as test response data are transferred in compressed form. The basic design of scan test with test compression is depicted in  FIG. 1 . With test compression, the ATE transfers compressed test stimuli to the device-under-test (DUT), where it is deflated by a decompressor circuitry. The scan-out data is likewise compacted by an on-chip compactor. This setup allows to feed a large number of short internal scan chains with only a limited number of external ATE channels. This is advantageous as a large number of (relatively) short scan chains reduces the number of scan shift cycles per pattern, thus reducing test time and test cost. 
     A major obstacle to efficient test response compaction are unknown values (x-values) captured by scan cells during test. If test responses with x-values are compacted, some of the outputs of the compactor may also take unknown values and the correctness of the compactor inputs cannot be verified at the compactor outputs. The presence of x-values hence reduces observability of (non-x) scan cells which may lead to reduced test quality and/or limited compaction rates. 
     To overcome these problems a number of ideas have been proposed. Some compactors are designed to tolerate a limited number of x-values, e.g. [3], [4], [5], [6], [7], [8]. In general, however, these solutions are only applicable to designs with very low x-densities. A slightly different approach is taken in [9], [10], where scan out data is rearranged to reduce x-value impact before it is fed into the compactor. 
     A second approach is to mask the x-values before they enter the compactor [11], [12], [13], [14], [15]. These compactors require the transfer of additional masking data to the DUT. Furthermore, these concepts usually entail over-masking, i.e. the overall observability of scan cells is decreased. This may result in a lower probability to detect non-targeted faults. 
     Thirdly, output selection may be used to circumvent x-values [16], [17], [18]. Recently, the use of ATE timing flexibility to observe a subset of the output values of an accelerated compactor has been proposed [19], [20]. 
     To further reduce test cost, circuits may be tested in parallel (multi-site testing). In that case, a number of DUTs shares the same input signals while transferring their test responses on separate channels each. 
     Furthermore, diagnosis data from volume testing is increasingly used for yield learning [21]. Hence, test compaction should also allow for efficient diagnosis. It has been demonstrated that test output data compacted into a single, 1-bit-wide output stream by an XOR-tree can be efficiently used for fault detection and diagnosis [22], [23]. However, these works considered x-free circuits only. 
     SUMMARY 
     A compaction solution is presented which allows an extreme response compaction (down to a single output) in presence of a high number of x-values. The solution is well suited for multi-site testing, can benefit from ATE timing flexibility, is design-independent and easily scales to an arbitrary number of scan chains. 
     According to one aspect of the invention, a circuit arrangement for controlling the masking of test and diagnosis data with X values of an electronic circuit with N scan paths is provided, wherein the test data are provided on insertion into the N scan paths by a decompressor with m inputs and N outputs, wherein m is &lt;N and wherein the masked test data are compacted by a compactor with N data inputs and n data outputs and m&lt;N applies, the arrangement comprising:
         a first circuit component for masking the test data, which comprises N data inputs for input of N-digit binary unmasked test data, N control inputs for input of the binary control signals for masking the corresponding digits of the test data and N outputs for outputting the values of the masked test data bits corresponding to the control signals,   a further circuit component with N binary inputs and N binary outputs for providing control signals, such that the N binary outputs are connected to the N control inputs of the first circuit component and wherein the first circuit component has a first serially loadable register with N memory elements, a second loadable register with likewise N memory elements for storing test pattern-dependent masking data and a third serially loadable register likewise with N memory elements for storing test-dependent masking data, wherein the following applies:
           the input of the ith memory element of the second register is connected, for i=1, . . . , N, logically directly with the ith input terminal of the further circuit component, if a first binary value is stored in the ith memory element,   the input of the ith memory element of the second register is connected, for i=2, . . . , N, with the output of the (i−1)th memory element of the second register, if a value is stored in the ith memory element of the first register which is different from the first binary value,   the output of the jth memory element of the second register is connected, for j=1, . . . , N−1, logically directly with the input of the (j+k)th memory element of this register, if the binary values b, b, . . . , b, −b are stored in the jth, (j+1)th, . . . , (j+k−1)th memory element of the third register, and   for k=1, . . . , N the output of the kth memory element of the second register is connected with the first input in each case of a kth logic gate with two inputs and one output, of which the second input is connected with the output of the kth memory element of the third register, wherein the output of the kth gate with two inputs and one output conveys the kth binary digit of the masking signal and is connected with the kth control input of the first circuit component, and wherein the logic gate with two inputs and one output in each case has a controlling value.   
               

     According to another aspect of the invention, a circuit arrangement for an electrical circuit to be tested is provided, the arrangement comprising:
         a test input signal generator, which generates a test input signal of width N, wherein only g&lt;N bits of the test input signal may be released per cycle,   terminals for connection to inputs and outputs of an electronic circuit to be tested, wherein the electronic circuit to be tested has N digital test inputs and M digital test outputs and wherein the terminals for the test inputs are connected to the test input signal and wherein the electronic circuit to be tested is controlled such that it outputs test responses at its test outputs,   a circuit component with M inputs and m outputs (M&gt;m) is present for compacting data,   a circuit component for masking X values, comprising the sub-components masking logic, mask register and loading circuit, wherein
           as a function of the state of the mask register, the masking logic is configured to allow masking of a portion of the outputs of the circuit to be tested   the mask register comprises one or more dynamically shortenable shift registers, whose state may be specified at least in part by way of the loading circuit   the loading circuit specifies one or more inputs for the mask register data, such that a freely selectable portion of the M outputs of the circuit to be tested form the data inputs of the mask register.   
               

     According to a preferred embodiment of the invention, the loading circuit is connected directly to at least one output of the test input signal generator. 
     In a preferred embodiment, the dynamically shortenable mask register comprises memory elements, which at least in part provide the masking information for the masking logic and which are connected to from as a whole a shift register, each memory element is configured to store one mask bit, at least one memory element i is configured to be excluded from the shift chain, i.e. that the initial value of the preceding element i−1 is forwarded combinatorially to the input of the following memory element i+1, the loading circuit consists of a series of memory elements IR j  and multiplexers MUX 1   j , the output of the memory element IR j  is connected to the input of the memory element IR j+1  and the control input of the multiplexer MUX 1   j , and one data input of the multiplexer MUX 1   j  is connected to the data output of a memory element k of the dynamically shortenable mask register and the other input is connected to a signal from the signal cluster and the output of the multiplexer MUX 1   j  is connected to the data input of the subsequent memory element k+1 in the mask register. 
     In still a further embodiment, the loading circuit and the dynamically shortenable shift register jointly consist of modules, the modules comprises memory elements IR i , MR i , Cr i , multiplexers MUX 1   i , MUX 2   i , and an AND gate, and the components are connected as follows: the memory element with the data input controls with its data output the multiplexer, the multiplexer allows the data signal or the data signal to pass to its output, which is connected with the input of the memory element and one input of the multiplexer, the output of the memory element is connected to the second input of the multiplexer and one input of the AND gate, the output of the multiplexer is the line, the output of the memory element with the input is connected to the second input of the AND gate and control input of the multiplexer, the line is the output of the AND gate, and wherein the lines form the inputs of the module and the lines form the outputs of the module, and wherein the loading and mask register logic are formed by chaining these modules, wherein output of a module i is connected with in each case the input of the module i+1 and wherein the input is connected with a signal from the signal cluster and the output is connected with the X masking logic via signal cluster. 
    
    
     
       BRIEF DESCRIPTION 
         FIG. 1  depicts an embodiment of a design of scan test compression. 
         FIG. 2  depicts an embodiment of outputs of scan chains filtered through a novel x-masking logic and subsequently compacted into a single, one-bit wide bitstream by an XOR-tree. 
         FIG. 3  depicts an XHCMR cell for a single scan chain. 
         FIG. 4  depicts an embodiment of a 4-element XHCMR with a 3-bit virtual mask register. 
         FIG. 5  depicts an embodiment of a selection of strobe positions within a scan cycle. 
         FIG. 6  depicts an embodiment of multiple DUTs in parallel on a single ATE. 
         FIG. 7  depicts a compaction ratio r for design B (cf. Table 2) depending on the number of scan chains. 
         FIG. 8  depicts the resulting fault coverage for the considered number of scan chains. 
         FIG. 9  depicts an embodiment of IR cells with additional multiplexers between the inputs respective outputs connecting successive HCMR cells. 
         FIG. 10  depicts a flow chart in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Following the invention will be described in further detail, by way of example, with reference to different embodiments. 
     The XHCMR-Compactor 
     In the proposed compactor the outputs of the scan chains are filtered through a novel x-masking logic and subsequently compacted into a single, 1-bit wide bitstream by an XOR-tree ( FIG. 2 ). The masking logic allows to define arbitrary, pattern-specific masks for filtering x-values and provides an efficient mechanism for the loading of x-masking information. 
     The x-masking is performed in the eXtended Hierarchically Configurable Mark Register (XHCMR). At the beginning of the test, the XHCMR is initialized once by loading two k-bit words (k denoting the number of scan chains) via IR in  and C in  (cf. Section II.1 and II.2). 
     During test, pattern-specific, reduced masking data can be loaded in parallel using the scan chains. An explicit mask-enable signal determines the clock cycles in which x-masking is to be performed. If ATE timing flexibility is used, the need for the mask control signal is eliminated (cf. Section 0). In the following three subsections we introduce the proposed XHCMR compactor in detail. Firstly, we illustrate advantages of using a hierarchical masking logic to reduce masking data volume, then we describe a novel approach to load the (reduced) masking data in parallel using the scan chains of the DUT. In the last subsection, we discuss options to utilize ATE timing flexibility. 
     Reducing Masking Data 
     The masking logic is employed to mask x-values captured by scan cells with arbitrary, pattern-specific masks. In known approaches [13], each mask defines for each scan chain whether this chain is to be masked, hence, the length of the x-mask is equal to the total number of scan chains. However, the majority of scan chains never capture any x-values. Therefore, masking data can be reduced if masking information is only loaded for those chains which are affected by x-values during one or more patterns. 
     For that reason, the XHCMR compactor employs a hierarchically configurable mask register (HCMR) [24], which configures the masks in two stages. At the beginning of the test, the subset of all the scan chains which potentially need to be masked is selected (first stage). Subsequently, to create a pattern-specific mask, only the masking information for the selected subset needs to be transferred. 
     To that end, a k bit word Zefining an arbitrary subset of l out of all k scan chains which can be masked during test is loaded into the HCMR via the input C in . Usually, this subset contains all scan chains which capture x-values in at least one pattern. The remaining scan chains cannot (and, by definition, need not to) be masked. 
     The choice of l maskable scan chains configures the HCMR to form a virtual mask register of length l. The virtual mask register VMR stores the pattern-specific x-masks which determine the scan chains to be masked in the respective test pattern. Consequently, for loading a new masking pattern, only l (instead of k) bits have to be loaded into the VMR. 
     A mask enable signal allows a per-clock activation of the mask. This control line is not necessary if ATE timing flexibility is used (cf. section II.3). 
     Parallel Loading of Mask Data 
     The architecture of the HCMR as presented in [24] provides only for serial loading of the mask information. This increases the test time if the length l of the x-masks exceeds the number n of flip-flops of the longest scan chain. In that case, additional l-n cycles are needed per test pattern to finish loading of the masking information. In this section, we present a new solution using parallel inputs to speed-up the loading of the mask information and eliminate the need for a separate input channel to transfer the x-mask data. 
     Ideally, the parallel loading mechanism should not require any additional data channels. For that reason, we propose to use the scan chains to feed the mask register. Even though there are k&gt;l scan chains, these are usually fed by a decompressor (cf.  FIG. 1 ). The decompressor limits the number of bits per slice whose values can be set independently of each other [25]. This number cannot exceed the number of decompressor inputs. To keep input bandwidth small, the number of decompressor inputs is usually only a fraction of the number of scan chains k, and also likely to be significantly smaller than the x-mask length l. 
     In the following, we assume that the values of g scan chains can be set independently of each other. Our solution uses g (g&lt;&lt;l) scan chains to load the mask information in parallel. This reduces the number of cycles required to load a mask to 
             ⌈     i   g     ⌉         
cycles.
 
     To facilitate parallel loading, we divide the l-bit wide virtual mask register VMR into g segments s 1 , s 2 , . . . , s g  of (nearly) equal length. Each of these segments forms a separate shift register. The segments s 1 , s 2 , . . . , s g  are fed by a single scan chain each, which allows to feed them simultaneously. However, both the length and the flip-flops constituting the VMR depend on the subset of scan chains actually affected by x-values. Hence, the position and length of the segments s 1 , s 2 , . . . , s g  are unknown at design time. Therefore, it we cannot use a fixed set of scan chains to form the intakes of s 1 , s 2 , . . . , s g . 
     In the following, we describe the internal structure of the XHCMR which enables the dynamic positioning of the g intakes. This allows to create sections of equal length and thus provides for efficient parallel loading. 
       FIG. 3  depicts an XHCMR cell for a single scan chain. The proposed structure consists of a standard HCMR cell [24] and an intake position register cell (IR cell) which defines the source for the input M in  of the associated HCMR cell. 
     A HCMR cells stores all masking related information for a given scan chain. If the scan chain never captures x-values, the HCMR cell is configured to be transparent, i.e. its input M in  is directly connected to its output M out  and the corresponding scan chain is never masked (m i =0). If, on the other hand, the scan chain may capture x-values, the HCMR cell is activated and acts like a flip-flop with the input M in  and output M out . Furthermore, value of the masking output m i  is determined by the flip-flop&#39;s state. The HCMR (cf.  FIG. 4 ) is made up by HCMR cells connected into a chain. It can thus be used to form a virtual mask register (VMR) by activating only a subset of its cells. 
     To split the VMR into g segments, the IR cells add additional multiplexers MUX, (0≦i≦k) between the inputs respective outputs connecting successive HCMR cells (cf.  FIG. 3  and  FIG. 9 ). Each MUX i  is controlled by the corresponding flip-flop IR i  and switches between the output M out  of the preceding HCMR cell (equivalent to input int i ) and the external intake ext i . If the value of the flip-flop IR i  is 1, then the (external) source ext i  is the input of the HCMR cell i. Thus, the VMR is splitted at this position and a new segment with a new external intake is created. If, on the other hand, IR i  equals 0, then the HCMR cell i is directly connected to its predecessor (as in a standard HCMR). In this way, the standard HCMR can be dynamically divided into a number of shorter segments s 1 , s 2 , . . . , s g . By feeding the input ext i  with the i-th scan chain, the segments can be loaded simultaneously through the scan chains. 
     The IR cells form a shift register, the intake position register IR, with the global input IR in . The contents of the register IR thus defines the position of the intakes for parallel loading of the virtual mask register VMR. Like the subset of scan chains selected as maskable, the configuration of the IR remains unchanged during test. 
       FIG. 4  illustrates a 4-element XHCMR with a 3-bit virtual mask register VMR (formed by HCMR cells  1 ,  3  and  4 ). The configuration of the intake-position register divides the VMR into two separate segments s 1  and s 2  with the two intakes ext 1  and ext 4 , respectively. Due to the selection of maskable scan chains and IR configuration, the first segment s 1  is made up of HCMR cell  1  and HCMR cell  3 , while the second segment s 2  is made up by HCMR cell  4  only. HCMR cell  2  is in neither section (dynamically disabled) as the corresponding scan chain is not in the subset of maskable chains. In this example setup, the 3-bit mask can be loaded in only two clock cycles using ext 1  and ext 4 . 
     Putting it all together, the overall test sequence using XHCMR can be summarized as follows:
     (1) select virtual mask register elements (VMR) (k cycles)   (2) initialize intake position register (k cycles)   (3) load x-mask for patter P into VMR via scan chains (1/g cycles)   (4) scan-shift for pattern P (n≧1 cycles)   (5) if P&lt;#pattern goto (3)   

     Compared to the basic HCMR architecture, reloading of the x-masks using XHCMR is reduced by the factor g. 
     The architecture from [24] and the XHCMR architecture were synthesized with the Synopsys Design Compiler (Version Z-2007.03-SP5) using the lsi — 10k library. Table 1 displays area consumption per scan chain in NAND-equivalents. Compared to the HCMR, the XHCMR architecture reduces load time overhead and eliminates the need for an additional mask load channel with comparable per-cell area consumption. 
                                                 TABLE 1                   Comparison of HCMR-architecture and XHCMR architecture       area (in NAND-equivalents)                HCMR cell               (with shadow register (cf. [24]))   XHCMR cell                        Combinational area   6.0   8.0       Non-combinational area   25.0   27.0       Total cell area   31.0   35.0                    
Eliminating Control Signals Using Timing Flexibility
 
     Modern automated test equipment (ATE) supports placement of several strobe positions within a single test cycle for a given channel. A practical solution using timing flexibility in conjunction with an accelerated compactor to reduce the impact of x-values was presented in [19], [20]. In this work, we propose to use the timing flexibility to eliminate the need for an explicit mask control signal. 
     In the standard approach, the mask enable signal is used to decide whether the current x-mask should be applied at the current test cycle. If the ATE timing flexibility is used, both the masked and the non-masked test response are provided during each scan-out cycle. The decision whether to apply the mask can then be transformed into the decision about the time in which the test response is strobed by the ATE. Hence, the mask enable signal is eliminated while the volume of test data to be evaluated on the ATE remains constant, thus increasing the overall compaction ratio. 
     Please note that a strobe position is stored on the ATE for each clock cycle and each pin irrespective of whether the strobe position is always the same or not. Hence, using multiple different strobe positions does not affect the strobe position data volume. 
     To provide both a masked and a non-masked test response within a single cycle, an internal mask enable signal is used. This internal signal is alternating between 0 and 1 with a period equal to a single scan shift cycle, which effectively means that the signal is equal to the scan clock signal. 
     By programming the strobe positions for each scan cycle, either the masked or non-masked compactor output can be evaluated by the ATE. As only two different strobe positions are required, this technique can be used on all current ATE models. 
       FIG. 5  displays the selection of strobe positions within a scan cycle. The internal mask enable signal activates the masking logic during the second half of each clock cycle. Correspondingly, two strobe positions s 0  and s 1  have to be defined on the ATE. If the masked test response is to be evaluated, strobing has to be performed at time s 1 . If a masking of the test response is not desired, strobing should be performed at s 0 . In  FIG. 5 , the test response in cycle i is evaluated without masking (strobed at s 0 ), while in cycle i+1 the masked test response is evaluated (strobed at s 1 ). 
     Improving Compaction Ratio Further 
     Multi-Site Testing 
     Multi-site testing aims at reducing test cost by testing multiple DUTs in parallel on a single ATE. The parallel testing increases the number of DUTs testable by a single ATE in a given time. 
     All DUTs can share the same input signals (shared driver [2]), while output signals have to be individually transferred to and processed by the ATE to allow identification of failing chips. The power of multi-site testing stems from the fact that the sharing of input stimuli makes both test stimuli memory and the number of output channels on ATE independent of the number of DUTs tested (cf.  FIG. 6 ). The feasibility of testing up to 8 DUTs in parallel on a low pin count ATE in a production environment was demonstrated in [2]. 
     As the number of output channels of the ATE is independent of the number of DUTs tested, parallelism is limited by the number of input channels required to process the test responses of individual DUTs. For that reason, a solution suited for multi-site testing requires a low output pin count on each DUT, while the number of required input pins is of much less importance. 
     The proposed compactor solution requires only a single 1-bit wide tester channel to transfer the test responses to the ATE. The input channels providing the mask information and test stimuli can be shared. Thus, in multi-site testing the overall compaction ratio of the proposed compactor scales well with the number of DUTs tested in parallel (see experimental results). 
     Determining an Optimal Scan Chain Configuration 
     The overall compaction ratio of the XHCMR compactor is the ratio between uncompacted scan-out data and XHCMR compactor input and output data. If the number of scan chains is increased, the amount of output data is reduced as a higher compaction is achieved by the XOR-tree. On the other hand, input data increases, as more scan chains likely cause a longer virtual mask register (VMR). In this section, we present a formula that can be used to compute the optimal number of scan chains in a standard test set-up. 
     The overall compaction ratio r of the XHCMR compactor for single-site testing without ATE-timing flexibility can be calculated as
 
 r =( p·k·n )/( p·n+ 2 k+p·n+p·l ))  (1)
 
where p is the number of test patterns, k is the number of scan chains, n is the number of flip-flops of the longest scan chain and l is the length of the virtual mask register VMR.
 
     The number of scan chains k for which the maximum overall compaction ratio r is achieved can be computed based on (1). For the sake of simplicity we assume:
         All scan chain have (almost) the same length. Thus, the number c of scan-flip flops in the design is approximately c=k·n.   The length l of the VMR scales linearly with the total number of scan chains k, m·k, with 0≦m≦1.   The reconfiguration of the scan chains does not affect the number of test patterns p. Transforming (1) using these assumptions we obtain       

     
       
         
           
             
               
                 
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     Since the values of p, c and m are known (and constant) for a specific design, the nominator is also a constant; therefore, the compression ratio r increases if the denominator of equation (2) decreases. 
     To find the minimum of the denominator of equation (2) depending on the value of k, we compute the first and second derivative of the denominator d(k), denoted as d(k)′ and d(k)″ in equation (3) and (4): 
     
       
         
           
             
               
                 
                   
                     
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     Using the derivatives d(k)′ and d(k)″ it can be easily shown that equation (5) describes a minimum of d(k) if k&gt;0. Thus, (5) can be used to calculate the number of scan chains for the proposed XHCMR compactor to maximize the compression ratio r. 
     
       
         
           
             
               
                 
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                   = 
                   
                     
                       
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     In a similar way, formulas to compute the optimal scan chain configuration taking ATE timing flexibility and/or multi-site testing into account can be derived. 
       FIG. 7  illustrates the compaction ratio r for design B (cf. Table 2) depending on the number of scan chains. The solid line displays the compaction ratio for the standard test set-up with a maximum of 208x at k=833 scan chains. Thus, in this case an optimal scan chain configuration implements 833 scan chains of length ˜125. The curve indicates a wide range of nearly maximal compression. For design B, a compression ratio ≧200 can be achieved with scan chain configurations ranging from 620 to 1100 scan chains, providing considerable flexibility for the test engineer. 
     Using ATE timing flexibility as presented in section 0, the compression ratio can be further improved peaking at k=589 scan chains with r=294x. 
     Methodology 
     To assess the effectiveness of the proposed compactor, its performance on three different industrial designs is investigated. For production test, all three designs use on-chip decompression/compaction provided by a commercial test suite. For our experiments, we used the circuits&#39; original scan chain configuration. Instead of the original compactor, all scan chain outputs are compacted into a 1-bit stream by a XHCMR compactor. Fault coverage was determined by performing a full (stuck-at) fault simulation using the existing stuck-at test set tuned to the commercial test suite. 
     Two masking approaches were studied: full masking, which masks all x-values, and a second, less restrictive masking method, which we called smart masking. The idea of smart masking is that over-masking of output care bits can be reduced by not masking all x-values. 
     As output care bit information was not readily accessible, we used the fault simulator&#39;s ability to define any given line to be a virtual output to directly observe the scan cells&#39; inputs. The care bit positions were then computed using this information in conjunction with the scan chain configuration data. 
     Masking information for smart masking was computed using a simple heuristic algorithm. The algorithm processes the patterns sequentially. For each pattern, the mask is initially empty and all yet undetected faults causing errors within that pattern are listed in arbitrary order in the fault queue. The first fault is retrieved from the fault queue. If it is detectable with the current mask, processing continues with the next fault. Otherwise, the mask is extended to allow detection of the fault effect of the considered fault in at least one output slice and the next fault is processed. This is repeated for all faults in the fault queue. While the extension of the mask may cause a masking of faults considered earlier, this heuristic works remarkably well (see experimental results). 
     The fault coverage was determined by resimulating the test set with the computed masking information. By using the test set actually employed in production testing (albeit with another compactor), fault detection is limited to faults detected by the predefined test set. On the other hand, our results give a realistic impression of the compactor&#39;s performance without customized tooling support. It seems likely that a customized ATPG algorithm tailored to the XHCMR compactor would further improve the fault detection rate and/or reduce pattern count. 
     Experimental Results 
     In this section, we investigate the performance of the proposed compactor with respect to overall scan cell observability and fault coverage. Basic information for the considered designs is given in table 2. The designs A and B have been presented in [24]. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 basic design information 
               
             
          
           
               
                   
                 design A 
                 design B 
                 design C 
               
               
                   
                   
               
             
          
           
               
                   
                 # scan flip-flops 
                 139,597 
                 104,509 
                 33,296 
               
               
                   
                 # scan chains 
                 515 
                 1,360 
                 171 
               
               
                   
                 longest scan chains 
                 295 
                 82 
                 198 
               
               
                   
                 # test pattern 
                 503 
                 9,188 
                 5,654 
               
               
                   
                 x-probability 
                 0.43% 
                 1.78% 
                 2.75% 
               
               
                   
                 # chains affected by 
                 256 
                 409 
                 5,654 
               
               
                   
                 x-values 
               
               
                   
                   
               
             
          
         
       
     
     Table 3 summarizes the resulting overall compaction ratio r (cf. equation (1)) for the given designs using their original scan chain configuration. In standard test set-up (first row) the XHCMR compactor achieves compaction ratios up to 194x (design B) for the given industrial designs. Even for design C, which implements only a small number of scan chains, a compaction ratio of more than 70x is achieved. 
     Compression ratios can be further increased by using ATE timing flexibility and/or multi-site testing. Testing four DUTs in parallel, compression of up to 544x (design B) can be realized. If ATE timing flexibility is used in combination with 4x multi-site test, compaction ratios ranging from 155x (design C) to 605x (design B) are achieved (last row). 
     
       
         
               
             
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 comp. ratio r for different test setups with XHCMR compactor 
               
             
          
           
               
                   
                 compaction ratio r 
               
             
          
           
               
                 test setup 
                 design A 
                 design B 
                 design C 
               
               
                   
               
               
                 standard setup 
                 179.36x 
                 194.57x 
                  71.43x 
               
               
                 ATE timing flexibility 
                 275.21x 
                 227.06x 
                 122.66x 
               
               
                 MST 4 DUTs 
                 350.86x 
                 544.57x 
                 126.81x 
               
               
                 MST4 DUTs + ATE timing 
                 422.89x 
                 605.14x 
                 155.66x 
               
               
                 flexibility 
               
               
                   
               
             
          
         
       
     
     In the first experiment we investigated the fraction of observable non-x-cells if all x-values are masked (full masking) using the XHCMR masking logic. A high observability of the scan cells indicates a high fault coverage for a given test set, as (both targeted and non-targeted) errors are then likely to be observed at the compactor&#39;s output. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 observable non-x scan cells using full x-masking 
               
             
          
           
               
                   
                 design A 
                 design B 
                 design C 
               
               
                   
                   
               
             
          
           
               
                   
                 observable non-x cells 
                 91.78% 
                 93.30% 
                 91.89% 
               
               
                   
                   
               
             
          
         
       
     
     For all three designs, the fraction of observable scan cells is high, ranging from 91.7% to 93.3% (cf. table 4), in spite of large compaction ratios (171-to-1 to 1360-to-1) and medium (1.78%) to high (2.75%) x-densities. Thus, x-masking has a low impact on the number of observable scan cells even if all x-values are masked during test. 
     In the second experiment, the fault coverage based on care bit information to investigate the test quality for design C was specifically measured. Using XHCMR with the original scan chain configuration, a compaction ratio of ˜71x is achieved. Two masking approaches were used: full masking masks all x-values during test while smart masking (cf. section IV) is less restrictive. Depending on the masking approach, 99.5% and 99.8% of all faults (100% is equal to all faults covered by original test set (cf. section IV)) are detectable at the compactor output for full masking and smart masking, respectively. 
     To assess the impact of further increases of the compaction ratio, we also investigated the fault coverage for a higher number of scan chains. To that end, we rearranged the scan chains of design C to get 2, 3, and 4 times as many scan chains as in the original configuration.  FIG. 8  depicts the resulting fault coverage for the considered number of scan chains. For both masking methods the fault coverage is very high and independent of the number of scan chains. This indicates that a scan chain configuration which maximizes compression ratio can be chosen without compromising test quality. 
     An example how the invention is included in the test architecture is given in  FIG. 10 . 
     A new compactor solution for very high compaction ratios (&gt;200x) in presence of many x-values without compromising test quality was presented. This is accomplished by a highly effective x-masking register which only targets scan chains actually capturing x-values. The x-masking information can be loaded in parallel using the DUT&#39;s scan chains and built-in decompressor logic. 
     A novel technique uses ATE timing flexibility to eliminate the need for a mask control channel and increase the overall compaction ratio. The compactor architecture is design independent. Due to its regular structure, it can readily be implemented as a push-button solution. 
     The proposed single-output compactor is very well suited for multi-site testing, enabling compression ratios of ˜500x in 4-DUT multi-site testing setups. 
     The features disclosed in this specification, claims and/or the figures may be material for the realization of the invention in its various embodiments, taken in isolation or in various combinations thereof. 
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