Patent Publication Number: US-7913131-B2

Title: Scan chain cell with delay testing capability

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of integrated circuit testing. More particularly, this invention relates to scan chain cells which are used to apply stimulus signals to, and capture result signals from, signal nodes within integrated circuits. 
     2. Background of the Invention 
     It is known to provide scan chain cells arranged in serially connected scan chains through which stimulus signal values or result signal values may be serially clocked. This is a convenient way to test complex integrated circuits. As the complexity of integrated circuits increases, they are often formed of a collection of different functional units with some of those functional units being disposed within the integrated circuit such that it is not possible to directly apply signals as inputs to those functional units through an external connection to the device. In these circumstances, scan chain cells are particularly useful for accessing the points (nodes) within an integrated circuit at which it is desired to apply a stimulus signal and from which it is desired to capture a result signal when those points are not otherwise externally accessible. 
     There is an ongoing progression within integrated circuit design to use increasingly small process sizes. These small process sizes enable higher densities of circuit gates to be achieved, reduce power consumption and increase performance. A drawback with the use of such smaller processes, e.g. 45 nm process sizes and below, is that the variability between individual gates becomes a more significant factor resulting in a lower yield of properly operating integrated circuits. In this context, test, such as manufacturing test, is becoming increasingly significant. One important aspect of integrated circuit performance that needs to be tested is the “at speed” performance to determine that processing logic within the integrated circuit is capable of operating at its design speed or to detect other types of defect that manifest themselves as an excessive delay. One way of viewing this test is that it is necessary to determine that the functional logic is capable of responding to changes in values of input signals sufficiently rapidly to generate the appropriate output signals without too large a delay which would cause an error in operation. This type of testing can be termed “delay testing”. 
     A problem exists in using conventional scan chain cells to apply stimulus signals for such delay testing. Scan chain cells are typically provided in a serially connected fashion and use a scan clock to shift signal values through the scan chain at a comparatively slow speed. Accordingly, changing a signal value at a stimulus node within an integrated circuit from one signal value to another signal value so as to cause a transition and test the delay time through a functional circuit is difficult to achieve by serially advancing differing signal values through the scan chain cell since the scan clock is typically not fast enough, and the gates within the scan cell chain are often made small and weak for area saving and power saving reasons. Even if the scan system is fast enough to shift at speed, there are other reasons why this may be undesirable, e.g. a requirement for a separate scan enable. It may also be undesirable to have to add multiple state holding elements within a wrapper cell to support rapid transition generation. 
     SUMMARY OF THE INVENTION 
     Viewed from one aspect the present invention provides an integrated circuit comprising: 
     a scan chain cell having a functional path used in non-test operation between a functional input and a cell output, said functional path being controlled by a functional clock signal; and 
     functional circuitry clocked by said functional clock signal and coupled to said cell output so as to receive an input signal to be processed by said functional circuitry; wherein 
     said scan chain cell has: 
     inverting circuitry coupled to said cell output to generate an inverted signal having a value opposite to said input signal; 
     inverted value selecting circuitry coupled to said inverting circuitry and responsive to a test selecting signal to select said inverted signal as a next signal output from said cell output; and 
     output signal holding circuitry responsive to a hold signal to hold unchanged a signal value at said cell output. 
     The technique recognises that the scan chain cell itself can generate an appropriate inverted value which is available within the scan chain cell for use as the next signal value to be applied to the functional path thereby generating a transition, the effect of which can be monitored. This combination of mechanisms provides a relatively low overhead way of achieving reliable delay testing of functional circuitry (the circuitry for performing normal non-test operations) with good coverage of potential fault situations which can occur within the integrated circuit. The scan chain cell also provides a signal holding capability whereby a hold signal applied to the scan chain cell will control it to hold its output while the functional clock is toggled. This is useful in sensitizing paths to transitions in other signals. 
     The output signal holding circuitry may act by gating the action of the functional clock signal within the scan chain cell. 
     Whilst it will be appreciated that the scan chain cell could be used in a variety of different ways within an integrated circuit to apply the stimulus discussed above, it is particularly suited for use within systems where the scan chain cell is part of a wrapper scan chain providing test access to signal nodes that are not directly accessible from outside the integrated circuit. These nodes may supply their stimulus signal outwardly and/or inwardly (extest and/or intest). Complex integrated circuit designs with large functional blocks, such as processor cores, embedded within them and not directly accessible from the outside are becoming more common and the use of wrapper scan chains to test such functional blocks is an increasingly useful technique, particularly when combined with the delayed testing capability provided by the present technique. The wrapper scan chains can be used to test logic within a core (intest) and also to test logic external of such a core (extest). The methodology applies to both cases. 
     In order to reduce the control signal overhead associated with the present technique, the testing select signal can be shared by a plurality of scan chain cells, (up to all of the scan chain cells) within a wrapper scan chain. The scan chain cells could however be provided with separate test select signals. 
     In a similar way, in order to reduce overhead it is possible that the hold signal can be shared by a plurality of scan chain cells (up to all of the scan chain cells) within the wrapper scan chain. Different combinations of shared/non-shared test select and hold signals are possible. Sharing the hold signal reduces the ability to select particular scan chain cells to be held while others toggle in value. The held values can sensitize paths to toggling signals and thus the more the hold signals are shared the less specific can be the sensitization. It is also possible that in some systems it may be desirable to use a plurality of hold signals which are independent from one another so that some of the scan chain cells can be held and the inverted signal used to generate a transition, while other of the scan chain cells are not held in this way. This can be useful in increasing the amount of fault coverage achievable since it may be appropriate to not transition some signal nodes within the design in order to sensitise other functional circuits to the delay test that is to be performed. With a shared hold signal fault coverage may still be increased as a held node may serve to sensitise a node which does not have a scan chain cell as the head state element of its path and so is not being held. 
     In some embodiments of the scan chain cell, the scan path between a scan in port and the cell output can be controlled by a separate scan clock signal and this clock signal may also be gated by the clock gating circuitry in response to the hold signal. The gating of the scan clock signal in this way is useful for holding the scan chain cell in the desired state to perform delay testing irrespective of other signals and manipulations which may be being performed using the scan clock signal. 
     Whilst it will be appreciated that the present technique is not dependant upon the relative speed of the scan path and the functional path (e.g. it can be used to avoid the need to provide multiple state holding elements within the scan cells in order to support rapid transition generation), it does have particular use when the scan path is slower than the functional path as in the circumstances the use of the scan path to generate transitions for delay testing is difficult. However, the technique can still be used when the scan path is capable of operating at full speed, e.g. as a way of avoiding the need for separate scan enables to the wrapper. 
     Whilst the inverting value selecting circuitry can take a wide variety of different forms, one form that may be conveniently used is that of a multiplexer switched by the test selecting circuitry. 
     It will also be appreciated that the clock gating circuitry can take a wide variety of different forms depending on the particular implementation of the scan chain cell. One convenient form is the use of one or more OR gates logically combining the functional clock signal with the hold signal. It will be appreciated that other forms of clock gating circuitry are also possible. 
     Viewed from another aspect the present invention provides an integrated circuit comprising: 
     scan chain cell means having a functional path used in non-test operation between a functional input and a cell output, said functional path being controlled by a functional clock signal; and 
     functional circuit means clocked by said functional clock signal and coupled to said cell output so as to receive an input signal to be processed by said functional circuit means; wherein 
     said scan chain cell means has: 
     inverting means coupled to said cell output for generating an inverted signal having a value opposite to said input signal; 
     inverted value selecting means coupled to said inverting circuitry and responsive to a test selecting signal for selecting said inverted signal as a next signal output from said cell output; and output signal holding means responsive to a hold signal for holding unchanged a signal value at said cell output. 
     Viewed from a further aspect the present invention provides a scan chain cell for coupling via a cell output to functional circuitry clocked by a functional clock signal, said scan chain cell comprising: 
     a functional path used in non-test operation between a functional input and said cell output, said functional path being controlled by said functional clock signal; 
     inverting circuitry coupled to said cell output to generate an inverted signal having a value opposite to said input signal; 
     inverted value selecting circuitry coupled to said inverting circuitry and responsive to a test selecting signal to selected said inverted signal as a next signal output from said cell output; and 
     output signal holding circuitry responsive to a hold signal to hold unchanged a signal value at said cell output. 
     Viewed from a further aspect the present invention provides a method of testing an integrated circuit with a scan chain cell, said method comprising the steps of: 
     controlling with a functional clock signal a functional path used in non-test operation through said scan chain cell between a functional input and a cell output; 
     controlling with a scan clock signal a scan path through said scan chain cell between a scan input and said cell output; 
     receiving and processing an input signal from said cell output with functional circuitry clocked by said functional clock signal; 
     generating within said scan chain cell an inverted signal having a value opposite to said input signal; 
     in response to a test selecting signal, selecting within said scan chain cell said inverted signal as a next signal output from said cell output; and 
     in response to a hold signal, holding unchanged a signal value at said cell output. 
     The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an integrated circuit using wrapper scan chains; 
         FIG. 2  schematically illustrates the use of scan chain cells to apply a signal transition to functional circuitry so as to delay test that functional circuitry; 
         FIG. 3  schematically illustrates a scan chain cell with an in-built delay testing capability in accordance with one example embodiment of the present technique; 
         FIG. 4  is a signal timing diagram illustrating the relative timing of various signals controlling the operation of the scan chain cell of  FIG. 3  when performing delay testing; 
         FIGS. 5-7  illustrate the scan chain cell of  FIG. 3  at various times during signal transition generation for delay testing; 
         FIGS. 8 and 9  illustrate the scan chain cell of  FIG. 3  being used to hold a signal value output while a functional clock signal is advanced; 
         FIG. 10  is a flow diagram schematically illustrating a delay testing methodology in accordance with the present techniques; 
         FIG. 11  schematically illustrates the use of two wrapper scan chain cells with separate hold signals; 
         FIG. 12  is a signal timing diagram for use in understanding a mode of use of the arrangement of  FIG. 11 ; and 
         FIG. 13  schematically illustrates another possible use of the wrapper scan chain cell. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates an integrated circuit  2  in the form of a system-on-chip integrated circuit. This integrated circuit  2  includes a plurality of functional blocks  4 ,  6 ,  8 ,  10 . These functional blocks  4 ,  6 ,  8 ,  10  can take a wide variety of different forms and may be produced using designs from a variety of different companies. As an example, the functional block  8  may be a memory designed by one company with the functional block  10  being a processor core designed by another company. 
     In  FIG. 1  the functional block  10  in the form of a processor core has associated wrapper scan chains  12  and  14  for respectively applying stimulus signals to the core  10  and capturing result signals from the core  10 . In this example it will be seen that separate wrapper scan chains  12  and  14  are provided for inputs and outputs, but it will be appreciated that many other different configurations are possible and the wrapper scan chains  12  and  14  could be combined in the form of a single wrapper scan chain. It will be noted that, as indicated in  FIG. 1 , the input signals from the wrapper scan chain  12 , and the output signals from the wrapper scan chain  14 , link to other points within the system-on-chip integrated circuit  2  and are not directly connected to the exterior of the integrated circuit  2 . The scan chain cells  16  can be used to test both the core  10  and external logic directly connected to the core  10 . For this reason, it is not possible to directly apply at speed signal transitions to these points by using a transition in an externally applied signal. The individual scan chain cells  16  which are serially connected to provide the wrapper scan chains  12 ,  14  are illsuited to generating such at speed transitions by serially advancing different signal values between the scan chain cells  16 . 
     It will be appreciated that the processor core  10  will itself comprise many functional circuits which can be subject to delay testing in accordance with the present techniques. The present techniques are not restricted to the testing of any particular type of functional block or functional circuit within a functional block. 
       FIG. 2  schematically illustrates a functional circuit  18  which is to be subject to delay testing. A scan chain cell  20 , which is part of a wrapper scan chain  12 ,  14 , is used to apply a signal transition to one of the inputs of the functional circuit  18 . The functional circuit  18  responds to this signal transition and performs its processing so as to generate an output signal that may also transition when the functional circuit  18  is operating correctly. This output signal value is captured in a further scan chain cell  22 . The capture time of the scan chain cell  22  can be adjusted and accordingly the time taken for the transition to occur in the output signal of the functional circuit  18  can be determined by sampling the output at various different delay times starting from the transition in the input signal. In this way, a determination can be made as to the processing delay associated with the functional circuit  18  and its correct operation can be verified. Such delay testing may be performed as part of manufacturing test may be performed as part of design testing or both. It will be seen that the functional circuit  18  responds to a functional clock signal clk in order to perform its signal processing. 
       FIG. 3  illustrates a scan chain cell  24  having an in-built delay testing capability. The scan chain cell includes three latch elements  26 ,  28 ,  30  that together form a functional signal latch  26 ,  28  and a scan latch  30 ,  28 . The functional signal latch  26 ,  28  is provided by the combination of the latch elements  26  and  28  with the functional path through this functional latch  26 ,  28  being between the D input signal and the Q output signal. The latch elements  26 ,  28  forming the functional latch are clocked by the functional clock signal clk so as to permit a functional signal to propagate therethrough and be held therein. The latch elements  30 ,  28  together provide a scan chain latch  30 ,  28 . This scan chain latch  30 ,  28  is part of a serial scan chain and has a serial input SI and a serial output SO. A scan clock signal sclk controls advancing of the serial signals through the scan chain latch  30 ,  28  and holding of wrapper scan chain signals within the scan chain latch  30 ,  28 . 
     Also provided within the scan chain cell  24  is an inverter  32  which inverts the output signal on the cell output Q to generate an inverted signal which is supplied to a multiplexer  34 . The inverter  32  performs the role of the inverted circuitry discussed above and the multiplexer  34  performs the role of the inverting value selecting circuitry discussed above. This is only one example of the ways in which these circuit elements may be provided. The output from the inverter  32  is selectively applied as the input to the functional latch  26 ,  28  as a replacement for the functional input signal D when such behaviour if selected by a delay test enable signal. The delay test enable signal in this example provides the test selecting signal described above. 
     Two OR gates  36 ,  38  serve to respectively gate the functional clock signal clk and the scan signal sclk under control of a hold signal. The OR gates  36 ,  38  provide the clock gating circuitry discussed above in this example embodiment. 
       FIG. 4  is a signal timing diagram illustrating the values of different signals controlling the operation of the scan chain cell  24  when performing delay testing. In this example the scan clock sclk and the functional clock clk are separate, but in other possible designs (e.g. muxD) the clock could be shared for both functions. This is one example of how such delay testing may be controlled. 
     The use of a shared hold signal may increase fault coverage by virtue of the held nodes serving to sensitise other nodes which do not have any other scan chain cell and so are not being held.  FIG. 4  illustrates various timing points as the delay testing operation progresses and these are respectively marked A, B, C, D, and E. The state and operation of the scan chain cell  24  at these different times A, B, C, D and E will be discussed in relation to  FIGS. 5-9 . 
       FIG. 5  illustrates the operation of the scan chain cell  24  at time A. At this time the scan clock signal sclk is asserted, the hold signal is not asserted and a scan chain input signal SI, being in this example a “1” value, propagates through the latch elements  30 ,  28  and is applied at the cell output Q. 
       FIG. 6  illustrates the operation of the scan chain cell  24  at time B. At this time, the delay test enable signal switches the multiplexer  34  to select the output of the inverter  32  to be fed back into the functional latch  26 ,  28 . As this inverted signal generated by the inverter  32  is generated within the scan chain cell  34  itself it can be made more rapidly available and is held ready within the functional latch  26 ,  28  to be applied to the cell output Q in place of the currently driven output signal “1” at the next functional clock. The inverter  32  need not be within a library cell, but it would be faster provided that way. Also the wrapper cell can comprise more than one library element. 
       FIG. 7  illustrates the operation of the scan chain cell  24  at time C. At this time the functional clock signal clk is asserted. This triggers capture in scan chain cell  22  of the output of the functional circuit  18 , which is to be subject to delay test, and that has resulted from the processing of the signal value “1”. At the same time, the latch element  28  is opened so as to permit the inverted signal “0” to appear at the cell output Q and to start the processing of the “0” signal value by the functional circuit  18 . The propagation delays through the latch element  28  and through the functional circuitry  18  are such that the value captured in scan chain cell  22  at time C with not be influenced by the “0” thereby avoiding a false result. The next functional clock signal clk pulse illustrated in  FIG. 4  will cause capture of the result of the processing of the inverted value “0”. This tests a “1” to “0” transition and whether the result of this transition propagates through the functional circuitry within the functional clock period. The generation of the inverted value within the scan chain cell  24  itself permits this next functional clock signal to be applied “at speed” with an assured signal transition in the scan chain cell  24  output Q. 
       FIG. 8  illustrates the operation of the scan chain cell  24  when acting to hold a signal output. At this time a hold signal is asserted. This hold signal acts with the OR gates  36 , 38  to gate the functional clock signal clk and the scan clock signal sclk such that both the functional path through latch elements  26 ,  28  is blocked as well as the scan path between latch elements  30  and  28 . The scan chain cell  24  thus holds its output, in this case a value of “0”. 
       FIG. 9  illustrates the operation of the scan chain cell  24  when the functional clock clk is asserted while the hold signal is active. In this case the functional path  26 ,  28  and the scan path  30 ,  28  are both blocked. The functional clock signal nevertheless triggers capture of the processing results from the functional circuitry  18 . Thus, a known constant signal value, e.g. “0”, is applied to the functional circuitry  18  while the functional clock signal clk is advanced. This can be useful in sensitizing paths for test. 
     In one scenario separate hold signals can be provided to the wrapper scan chain cells at respective different inputs to some functional circuitry under test so that one of these inputs may be held constant while the other is transitioned. The constant value may be needed to sensitize the functional circuitry being tested. 
     In another scenario the functional circuitry may have some input(s) which is not supplied via wrapper scan chain cells and cannot be held at a fixed value. In this scenario, one or more input signals which do pass through wrapper scan chain cell can be held constant, potentially to sensitize the circuitry to test the delay associated with the input signal(s) that is not held. 
       FIG. 10  is a flow diagram schematically illustrating delay testing in accordance with one example of the present technique. At step  40  the scan clock signal is used to shift a known signal value “1” into a wrapper cell. At step  42  the delay test enable signal is asserted and an inverted signal value “0” is selected and made available within the scan chain cell  24  for use as the next value to the output there from. This inverted signal value is generated within the scan chain cell  24  itself (in this example by the inverter  32 ); the inverter  32  need not be within the same library cell. The hold signal remains low throughout as the scan chain cell is being used to generate a transitioning value rather than a held value. 
     At step  44 , the result of the processing by the functional circuitry  18  of the signal value “1” is captured. The functional clock clk at this time also serves to move the inverted signal value “0” to the cell output Q and the functional circuitry  18  starts to respond to this new signal value “0” (i.e. responds to a “1” to “0”). 
     At step  48  the functional clock clk is clocked again so as to trigger capture at the output of the functional circuitry  18  of the result of processing the inverted signal “0” which was presented at the cell output Q. This second functional clock signal clk follows “at speed” since the inverted signal value was generated within the scan chain cell (which can comprise more than one library cell) itself and so is rapidly available. 
     At step  50  the output of the functional circuitry  18  in response to the transition of its input signal value is captured. Step  52  then determines the functional circuitry  18  responded to the transition in the relevant input signal value within a maximum permitted delay time. It will be appreciated that the step  52  will typically be performed off-chip once the observe signal value has been recovered and may be analysed with the appropriate testing tools and software. 
       FIG. 11  schematically illustrates the use of two wrapper scan chain cells with separate hold signals. It is desired to test the signal path extending between wrapper cell  54  through processing logic  56  to a further cell  58 . As part of this path to be tested there is a gate  60  which receives an input from a wrapper scan chain cell  62  that must have a predetermined value if the path under test is to be unblocked. This gate is schematically illustrated in  FIG. 11  as an AND gate. It will be appreciated that the arrangement of the gate  60  and the processing logic  56  is schematic and illustrative of what in practice may be a much more complicated arrangement. At an overall level the desired operation is that the wrapper scan chain cell  62  should hold a value at its output which allows the path under test to be unblocked while the wrapper scan chain cell  54  generates an at-speed signal level transition which propagates through the gate  60  and the processing logic  56  to cell  58  where it is captured. In this way the path delay can be tested. 
       FIG. 12  is a signal timing diagram illustrating control and data signals which arise in the above scenario when performing path delay testing. It will be noted that the wrapper scan chain cell  54  has a hold signal hold 0  and the wrapper scan chain cell  62  has a separate hold signal hold 1 . During a shift in phase of operation the clock signal is controlled so as to serially scan in desired values into the wrapper scan chain cells  54 ,  62  in accordance with conventional serial scan chain operation. In this example, high values “1” are scanned into both of the wrapper scan chain cells  54 ,  62 . When these data values are in place, the hold signal hold 1  of wrapper scan chain cell  62  is asserted and the delay_enable signal is asserted in order to activate the delay testing mode for the wrapper scan chain cell  54 . When the clock signal clk is then advanced as previously described the wrapper scan chain cell  54  will generate a transition at its output and the processing result from that output can be captured within the cell  58  following the normal clock period delay in order to determine whether the path delay time through the processing logic  56  and the gate  60  is too long. During this operation the output of the wrapper scan chain cell  54  will toggle with each rising edge of the clock signals clk since its whole signal hold 0  is not asserted. In contrast, as the hold signal hold 1  for the wrapper scan chain cell  62  is asserted, its output will remain constant and accordingly the signal path through the gate  60  will remain open and the circuitry accordingly sensitised for the delay testing of the path through gate  60  and processing logic  56 . Use of separate hold signals in this way allows particular processing paths to be sensitised for delay testing and accordingly, test coverage may be increased. 
       FIG. 13  shows functional circuitry  64  with one input signal coming from an internal cell  66  which is not part of the wrapper and cannot be controlled by wrapper control signals. Another input signal to the functional circuitry  64  comes from a wrapper cell  68  which can be controlled to output and hold (during functional clock clk clocking) a known signal value thereby sensitizing the functional circuitry  64  to the delay associated with the transitions in the signal from the internal cell  66 . 
     Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.