Abstract:
A semiconductor integrated circuit device which shortens the time required for testing a divided logic circuit to reduce test cost and a fault-detecting method therefor. The logic circuit is divided into N logic blocks using N+1 scan paths comprises of scan flip-flops each having selectors for selectively picking up the output signals of storage elements which are fed back to the storage elements. A common scan operation may then be carried out on these logic blocks (Logic  1 -to Logic N), and a testing operation may be continuously carried out on the logic blocks. The present invention preferably eliminates the overlaps in conventional scan operations, resulting in a shorter test time.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to Japanese Patent Application No. 2001-151983 filed on May 22, 2001.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a semiconductor integrated circuit device, and, more particularly, the present invention relates to a semiconductor circuit device which has a logic circuit including storage elements and a fault-detecting method of a semiconductor integrated circuit device.  
           [0004]    2. Description of the Background  
           [0005]    A conventional method for scanning a logical circuit has been widely used to detect faults, such as a stuck-at fault, in a logic circuit. This conventional method can efficiently detect faults because the value of a flip-flop (hereinafter, “FF”) can be directly handled using this method.  
           [0006]    The process for detecting a fault (hereinafter, “testing”) of a scanned logic circuit will now be explained with reference to the accompanying drawings.  
           [0007]    [0007]FIG. 17 is a circuit diagram showing a scan FF as used in the prior art. This example is a scan FF with a multiplexer (hereinafter, “MUX”), or a MUX-type scan flip-flop (FF).  
           [0008]    In this configuration, MUX g 1702  is connected with input terminal D of a master latch g 1701  constituting a FF. A signal (hereinafter, “input signal from logic circuit”) which comes from logic gates (hereinafter, “user logic circuit”) for normal operation at the preceding stage is input through an input signal line into g 1702 . Also, a signal for scan from the FF at the preceding stage (hereinafter, “scan-in signal”) is input through a scan-in signal line. Further, a control signal for selecting whether g 1701  picks up the input signal from logic circuit or the scan-in signal (hereinafter, “scan-enable signal”) is input through a scan-enable signal line. Input terminal D of a slave latch g 1703  is connected with output terminal Q of g 1701 . The output terminal of g 1703  is connected with an output signal line which conveys a signal (hereinafter, “output signal”) to the user logic circuit or a scan FF in a successive stage.  
           [0009]    [0009]FIG. 18 shows a logic circuit which is scanned using the prior art. This is an example of a scanned logic circuit in which multiple MUX-type scan FFs are connected. In this configuration, scan-out signal lines of MUX-type scan FFs g 1801 , g 1802  are connected with the scan-in signal lines of g 1802  and g 1803 , respectively; this constitutes a signal path (hereinafter, “path”). This path is referred to as a “scan path.” 
           [0010]    The scan-in signal line of g 1801  is connected with the terminal (hereinafter, “scan-in terminal”) for inputting a scan-in signal from the outside of the semiconductor integrated circuit chip while the scan-out signal line of g 1803  is connected with the terminal (hereinafter, “scan-out terminal”) for outputting a scan-out signal to the outside of the semiconductor integrated circuit chip (hereinafter, “chip”).  
           [0011]    A test procedure which utilizes scan FFs preferably takes place in the following sequence: (1) an operation for substituting an initial value for the test into each FF in the logic circuit (“scan-in operation”); (2) an operation for inputting the initial value from each FF into the user logic circuit and letting each FF pick up the result data for the test as an output from the user logic circuit (“testing operation”); and (3) an operation for collecting the result data from each FF (“scan-out operation”). This sequence may be repeated. In FIG. 18, a 1804 , a 1805 , and a 1806  represent signal flows for scan-in operation, testing operation, and scan-out operation, respectively.  
           [0012]    [0012]FIG. 19 is a timing diagram showing the operation of a scan FF (g 1704 ) as used in the prior art. First, for the scan-in operation, a scan-enable signal is set at High in order to enable each FF to pick up a scan-in signal. Also, in order to substitute the initial value for a test value in each FF, a transition of the system clock signal is made more than one time (s 1901 ) to perform shift operations through -the scan path.  
           [0013]    Next, for the testing operation, in order to enable each FF to pick up an output signal, the scan-enable signal is set at Low (s 1902 ). Also, a transition of the system clock signal is made a first time to input the initial value for the test into the user logic circuit and a second time for the result data for the test to be picked up by each FF (s 1903 ).  
           [0014]    For the scan-out operation, the scan-enable signal is set at High again in order to enable each FF to output a scan-out signal (s 1904 ). Also, for collection of the result data for the test from each FF, a shift operation is done as in the scan-in operation.  
           [0015]    [0015]FIG. 20 is a circuit diagram showing an exemplary internal circuit of a scan FF (g 1704 ) as used in the prior art.  
           [0016]    However, in a scan-in operation and a scan-out operation (collectively referred to as “scan operation”) the amount of state changes of the logic circuit tends to be higher than in normal user operation. Therefore, as pointed out in IEEE Computer vol.32, no.11 (p.61, 1999), with the growing tendency toward device miniaturization, fault-detecting errors due to excessive voltage drops or chip damage due to heat generation may occur.  
           [0017]    As potential solutions to address this problem, some methods have been suggested; the Proceedings of the 11 th  International VLSI Test Symposium (pp.4-9, 1993) include one such method in which the logic circuit is divided into blocks which are then tested sequentially.  
           [0018]    The way in which the logic circuit is divided into blocks and each block is tested will now be described with reference to the relevant accompanying drawings.  
           [0019]    [0019]FIG. 21 shows a logic circuit which is divided according to the prior art. In this example, the logic circuit is divided into N blocks using N+1 scan paths which consist of MUX-type scan FFs (wherein N is a natural number).  
           [0020]    In this configuration, a selector g 2102  which controls the operation of Logic  1  at the following stage is connected with the output signal line of a scan FF g 2101 . An output signal for normal operation and testing operation which comes from g 2101  is input into g 2102 . In addition, a fixed signal for pausing Logic  1  is input through a boundary scan (FF) g 2103 . Further, a signal (pausing signal) for selecting whether to output the output signal from g 2101  or the fixed signal through g 2103  to Logic  1  is input through Pausing Signal Line  1 . The same type of selector is connected with the output signal lines of the other scan FFs to control the operation of Logic  1  through Logic N.  
           [0021]    [0021]FIG. 22 is a timing diagram showing the operation of the logic circuit which is divided according to the prior art. Prior to the testing operation for Logic  1 , Pausing Signal  1  is set at Low so that the initial value for the test as supplied from Scan-In Signal Line  1  can be input for Logic  1  (s 2201 ). For pausing Logic  2  to Logic N, Pausing Signal  2  to Pausing Signal N are set at High (s 2202 ). Thereafter during the testing operation for Logic  1 , the result data for the test of Logic  1  is stored in the scan FF connected with Scan-In Signal Line  2 . Next, the result data for the test of Logic  1  is collected by scan operation (s 2203 ). This is followed by similar operation sequences for Logic  2  through Logic N.  
           [0022]    Regardless, since there are overlaps of scan operations by scan FFs in the boundaries of the divided logic circuit, this method requires a longer test time. This counteracts the advantage of scanning because the time required for scan operation generally accounts for most of the overall test time.  
           [0023]    As exemplified in FIG. 22, although a scan operation which uses Scan-In Signal Line  2  as an input can be accomplished during the test of Logic  1  (s 2204 ), it must be done again during the test of Logic  2  (s 2205 ). This results in an increase in the cost required for the test (the “test cost”).  
           [0024]    As another potential solution, “Design for At-speed Test, Diagnosis and Measurement” (Kluwer Academic Publishers, p.24, 1999) suggests a method of decreasing the frequency in scan operation to reduce power consumption. In this method, the frequency (s 1903 ) of the system clock signal in the testing operation is not decreased—only the frequency (s 1901 ) of the system clock signal in the scan operation is decreased.  
           [0025]    However, this method cannot reduce power consumption in the testing operation. As discussed above, the prior art has not properly addressed the problem of the extensive amount of time required for fault detection of a divided Logic circuit.  
         SUMMARY OF THE INVENTION  
         [0026]    In at least one embodiment, the present invention preferably provides a semiconductor integrated circuit device in which the test cost can be reduced by reducing the required test time in relation to the prior art. The present invention also comprises fault-detecting methodologies thereof.  
           [0027]    According to at least one preferred embodiment of the present invention, a semiconductor integrated circuit device comprises a storage element which constitutes a scan flip-flop, wherein the storage element has a selector for selectively picking up its output signals fed back thereto.  
           [0028]    According to at least one embodiment of the present invention, a semiconductor integrated circuit device comprises storage elements and logic gates which constitute a scan flip-flop, wherein a first signal, the output signal of a first storage element, and a second signal are input into a first logic gate. The output signal of the first logic gate, and a third and a fourth signal are input into a second logic gate, and the output signal of the second logic gate and a fifth signal are input into the first storage element.  
           [0029]    The above configuration may be characterized in that the output signal of the first storage element, the output signal of the second storage element, and the fourth signal are input into a third logic gate, and the output signal of the third Logic gate and the fifth signal are input into the second storage element. Additionally, the output signal of the first storage element and the fifth signal may be input into the second storage element.  
           [0030]    According to at least one embodiment of the present invention, the above configuration is characterized in that the first logic gate consists of a first selector for selecting, through the first signal, whether to pick up the output signal of the first storage element or the second signal, and the second logic gate consists of a second selector for selecting, through the fourth signal, whether to pick up the output signal of the first logic gate or the third signal. Also, the embodiment may be characterized in that the third logic gate consists of a third selector for selecting, through the fourth signal, whether to pick up the output signal of the first storage element or the output signal of the second storage element.  
           [0031]    According to another embodiment of the present invention, a semiconductor integrated circuit device comprises storage elements and logic gates which constitute a scan flip-flop, wherein: a first signal, a second signal, and a third signal are input into a first logic gate; a fourth signal and a fifth signal are input into a second logic gate; and the output signal of the first logic gate and the output signal of the second logic gate are input into the first storage element. Additionally, the output signal of the first storage element and the output signal of the second logic gate may be input into the second storage element.  
           [0032]    According to another embodiment of the present invention, a semiconductor integrated circuit device comprises storage elements and logic gates which constitute a scan flip-flop, wherein: a first signal, a second signal, and a third signal are input into a first logic gate; a fourth signal and a fifth signal are input into a second logic gate; the output signal of the second logic gate is input into a third logic gate; and the output signal of the first logic gate and the output signal of the third logic gate are input into the first storage element. Additionally, either the fourth signal or the fifth signal, whichever makes more signal transitions than the other, is input into a gate terminal as another terminal of one of the transistors constituting the second logic gate which has a terminal connected with the output terminal of the second logic gate.  
           [0033]    According to still another embodiment of the present invention, a semiconductor integrated circuit device comprises storage elements and logic gates which constitute a scan flip-flop, wherein: a first signal, a second signal, and a third signal are input into a first logic gate; the output signal of the first logic gate is input into a second logic gate; the output signal of the first logic gate, a fourth signal, and a fifth signal are input into a third logic gate; the output signal of the second logic gate, the fourth signal, and the fifth signal are input into a fourth logic gate; and the output signal of the third logic gate and the output signal of the fourth logic gate are input into the first storage element.  
           [0034]    In at least one embodiment of a fault-defecting method of the present invention, the fault-detecting method of a semiconductor integrated circuit device having a logic circuit including storage elements is characterized in that the logic circuit is divided into a plurality of circuit blocks and a common scan operation is carried out on these circuit blocks. A testing operation is also carried out on every circuit block on a time-division basis.  
           [0035]    According to another embodiment of the present invention, a fault-detecting method of a semiconductor integrated circuit device having a logic circuit including storage elements is characterized in that the logic circuit is divided into N circuit blocks using (N+1) scan paths composed of scan flip-flops, and common scan operation is carried out on these circuit blocks. A testing operation is continuously carried out on the circuit blocks.  
           [0036]    According to still another embodiment of the present invention, a fault-detecting method of a semiconductor integrated circuit device is characterized in that the logic circuit in the semiconductor integrated circuit device is divided into N circuit blocks using (N+1) scan paths composed of scan flip-flops each having selectors for selectively picking up storage elements&#39; output signals which are fed back to the storage elements, and a common scan operation is carried out on these circuit blocks. A testing operation is preferably continuously carried out on the circuit blocks. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    For the present invention to be clearly understood and readily practiced, the present invention will be described in conjunction with the following figures, wherein like reference characters designate the same or similar elements, which figures are incorporated into and constitute a part of the specification, wherein:  
         [0038]    [0038]FIG. 1 illustrates the circuit structure of a semiconductor integrated circuit device according to a first embodiment of the present invention;  
         [0039]    [0039]FIG. 2 is a timing diagram showing the operation according to the first embodiment;  
         [0040]    [0040]FIG. 3 illustrates the circuit structure of a semiconductor integrated circuit device as an application example according to the first embodiment;  
         [0041]    [0041]FIG. 4 is a timing diagram showing how the semiconductor integrated circuit device shown in FIG. 3 operates;  
         [0042]    [0042]FIG. 5 is a circuit diagram showing an example of internal circuitry of a scan FF described in connection with the first embodiment;  
         [0043]    [0043]FIG. 6 illustrates the circuit structure of a semiconductor integrated circuit device according to a second embodiment of the present invention;  
         [0044]    [0044]FIG. 7 illustrates the circuit structure of a semiconductor integrated circuit device according to a third embodiment of the present invention;  
         [0045]    [0045]FIG. 8 is a timing diagram showing the operation according to the third embodiment;  
         [0046]    [0046]FIG. 9 illustrates the circuit structure of a semiconductor integrated circuit device as an application example according to the third embodiment;  
         [0047]    [0047]FIG. 10 is a timing diagram showing how the semiconductor integrated circuit device shown in FIG. 9 operates;  
         [0048]    [0048]FIG. 11 illustrates the circuit structure of a semiconductor integrated circuit device according to a fourth embodiment of the present invention;  
         [0049]    [0049]FIG. 12 is a circuit diagram showing an example of the internal circuitry of a semiconductor integrated circuit device according to the fourth embodiment;  
         [0050]    [0050]FIG. 13 is a top view showing an exemplary layout of scan FFs described in connection with the fourth embodiment;  
         [0051]    [0051]FIG. 14 illustrates the circuit structure of a semiconductor integrated circuit device according to a fifth embodiment of the present invention;  
         [0052]    [0052]FIG. 15 illustrates the circuit structure of a semiconductor integrated circuit device according to a sixth embodiment of the present invention;  
         [0053]    [0053]FIG. 16 is a graph showing peak current values in the testing operation concerning the fourth embodiment;  
         [0054]    [0054]FIG. 17 shows the circuit structure of a conventional scan FF;  
         [0055]    [0055]FIG. 18 shows a logic circuit scanned by conventional methods;  
         [0056]    [0056]FIG. 19 is a timing diagram showing the operation of a conventional scan FF;  
         [0057]    [0057]FIG. 20 shows the structure of internal circuitry of a conventional scan FF;  
         [0058]    [0058]FIG. 21 shows a logic circuit as divided by conventional methods; and  
         [0059]    [0059]FIG. 22 is a timing diagram showing the operation of the logic circuit as divided by conventional methods. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0060]    It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements that may be well known. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. The detailed description will be provided hereinbelow with reference to the attached drawings.  
         [0061]    Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.  
         [0062]    [0062]FIG. 1 illustrates the general structure of a semiconductor integrated circuit device according to a first exemplary embodiment of the present invention. This example applies the present invention to a conventional MUX-type scan FF.  
         [0063]    In this configuration, a selector is inserted before each latch constituting a conventional scan FF (g 1704 ).  
         [0064]    Specifically, a scan-in signal line is connected with a selector g 302  which, under the control of a scan-enable signal line, selects whether to output a scan-in signal or the output signal of a master latch g 301  constituting a scan FF.  
         [0065]    Also, input terminal D of g 301  is connected to a selector g 303  which, under the control of a data-holding signal line, selects whether to output the input signal from the logic circuit or the output signal of g 302 . Further, input terminal D of a slave latch g 304  constituting the scan FF is connected with a selector g 305  which, under the control of the data-holding signal line, selects whether to output an output signal of g 301  or an output signal of g 304 .  
         [0066]    [0066]FIG. 2 is a timing diagram showing the operation of the first embodiment. Initially, prior to the testing operation, the data-holding function incorporated in the scan FF g 306  according to the present invention is activated. Meanwhile, the clock signal is stopped (s 401 ) and the state of the data-holding signal is changed from Low to High (s 402 ) so that even when there is a transition of the clock signal, the output signals of g 301  and g 304  can be held (s 403 ).  
         [0067]    Completion of the above-mentioned signal control requires a waiting time which corresponds to a signal propagation delay equivalent to one gate stage. However, the resultant increase in the overall test time which is caused by this waiting time is negligible. In general, the number of clock signal transitions necessary for the scan operation is hundreds to thousands of times as large as that for the testing operation, and the time required for the scan operation accounts for most of the test time. Also, the clock and data-holding signals can be easily generated using conventional techniques. Specifically, such signals can be fed from outside of the chip using a tester or using an oscillation circuit with an added counter inside the chip.  
         [0068]    After completion of the testing operation, the data-holding function is deactivated. Meanwhile, the clock signal is stopped, and the state of the data-holding signal is changed from High to Low (s 404 ) so that the holding condition of the output signals of g 301  and g 304  is cancelled. With the data-holding signal in the Low state, the testing operation is carried out in the same way as in a conventional scan FF.  
         [0069]    [0069]FIG. 3 illustrates the circuit structure of a semiconductor integrated circuit device as an application example according to the first embodiment.  
         [0070]    In this example, the logic circuit is divided into N logic blocks (circuit blocks) using N+1 scan paths comprised of scan FFs (g 201 ) described in connection with the first exemplary embodiment.  
         [0071]    [0071]FIG. 4 is a timing diagram showing how the semiconductor integrated circuit device shown in FIG. 3 operates. The testing operation takes place in the order of proximity to the output of the whole logic circuit, namely from Logic N to Logic  1 . Initially, before the testing operation for Logic N, data-holding signal N is set at Low (s 101 ) so that the initial value for the test as supplied from scan-in signal line N can be input for Logic N. Data-holding signal N+1 is set at Low (s 102 ) so that the scan FF connected to scan-in signal line N+1 can collect the result data for the test of Logic N. Further, in order to pause Logic  1  to Logic N−1, data-holding signal  1  to data-holding signal N−1 are set at High (s 103 ).  
         [0072]    Thereafter, in the testing operation for Logic N, the result data for the test of Logic N is stored in the scan FF connected to scan-in signal line N+1. The result data for the test of Logic N remains held until data-holding signal N becomes Low again. After this, a similar operational sequence is followed for Logic N−1 through Logic  1 . As described above, according to this configuration, a common scan operation is performed for Logic N to Logic  1  of the divided logic circuit and a testing operation can be continuously carried out on these logic blocks (s 104 ).  
         [0073]    Therefore, this configuration eliminates the overlaps of the scan operation that are seen in the conventional methods without causing fault-detecting errors due to excessive voltage drops or chip damage due to heat generation. Consequently, the overall test time may be shortened. Furthermore, since the testing operation begins with a logic block nearest to the output of the whole logic circuit, an increase in the chip area can be minimized. In other words, according to the first exemplary embodiment, only two additional selectors for holding data (g 302  and g 305 ) are used for the scan FF. However, if the order in which the testing operation takes place is not specified, the master latch (g 301 ) and slave latch (g 304 ) must be doubled.  
         [0074]    [0074]FIG. 5 is a circuit diagram showing an example of the internal circuitry of the scan FF (g 306 ) described in connection with the first exemplary embodiment. G 301  to g 305  in FIG. 5 correspond to those in FIG. 1.  
         [0075]    [0075]FIG. 6 illustrates the circuit structure of a semiconductor integrated circuit device according to a second exemplary embodiment of the present invention. This example applies the present invention to a MUX-type scan latch. The scan latch has a structure in which output terminal Q of a master latch g 601  is connected to input terminal D of a slave latch g 602  dedicated to scan.  
         [0076]    The present invention can be applied not only to the scan FF but also to the scan latch as in the case of the first exemplary embodiment. Specifically, the scan-in signal line is connected with a selector g 603  which, under the control of the scan-enable signal line, selects whether to output the scan-in signal or the output signal of g 601 . Further, input terminal D of g 601  is connected with a selector g 604  which, under the control of the data-holding signal line, selects whether to output the input signal from the logic circuit or the output signal of g 603 .  
         [0077]    [0077]FIG. 7 illustrates the circuit structure of a semiconductor integrated circuit device according to a third exemplary embodiment of the present invention.  
         [0078]    This embodiment applies the present invention to a MUX-type scan FF. In this configuration, a two-input AND gate g 703  which is controlled by a “data-holding inverted signal line” for the propagation of an inverted signal with respect to the data-holding signal is inserted in the scan FF&#39;s clock signal line.  
         [0079]    This configuration provides a data-holding function similar to that of the first exemplary embodiment without the feedback of the output signal from the master latch to the selector as was needed in the first exemplary embodiment. Therefore, when the data-holding inverted signal is set at Low, the transition of the clock signal is prevented from being input to the master latch g 701  and slave latch g 702  which both comprise the scan FF.  
         [0080]    [0080]FIG. 8 is a timing diagram showing the operation according to the third exemplary embodiment. Initially, prior to the testing operation, the data-holding function incorporated in the scan FF according to the present invention is activated. Meanwhile, the clock signal transition is stopped (s 801 ) and the state of the data-holding inverted signal is changed from High to Low (s 802 ) so that even when there is a transition of the clock signal, the output signals of g 701  and g 702  can be held (s 803 ). Completion of the above-mentioned signal control requires a waiting time. However, an increase in the overall test time which is caused by this waiting time is negligible as in the case of the first exemplary embodiment.  
         [0081]    Clock and scan-enable signals can be easily generated using conventional techniques. After completion of the testing operation, the data-holding function is deactivated. At the same time, the clock signal is stopped, and the state of the data-holding inverted signal is changed from Low to High (s 804 ) so that the holding condition of the output signals of g 701  and g 702  is cancelled. With the data-holding signal in the Low state, the testing operation is carried out in the same way as in a conventional scan FF.  
         [0082]    [0082]FIG. 9 illustrates the structure of a semiconductor integrated circuit device as an application example according to the third exemplary embodiment. In this example, the logic circuit is divided into N logic blocks where scan FFs g 901  described in connection with the third exemplary embodiment and conventional scan FFs g 902  are mixed.  
         [0083]    In this configuration, a two-input AND gate g 903  which is controlled by a data-holding inverted signal line is inserted in the clock signal line connected with g 902 . This configuration enables g 902  to provide a data-holding function similar to that of g 901 . In addition, the number of transistors which constitute a scan FF may be smaller than the third embodiment, which may lead to a smaller chip area and reduced power consumption.  
         [0084]    [0084]FIG. 10 is a timing diagram showing the operation of the semiconductor integrated circuit device shown in FIG. 9.  
         [0085]    In this example, a data-holding inverted signal is set to stop the clock signal transition. Therefore, as in the case shown in FIG. 3, common scan operation is carried out on Logic N to Logic  1  of the divided logic circuit, and a testing operation can be continuously carried out on the logic blocks (s 1001 ).  
         [0086]    Here, the testing operation takes place in the order of proximity to the output of the whole logic circuit, namely from Logic N to Logic  1 . Initially, before the testing operation for Logic N, data-holding inverted signal N+1 is set at High (s 1002 ) so that the initial value for the test as supplied from scan-in signal line N can be input for Logic N. Data-holding inverted signal N+1 is set at High (s 1003 ) so that the scan FF connected to scan-in signal line N+1 can collect the result data for the test of Logic N. Further, in order to pause Logic  1  to Logic N−1, data-holding inverted signal  1  to data-holding inverted signal N−1 are set at Low (s 1004 ).  
         [0087]    Thereafter, in the testing operation for Logic N, the result data for the test of Logic N is stored in the scan FF connected with scan-in signal line N+1. The result data for the test of Logic N remains held until data-holding signal N becomes Low again. After this, a similar operational sequence is followed for Logic N−1 to Logic  1 .  
         [0088]    [0088]FIG. 11 illustrates the structure of a semiconductor integrated circuit device according to a fourth exemplary embodiment of the present invention. In this example, a clock gate (g 903 ) as described in connection with the third exemplary embodiment is incorporated in a MUX-type scan FF.  
         [0089]    In this configuration, a two-input NAND gate g 1101  which is controlled by a data-holding inverted signal line is inserted after a system clock terminal. This configuration permits a reduction in the number of gate stages provided on the clock signal line as compared with the third exemplary embodiment. In other words, while the third exemplary embodiment uses three gate stages (g 903 , g 2001 , and g 2002 ), this embodiment uses only two gate stages g 1101 , g 1102 .  
         [0090]    In addition, the size of a transistor which comprises a scan FF may be smaller than in the third exemplary embodiment, which could lead to a smaller chip area and reduced power consumption. Since there are two gate stages on the clock signal line as in the conventional type of scan FF, the scan FFs according to the present invention and the conventional type of scan FFs may be mixed, thereby preventing an increase in clock skews.  
         [0091]    [0091]FIG. 12 is a circuit diagram showing an example of the internal circuitry of a semiconductor integrated circuit device according to the fourth exemplary embodiment. In this example, the clock gate g 1101  and inverter gate g 1102  as described in connection with the fourth exemplary embodiment are comprised of transistors.  
         [0092]    In this configuration, the clock signal line is connected to the gate terminal of transistor t 1201  (comprising g 1101 ) whose drain terminal is connected with the output terminal of g 1101 . In this configuration, the delay time in clock signal propagation is shorter than when the clock signal line is connected with the gate terminal of transistor t 1202  whose drain terminal is not connected with the output terminal of g 1101 .  
         [0093]    [0093]FIG. 13 is a top view showing an example of the layout of scan FFs described in connection with the fourth exemplary embodiment. For better illustration, voltage supply lines, diffusion layers, and poly-silicon gate layers are shown, but connection lines between poly-silicon gate layers and diffusion layers are omitted.  
         [0094]    In this configuration, g 1101  and g 1102  share Vdd voltage supply line v 1203  and Gnd voltage supply line v 1205 . This makes it possible to decrease the width of diffusion layers, resulting in chip area reduction.  
         [0095]    [0095]FIG. 14 illustrates the structure of a semiconductor integrated circuit device according to a fifth exemplary embodiment of the present invention. This example applies the present invention to a MUX-type scan latch. The scan latch has a structure in which output terminal Q of a master latch g 1401  is connected with input terminal D of a slave latch g 1402  dedicated to scan.  
         [0096]    The present invention may be applied not only to a scan FF but also to a scan latch as in the case of the third exemplary embodiment. Specifically, a two-input AND gate g 1403 , which is controlled by the data-holding inverted signal line, is inserted in the clock signal line. The output signal line of g 1403  is connected to the clock terminal of g 1401 , and the clock signal line is connected to the clock terminal of g 1402 .  
         [0097]    [0097]FIG. 15 illustrates the structure of a semiconductor integrated circuit device according to a sixth exemplary embodiment of the present invention. In this example, the clock gate (g 1403 ) as described in connection with the fifth exemplary embodiment is built in the master latch (g 1401 ) of a MUX-type scan latch (g 1401 ).  
         [0098]    As in the fourth exemplary embodiment, the present invention may be applied even when the clock signal of the master latch is driven by NAND gates (g 1501  and g 1502 ) rather than an inverter gate. Specifically, this may be achieved using three-input NAND gates as g 1501  and g 1502  instead of two-input NAND gates and connecting data-holding inverted terminals to the respective gates.  
         [0099]    [0099]FIG. 16 is a graph showing peak current values in the testing operation concerning the fourth exemplary embodiment. When the whole logic circuit is tested at one time, the peak values of currents consumed by the logic circuit during the testing operation are at least twice as high as those during normal operation. In order to lower the peak current values in testing operation, the logic circuit should be divided. According to the present invention, it is divided into three blocks.  
         [0100]    Accordingly, when the technique suggested by the present invention is used, the test time can be reduced to one half or less of that required using conventional techniques. As explained above, the present invention may also reduce the test cost by making the test time shorter than in the conventional art.  
         [0101]    Nothing in the above description is meant to limit the present invention to any specific materials, geometry, or orientation of parts. Many part/orientation substitutions are contemplated within the scope of the present invention. The embodiments described herein were presented by way of example only and should not be used to limit the scope of the invention.  
         [0102]    Although the invention has been described in terms of particular embodiments in an application, one of ordinary skill in the art, in light of the teachings herein, can generate additional embodiments and modifications without departing from the spirit of, or exceeding the scope of, the claimed invention. Accordingly, it is understood that the drawings and the descriptions herein are proffered by way of example only to facilitate comprehension of the invention and should not be construed to limit the scope thereof.