Patent Publication Number: US-7900103-B2

Title: Scan chain architecture for increased diagnostic capability in digital electronic devices

Description:
FIELD OF THE INVENTION 
     The present invention relates in its more general aspect to an improved scan chain architecture, belonging to the field of Design For Testability techniques, such as may be used on digital devices in order to increase the diagnostic capability of fault categories which prevent the device scan chain to shift. 
     BACKGROUND OF THE INVENTION 
     As is known in this specific technical field, the complex System-on-Chip Integrated Circuits developed use the “Design For Testability” structured technique of the scan chains to allow the Automatic Test Patterns Generation (ATPG) tool to reach the very high standard quality currently required by the customer and, at the same time, reduce the generation time of the patterns. 
     The currently available scan chain structures, inserted in the device using commercial tools, are built by stitching the device flip-flops to satisfy different constraints such as: minimization of routing (physical scan insertion), minimization of hold time problem, by hierarchical name, and so on. 
     At the end of the scan chain insertion phase, the structure that normally is inserted resembles the one shown in  FIG. 1 . The typical connections between two consecutive flip-flops  1  of a scan chain  9  are performed considering one or a combination of the above constraints. Provided that all flip-flops have both a normal output, identified as Q, and an inverted output QN, in some cases Q happens to be connected to the flip-flop input TI of the next flip-flop of the scan chain, while in the other cases the connection is between QN and TI. In other words, the outputs Q or QN of a given flip-flop are linked to the input of the subsequent flip-flop not according to a specific rule, but rather according to the specific requirement or requirements of the specific scan chain. 
     This limitation does not allow obtaining better performances from a scan chain set up according to the prior art techniques. Extensive analysis has been performed in attempt to find solutions, based on specific algorithms which help in finding the root cause of the problem (e.g. drive mapping method), but they very often require multiple patterns to be run related to the target defect, and specific tools which have to know the whole design structure during the phase in which failure data (obtained during testing phase) are post-processed to get the failure location of the failed device (i.e. an ATPG tool). 
     The difficulty is that of providing an improved connection configuration having characteristics allowing performing the diagnosis of possible failures while in some cases requiring the knowledge of the scan chain structure only, thus overcoming the limits which still affect the devices realized according to the prior art, and introducing only a small, and often null, area overhead into the hardware structure. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is therefore an object of present invention to modify the scan chain structure of the known type by using dedicated logic circuitry to connect consecutive flip-flops belonging to a same scan chain. In this manner it is possible to improve the diagnostic capability by a more efficient use of the asynchronous initialization of the scan chain flip-flops. 
     Moreover, to perform a diagnosis of the failures, the status of the scan chain flip-flops may be initialized using an asynchronous pin (set or reset), which is typically present in all the flip-flops adopted in digital design and connected to the external reset or set when the scan mode is enabled. 
     In other words, the invention relates to a modification of the logic circuitry connecting consecutive flip-flops of the same scan chain, to achieve the diagnostic capability improvement which allows the identification of predetermined fault categories preventing the scan chain from properly shifting. Since all the flip-flops adopted in digital designs are typically asynchronously initialized using a dedicated set or reset pin, the above-proposed approaches address most digital devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics and advantages of the improved scan chain architecture according to the invention will be apparent from the following description of embodiments thereof given by way of indicative and non limiting example with reference to the annexed drawings. In such drawings: 
         FIG. 1  schematically shows the typical connections between consecutive flip-flops of a scan chain of known type, wherein the output or the inverting output of a given flip-flop can be connected to the input of the subsequent flip-flop; 
         FIG. 2  schematically shows a scan chain formed by flip-flop cells of known type but following the rules provided by the present invention; 
         FIG. 3  schematically shows a known flip-flop cell together with a modified cell according to the present invention and responsive to control signals disclosed in details in the following description; and 
         FIG. 4  schematically shows a scan chain structured with flip-flop cells modified according of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With specific reference to the example of  FIGS. 2 ,  3  and  4 , an improved scan chain architecture  10  according to the present invention is disclosed. 
     The architecture  10  may be structured with a scan chain of flip-flop cells of known type, for instance those indicated with  2  in  FIG. 1 , that are linked one to the other according to the rules provided by the present invention and disclosed hereinafter. 
     As an alternative, the architecture  10  may be structured with a scan chain of flip-flop cells  5  that have been modified with respect to known flip-flop cells  3 , as shown in  FIGS. 3 and 4 . 
     A low cost/capability approach will be disclosed first and a high cost/capability approach will be disclosed later. Furthermore a possible middle cost/capability approach will be disclosed as a sub-case of the low cost/capability one. 
     Briefly explained, the two main approaches are based on different ways of connecting together consecutive flip-flops cells in the same scan chain. The low cost/capability approach presents advantages in term of area overhead, while the high cost/capability approach presents advantages in terms of performance. 
     In disclosing the low Cost/Capability approach we will start by considering a library of known flip-flops cells  2  having both a Q output and a QN (inverted Q) output. It should be noted that this way of indicating the output of the cell  2  is unconventional and other alternative ways may be adopted. The only important thing is that the flip flop cell might have a main output Q or SO and inverted output QN or SON. If the inverted output is not present, it would be possible to insert an inverted connected to the main output thus obtaining an inverted logic value at the inverter output. 
     The difference between the Q, QN and SO, SON resides in their functional scope. The output Q, ON are functional outputs and fast signals while SO, SON are slower signals that are provided in the flip flop cells to be dedicated to the scan routing of the scan chain. However, the logic values of the output Q generally correspond to the logic value of the output SO and this is also true for the other outputs QN, SON. If the logic values of the output Q should not correspond to the logic value of the output SO, only the pins SO/SON could be used. 
     Generally speaking, the key point that allows improving the diagnostic capability with respect to a failure of the scan chain shifting is to have, for example, at every shift clock cycle, a toggling on the flip-flop output to identify and point out the element or event that broke the scan shifting. 
     Before proceeding further with the description, it may be worthwhile noting that the shifting operation is sequential to an asynchronous initialization phase wherein all the flip-flop cells of the chain are pre-charged to a predetermined status. In other words, there is typically the need to force an asynchronous set-reset phase before starting the toggling phase. 
     To achieve such a toggling, the only requirement is typically to execute a post analysis of the scan structure, i.e. after the scan insertion is performed, to slightly modify the scan chain architecture. 
     To do so it has been decided to take in consideration the last cell of the chain closer to the scan chain output (OUT) as the first element and proceeding back up to the first cell of the chain, the one closer to the input IN, that has been considered as the last element. 
     So, starting from the last scan chain element C 0  (the nearest to the scan chain output), it is desired to check if the status, for instance S 0 , of that element C 0  of the scan chain is asynchronously initialized to 0 (reset) or 1 (set). The next step is to check the status, for instance S 1 , of the previous flip-flop C 1  in the scan chain and the logic connection during the scan shift between these two flip-flops (S 1 -conn-S 0 : we can consider this connection equal to ‘1’ in case of inversion and ‘0’ in case of buffer—no inversion—). 
     Depending on the above parameters the connection between these two flip-flops C 0 , C 1  may be modified to have an inversion among them. To prevent potential problems in, the silicon structure that is already physically placed, as previously mentioned, the flip-flops may desirably have both the output Q and QN; as an alternative these flip-flop cells C 0 , C 1  might have a dedicated output of the kind SCAN OUT, that is identified with SO in  FIG. 3 , and is delayed to prevent a hold problem during the physical stitching. In this specific case using the output SO even an inverted output SON may desirably be present. 
     At the end, the rule to be applied can be summarized as follows:
 
 INV   x,x−1 =NOT( S   x   XOR   S   x−1   XOR   S   x - conn - S   x−1 )  (Eq. 1)
 
where INV x,x−1  is a logic function that is for modifying the connection. When INV x,x−1  is equal to 1 the connection between two consecutive flip-flops Cx, Cx−1 is inverted while, when INVx,x−1=0 the connection between two consecutive flip-flops Cx, Cx−1 is not inverted.
 
     In other words, given an original connection between two consecutive flip-flops Cx, Cx−1, the formula Eq. 1 determines if the connection between said two consecutive flip-flops Cx, Cx−1 has to be inverted with respect to the original connection (INV x,x−1=1), or if it has to be maintained equal to the original connection (INV x, x−1=0). 
     More particularly, the original connection is the connection between two consecutive flip-flops of the scan chain before the application of the formula INV x,x−1=NOT (Sx XOR Sx−1 XOR Sx-conn-Sx−1). As already stated before, after the application of the formula, and only if INV x,x−1=1, the two consecutive flip-flops of the scan chain are connected through a connection inverted with respect to the original connection; otherwise, after the application of the formula and if INV x,x−1=0, the two consecutive flip-flops of the scan chain remain connected through the original connection. 
     Another alternative or dual relation might be used in the case of different logic factors to achieve the same result. 
     The output of a good shift-out, after the external reset/set has been applied, will be a sequence of toggling at every shift clock cycles. 
     A modification in the connections between two consecutive flip-flops C 0 , C 1 , . . . Cx, Cx+1 . . . , Cn−1; Cn of the scan chain achieves an inversion of the input TI of the C x-th  flip-flop of the scan chain, depending on the status of the same flip-flop C x-th  and on the status of the previous C (x+1)th  flip-flop. Moreover, even the status of the connection between those flip-flops is taken into consideration. 
     Again, the flip-flops of the scan chain are here supposed to be numbered increasingly from the flip-flop cell C 0  closer to the scan chain output (OUT) to the one Cn closer to the scan chain input (IN). Provided that all flip-flops of the scan chain have both Q and QN (inverted Q) outputs, the connection between the flip-flops respects the following rules: the connection between two consecutive flip-flops Cx, Cx−1 is inverted, if INVx,x−1=1, while the connection between two consecutive flip-flops Cx, Cx−1 is not inverted, if INVx,x−1=0, according to the previous relation of Eq. 1 wherein S x , S x−1 , and S x -conn-S x−1  indicate the status of the two consecutive flip-flops after asynchronous set or reset initialization and of the logic relation of the original connection between them, respectively. In other words, provided that two consecutive flip-flops of the scan chain are connected with an original connection, the formula INVx,x−1 is processed, and if INVx,x−1=1, the original connection between the consecutive flip-flops Cx, Cx−1 has to be inverted, obtaining an inverted connection with respect to the original connection. If INVx,x−1=0, the original connection between the consecutive flip-flops Cx, Cx−1 does not have to be inverted, and the original connection is maintained. The term (S x -conn-S x−1 ) is considered 1 in case of an inversion and 0 in the case of a buffer (no inversion). This is substantially the low cost approach shown in  FIG. 2 . 
     Below there is a list of fault categories that can be identified and isolated by recording the failures occurring to the output of a scan chain. 
     Single Hold Problem 
     In this case it is possible to detect and isolate the fault because, starting from a definite relation with respect to the position where the hold time occurs in the scan chain, the output is inverted. 
     Cluster Hold Problems 
     This case is similar to the Single Hold problem, but in this case only the first element of the cluster (the one closer to the input of the scan chain Scan in) will be highlighted. This kind of problem could appear in case of clock tree buffer delayed with respect to the correct timing, when the flip-flops clocked by this driver are stitched consecutively in the scan chain (this is probable in the case of physical insertion). 
     Multiple Separated Single Hold Problems 
     In this case it is possible to detect and isolate every fault. The output sequence matches the expected value at some clock cycles, while being inverted at others, depending on the number of single Hold problem in the scan chain. 
     Double Consecutive Hold Problems 
     In this case it is not possible to isolate the faults, but only to detect their number. This is because the double hold problem, which probably will occur rarely because there should be a different skew among three consecutive flip-flops (the first one, CX+2, closer to scan chain input should have no delay cycles; the next flip-flop, CX+1, one delay cycle and the third one, CX, two delay cycles), does not generate a toggle on the output, but it only reduces the scan chain length. 
     Triple Consecutive Hold Problems 
     In this case it is possible to detect and isolate the faults closer to the scan chain output. As noticed above for the Double Consecutive Hold Problems, this case should have a low probability. 
     Single Stuck-at 0/1 on TI 
     In this case it is possible to detect and partially isolate the problem. In general, the reason for the impossibility of completely isolating the problem is related to the fact that the status of the scan chain elements, S x  and S x+1 , have a fixed relation between them, so that a couple of faults on S x  and S x−1  cannot be distinguished. The value of the fault depends on the location where the fault is detected and the set/reset asynchronous signal used to initialize the related flip-flops. 
     Multiple Stuck-at 0/1 on TI 
     In this case it is possible to detect and partially isolate the faults closer to the scan chain output. All the other problems cannot be analyzed because the scan chain is broken. 
     If the flip-flops having both Q and QN or the alternative SO and SON outputs are missing from the library, the approach proposed may be considered a middle cost approach. 
     As a matter of fact, in this possible case two alternative choices can be selected to implement the middle cost/capability approach described above: developing a dedicated library of flip-flops which implement the required double output (just as reference SON/QN in case SO/Q is available), or inserting an inverter between the output SO/Q and the input TI according to the relation explained above, so according to Eq. 1. 
     The High Cost/Capability approach is basically similar to the previously disclosed low cost approach but, to improve the diagnostic capability of the previous approach in the cases where the detection or the isolation is not possible, some modifications to the scan chain structure and cells are performed. 
     For implementing this second approach it would be suitable adopting flip-flops  5  having two separate outputs: one for the functional behavior Q and one for the scan chain stitching SO. 
     The simpler and smaller proposal for the implementation of the high cost/capability approach is to modify the basic flip-flop cells  3 , introducing a control signal SOCtrl and a further controlled output SO_SON. 
     Making reference to  FIG. 3  it may be appreciated that the cell  5  includes the cell  3  and a dedicated logic circuitry  6 , for instance a simple multiplexer circuit  6  enabled by the signal SOCtrl. This multiplexer  6  has a couple of inputs: A, that is equivalent to SO, and B, that is equivalent to the inverted value of SO (indicated as SON). 
     Assuming all flip-flops cells have at least a main output Q, for the functional behavior, and an output SO for the scan chain stitching, the dedicated logic circuitry  6  is suitably introduced to allow toggling of the SO output depending on the value of the control signal SOCtrl. More specifically SO_SON is equal to SO whenever SOCtrl=0, while SO_SON is equal to inverted SO whenever SOCtrl=1. In other words, there is no inversion when SOCtrl=0 and there is inversion when SOCtrl=1. 
     The logic structure of the modified cell  3  is shown in  FIG. 3  as cell  5  and in case of an asynchronously reset flip-flop (a cell with set capability requires similar modifications). 
     At this point it is necessary to analyze how the SOCtrl signal is controlled for every flip-flop of the device to increase the diagnostic capability with respect to the low cost/capability approach. The basic idea is that of having three different configurations: one behaves exactly like the previous one (i.e. one toggling for every flip-flop), the second approach generates or not an inversion depending on the flip-flop position in the scan chain (odd flip-flops have inversion, while even ones don&#39;t have the inversion, i.e. one toggle every two flip-flops), and a third configuration that is a complementary configuration with respect to the second one (i.e. even ones have inversion, while odd ones do not). 
     An implementation of a complete scan chain structure realizing the high cost/capability approach is shown in  FIG. 4 . Two control signals SOCtrl_ODD and SOCtrl_EVEN and the related inverted signals (INV_SOCtrl_ODD and INV_SOCtrl_EVEN) have been introduced, and have to be controlled during the scan chain integrity tests as sequentially described below. 
     The signals SOCtrl_ODD and INV_SOCtrl_ODD are used for the Flip-Flop cells being in odd position (C 1 , C 3 , . . . ) while the signals SOCtrl_EVEN and INV_SOCtrl_EVEN are used for the even positions (C 0 , C 2 , . . . ). 
     Each Flip-Flop cell is connected to the respective inverted (INV_SOCtrl . . . ) connection or non inverted (SOCtr_l . . . ) connection according to the rule reported in the equation 1; if an inverted connection is required (INV x,x−1  IS EQUAL TO 1) a connection to INV_SOCtrl_(ODD/EVEN) is established otherwise to the other signal SOCtrl_(ODD/EVEN). The first test to be performed is in line with the previous description and keeps the same functionality of the low cost/capability approach test. 
     In such a case it is enough to force both the control signal SOCtrl_ODD and SOCtrl_EVEN to the ground potential GND, and as a consequence the two inverted signals INV_SOCtrl_ODD and INV_SOCtrl_EVEN are forced to the potential value of the supply voltage VDD. 
     if this test should fail and some mismatches should not be isolated, additional tests can be applied. 
     A second test can be performed by forcing the potential value of the signal SOCtrl_ODD to VDD (thus INV_SOCTRL_ODD to GND) and the SOCtrl_EVEN to GND. This operation is enough to isolate double consecutive Hold Time, because there is always an inversion between two consecutive flip-flops that allows the isolation of the problem. 
     A third possible test is complementary to the second one, so SOCtrl_EVEN will be forced to VDD, while SOCtrl_ODD is forced to GND. This testing mode will confirm the failure location of the second test. Just for completeness sake it should be noted that an enable pin TE, not shown in the drawings, is normally used to enable between the input pin TI, starting the scan mode, and another input pin D, starting the user mode, Well, the testing mode of the present invention allows the isolation of the stuck-at 0 on TE closer to scan out, because applying these two tests the User Data will be the same (the reset state of the device flip-flops does not change) but the TI input will be complementary depending on the previous SOCtrl status. 
     The combination of these testing methods allows also the isolation of the Stuck-at 0/1 on TI closer to the scan chain output. This is because only one of these three configurations will change the logic behavior between the status S x  and S X+1  of two consecutive flip-flops. 
     Summarizing, the architecture proposed in  FIG. 4 , respect the previous described low/middle cost approach, allows isolating the Double Consecutive Hold Problem, the Triple consecutive Hold problem, and the Single Stuck-at 0/1 on TI, the isolation of the stuck-at 0 on TE faults closer to scan out covering in this way several possible sources of broken scan chain problems. 
     In particular, this structure allows the diagnosis and isolation of several typologies of Hold problems (also multiple), which can really be one of the main problems for the sub-micron technology. The only limitation to this structure is the case of Multiple Stuck-at in the same scan chain: in this case the diagnostic capability is the same as the low cost/capability approach. 
     With the architecture shown in  FIG. 4  it is also possible to investigate about an additional couple of faults related to the pin SOCtrl. These faults can be detected and isolated by applying the set of patterns depicted above because they own a special signature which will allow identifying this family of faults. 
     Of course, as may be appreciated by one skilled in the art, besides the approach proposed in  FIG. 4  other alternative structures are possible. For instance, instead of having four control lines as reported above, we can use just two lines (i.e. a SOCtrl_ODD and a SOCtrl_EVEN signals) and build a dedicated library in which, for every original flip-flop element, there are two modified flip-flops  5  each of them with different polarity of the SOCtrl signal. In this way, a modified flip-flop can be used instead of the inverted control line INV_SOCtrl_(ODD or EVEN) when an inverted control signal value is needed. In particular, in one flip-flop cell the output SO_SON is kept equal to SO whenever SOCtrl is set to GND, while in the other flip-flop cell this relation holds whenever SOCtrl is set to VDD. This approach will save wiring in the design but would require a more complex library. 
     Finally it worthwhile to note that the control signals in  FIG. 4  (SOCtrl_ODD, INV_SOCtrl_ODD, SOCtrl_EVEN, INV_SOCtrl_EVEN) may be provided by external pads or by a dedicated test logic portion.