Patent Publication Number: US-8536918-B2

Title: Flip-flop circuit, scan test circuit, and method of controlling scan test circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2011-074270, filed on Mar. 30, 2011, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     The present invention relates to a scan flip-flop circuit used when a scan test of a semiconductor integrated circuit is conducted, a test circuit using the scan flip-flop circuit, and a method of controlling the test circuit, and specifically, to a technique using a scan flip-flop circuit including two latch circuits of a master latch and a slave latch. 
     A scan test using an LSSD (Level Sensitive Scan Design) latch has already been known. A technique related to the scan test using the LSSD latch is disclosed in multiple literatures, including for example, “LSSD (Level Sensitive Scan Design)” [searched on Feb. 20, 2011] the Internet &lt;http://www.cedcc.psu.edu/ee497i/rassp — 43/s1d089.htm&gt;, “Chapter Design For Testability” [searched on Feb. 20, 2011] the Internet &lt;http://faculty.ksu.edu.sa/musaed/CEN491Doc/Scan-Path.ppt&gt;, and FIG. 6 of Japanese Unexamined Patent Application Publication No. 2005-308421. 
       FIG. 18  is a view showing a representative LSSD latch circuit disclosed in these cited literatures. As shown in  FIG. 18 , an LSSD circuit  800  includes a latch circuit  302  which is a Low level latch, and a latch circuit  303  which is a Low level latch. 
     The latch circuit  302  receives a data input D, a clock signal C, a scan shift data input I, and a scan clock signal A, and outputs data to a data output L 1 . The latch circuit  303  receives output data from the latch circuit  302  and a scan clock signal B, and outputs data to a data output L 2 . 
     The Low level latch (low-level latch) here means a circuit that captures data in a Low level of the clock signal and holds the data in a High level, and the High level latch (high-level latch) means a circuit that captures data in a High level of the clock signal and holds the data in a Low level. 
     In recent years, semiconductor integrated circuits such as an LSI (Large Scale Integration) include flip-flops (F/Fs) in accordance with a high-speed operation and an increase in size from the point of easiness of timing design. Typically, the F/F is formed of two stages of latch circuits of a master latch and a slave latch. Accordingly, in order to form an F/F of LSSD type using the LSSD latch shown in  FIG. 18 , a master latch is added to the input side of the LSSD latch. 
     Hereinafter, a related scan test using the F/F of LSSD type will be described in detail. In the specification, the term “normal operation” means a user circuit operation, the term “capture operation” means an operation to capture test data in a scan flip using a user circuit, and the term “scan shift operation” means the shift operation of test data from one flip-flop to the next flip-flop in the scan test operation. 
       FIG. 19  is a circuit view showing a Pos-type scan flip-flop circuit (hereinafter referred to as a Pos-type F/F), which is a related LSSD type. The Pos-type F/F here means an F/F that operates in synchronization with the rising edge of the clock signal. 
     As shown in  FIG. 19 , a Pos-type F/F  300  includes a latch circuit  301  which is a Low level latch, a latch circuit  302  which is a High level latch, and a latch circuit  303  which is a High level latch. 
     The latch circuit  301  receives a data input signal D by a data input and a normal clock signal C by a latch signal, and outputs a data signal Y. The latch circuit  302  receives a scan shift dedicated clock signal (scan shift clock signal) SA and the normal clock signal C by a latch signal, and receives a scan shift data input SI and the data signal Y which is the output from the latch circuit  301  by a data input. The latch circuit  303  receives a scan shift clock signal SB by a latch signal and a data output Q which is the output from the latch circuit  302  by a data input, and outputs a scan out signal SO. 
       FIG. 20  is a circuit view showing an example of detail of the latch circuit  302  of the Pos-type F/F  300  shown in  FIG. 19 . As shown in  FIG. 20 , the latch circuit  302  includes a latch circuit  310  which is a High level latch, a selector circuit  311 , and an OR circuit  312 . 
     The selector circuit  311  receives the scan shift data input SI and the data signal Y. The selector circuit  311  outputs the scan shift data input SI when the scan shift clock signal SA is in a High level, and outputs the data signal Y when the SA is in a Low level. The OR circuit  312  receives the scan shift clock signal SA and the normal clock signal C, and outputs the resulting signal to a latch signal of the latch circuit  310 . The latch circuit  310  receives the output from the selector circuit  311  by a data input and receives the output from the OR circuit  312  by the latch signal, and outputs a data output Q. 
       FIG. 21  is a timing chart showing an operation of a transition delay test using the Pos-type F/F  300  shown in  FIG. 19 .  FIG. 21  shows operations of the normal clock signal C, the scan shift clock signals SA and SB, the data input signal D, the scan shift data input SI, the output data signal Y of the latch circuit  301 , the output data Q of the latch circuit  302 , and the output scan out SO of the latch circuit  303 . Further, a scan shift cycle SS 1  indicates the last three cycle periods of the scan shift operation, a scan capture cycle SC indicates the scan capture operation period, and a scan shift cycle SS 2  indicates the first scan shift operation period after the scan capture operation. 
     First, the scan shift operation will be described using clock cycles C 1  and C 2 . In a period of the scan shift cycle SS 1 , the normal clock signal C becomes Low level. At time T 100 , since the scan shift clock signal SA is raised, the latch circuit  302  is in a through state and captures test data D 1  from the scan in terminal SI, and outputs the test data D 1  to the output data terminal Q. At time T 101 , since the scan shift clock signal SA is fallen, the latch circuit  302  holds the test data D 1  from the scan in terminal SI. 
     At time T 102 , since the scan shift clock signal SB is raised, the latch circuit  303  is in the through state and captures the test data D 1  from the output data terminal Q, and outputs the test data D 1  to the scan out terminal SO. At time T 103 , since the scan shift clock signal SB is fallen, the latch circuit  303  holds the test data D 1  from the output data terminal Q. 
     Next, the capture operation will be described using clock cycles C 4  and C 5 . At time T 110 , since the normal clock signal C is raised, the latch circuit  301  holds capture data capA (also shown as cA or A in the drawings) from the data input terminal D and outputs the capture data capA to the data signal Y. Further, at time T 110 , the latch circuit  302  captures capA which is the output signal data signal Y from the latch circuit  301 . 
     At time T 111 , since the normal clock signal C is fallen, the latch circuit  301  captures the capture data capB from the data input terminal D. Further, at time T 111 , the latch circuit  302  holds capA which is the output signal data Y from the latch circuit  301 . 
     At time T 112 , since the normal clock signal C is raised, the latch circuit  301  holds capture data capB (also shown as cB or B in the drawings) from the data input terminal D. Further, at time T 112 , the latch circuit  302  captures capB which is the output signal data signal Y from the latch circuit  301 . 
     At time T 113 , since the normal clock signal C is fallen, the latch circuit  302  holds capB which is the output signal data signal Y from the latch circuit  301 . 
       FIG. 22  is a circuit view showing a Neg-type scan flip-flop circuit (hereinafter referred to as a Neg-type F/F), which is a related LSSD type. The Neg-type F/F here means an F/F that operates in synchronization with a falling edge of the clock signal. 
     As shown in  FIG. 22 , a Neg-type F/F  400  includes a latch circuit  401  which is a High level latch, a latch circuit  402  which is a Low level latch, and a latch circuit  403  which is a Low level latch. 
     The latch circuit  401  receives a data input signal D by a data input, and receives a normal clock signal CB by a latch signal. The latch circuit  402  receives a scan shift clock signal SAB and the normal clock signal CB by a latch signal, and receives a scan shift data input SI and a data signal Y which is the output from the latch circuit  401  by a data input. The latch circuit  403  receives a scan shift clock signal SBB by a latch signal, receives a data output Q which is the output from the latch circuit  402  by a data input, and outputs the signal to a scan out signal SO. 
       FIG. 23  is a circuit view showing one example of detail of the latch circuit  402  of the Neg-type F/F  400  shown in  FIG. 22 . 
     As shown in  FIG. 23 , the latch circuit  402  includes a latch circuit  410  which is a Low level latch, a selector circuit  411 , and an AND circuit  412 . 
     The selector circuit  411  receives the scan shift data input SI and the data signal Y. The selector circuit  411  outputs the scan shift data input SI when the scan shift clock signal SAB is in the Low level, and outputs the data signal Y when the SAB is in the High level. The AND circuit  412  receives the scan shift clock signal SAB and the normal clock signal CB, and outputs the resulting signal to a gate signal of the latch circuit  410 . The latch circuit  410  receives the output from the selector circuit  411  by a data input, receives the output from the AND circuit  312  by a gate terminal, and outputs the signal to a data output Q. 
       FIG. 24  is a timing chart showing an operation of the transition delay test using the Neg-type F/F  400  shown in  FIG. 22 .  FIG. 24  shows operations of the normal clock signal CB, scan shift clock signals SAB and SBB, the data input signal D, the scan shift data input SI, the output data signal Y of the latch circuit  401 , the output data Q of the latch circuit  402 , and the output scan out SO of the latch circuit  403 . Further, a scan shift cycle SS 1  indicates the last three cycle periods of the scan shift operation, a scan capture cycle SC indicates the scan capture operation period, and a scan shift cycle SS 2  indicates the first scan shift operation period after the scan capture operation. 
     First, the scan shift operation will be described using the clock cycles C 1  and C 2 . As shown in  FIG. 24 , at time T 200 , since the scan shift clock signal SAB is fallen, the latch circuit  402  is in a through state to capture the test data D 1  from the scan in terminal SI, and outputs the test data D 1  to the output data terminal Q. At time T 201 , since the scan shift clock signal SAB is raised, the latch circuit  402  holds the test data D 1  from the scan in terminal SI. 
     At time T 202 , since the scan shift clock signal SBB is fallen, the latch circuit  403  is in the through state to capture the test data D 1  from the output data terminal Q, and outputs the test data D 1  to the scan out terminal SO. 
     At time T 203 , since the scan shift clock signal SBB is raised, the latch circuit  403  holds the test data D 1  from the output data terminal Q. 
     Next, the capture operation will be described using clock cycles C 4  and C 5 . At time T 210 , since the normal clock signal CB is fallen, the latch circuit  401  holds capture data capA from the data input terminal D, and outputs the capture data capA to the data signal Y. Further, at time T 210 , the latch circuit  402  captures capA which is the output signal data signal Y from the latch circuit  401 . 
     At time T 211 , since the normal clock signal CB is raised, the latch circuit  401  captures capture data capB from the data input terminal D. Further, at time T 211 , the latch circuit  402  holds capA which is the output signal data Y from the latch circuit  401 . 
     At time T 212 , since the normal clock signal CB is fallen, the latch circuit  401  holds the capture data capB from the data input terminal D. Further, at time T 212 , the latch circuit  402  captures capB which is the output signal data signal Y from the latch circuit  401 . 
     At time T 213 , since the normal clock signal CB is raised, the latch circuit  402  holds capB which is the output signal data signal Y from the latch circuit  401 . 
     In the meantime, some circuits including a DDR memory interface or a processor are intended to double throughput without increasing an existing clock frequency. Semiconductor integrated circuits including these circuits have a circuit configuration in which data is transmitted and received at each of the rising edge and the falling edge of clock signals. In short, such a semiconductor integrated circuit includes the Pos-type F/F and the Neg-type F/F connected each other. The problem in the scan F/F using such a typical LSSD latch is that it is impossible to perform a transition delay failure test between the Pos-type F/F and the Neg-type F/F when these F/Fs are connected each other. 
     Next, a scan test circuit configuration in a circuit in which the Neg-type F/F and the Pos-type F/F are connected will be described.  FIG. 25  is a view showing a scan test circuit configuration in a circuit in which the Neg-type F/F and the Pas-type F/F are connected. 
     As shown in  FIG. 25 , a scan test circuit  510  includes a Pos-type F/F  300 , a Neg-type F/F  400 , user combination circuits  1002  and  1003 , and a previous scan flip-flop circuit  1010 . 
     The Neg-type F/F  400  receives the output signal from the previous scan flip-flop circuit  1010  by a scan in terminal SI, a scan shift clock signal SA by a scan shift clock terminal SAB, a scan shift clock signal SB by a scan shift clock terminal SBB, the output from the user combination circuit  1003  by a data signal terminal D, and a normal clock signal C by a normal clock terminal CB. A data output terminal Q is input to the user combination circuit  1002 . 
     The Pos-type F/F  300  connects an SO which is the output signal from the Neg-type F/F  400  to a scan shift terminal SI, receives the output from the user combination circuit  1002  by a data input terminal D, the scan shift clock signals SA and SB by scan shift clock terminals SA and SB, respectively, and the normal clock signal C by a normal clock terminal C. 
       FIG. 26  is a timing chart showing a scan shift operation in a related scan test circuit shown in  FIG. 25 .  FIG. 26  shows changes of signal levels of a Neg-type F/F signal SIG 1  and a Pos-type F/F signal SIG 2  in the periods of the scan shift cycle SS and the scan capture cycle SC. 
     As shown in  FIG. 26 , the Neg-type F/F signal SIG 1  indicates signals input to the normal clock terminal CB, the scan shift clock terminals SAB and SBB, the scan in terminal SI and the data input terminal D which are input terminals of Neg-type F/F, and signals output from the data output terminal Q and the scan out terminal SO which are output terminals of Neg-type F/F. 
     The Pos-type F/F signal SIG 2  indicates signals input to the normal clock terminal C, the scan shift clock terminals SA and SB, the scan in terminal SI, and the data input terminal D which are input terminals of Pos-type F/F, and signals output from the data output terminal Q and the scan out terminal SO which are output terminals of Pos-type F/F. 
     The scan shift cycle SS includes clock cycles C 1 , C 2 , and C 3 . Further, the scan capture cycle SC includes clock cycles C 4  and C 5 . 
     At time T 300 , the scan out SO of Neg-type F/F outputs the test data D 1  from the scan in SI of Neg-type F/F. Further, at time T 300 , the Pos-type F/F captures the test data D 1  from the Neg-type F/F. 
     At time T 301 , the Pos-type F/F outputs the test data D 1  from the Neg-type F/F. 
     As described above, when the transition delay failure test between circuits in which the Neg-type F/F and the Pos-type F/F are connected is executed, the test data D 1  from the scan in SI of Neg-type F/F output at time  1300  in one clock cycle period C 1  is output from the scan out of Pos-type F/F at time T 301 , and thus the correct shift operation cannot be performed. 
     Specifically, it is originally required to supply, to the Neg-type F/F, the scan shift clock signals SAB and SBB shown in  FIG. 24  having the opposite polarity from that of the signals shown in  FIG. 26 , instead of the scan shift clock signals SAB and SBB shown in  FIG. 26 . However, as shown in  FIG. 25 , when the Neg-type F/F and the Pos-type F/F are connected, only one kind of scan dedicated clock signals SA, SB or SAB, SBB can be input, which raises the problem that the shift operation is not correctly performed. Accordingly, in the scan test circuit using the related F/F of LSSD type, an inverter for inverting scan dedicated clock signals are provided when the Neg-type F/F and the Pos-type F/F are connected. 
     The use of the F/F of related LSSD type eliminates the need to secure the hold time (hold free). However, in recent years, the wiring width and the wiring pitches of the semiconductor integrated circuit including LSI have been decreased due to the high-speed operation and miniaturization of the process. Thus, it is necessary to perform the delay failure test in the actual operating frequency in order to screen wiring which may be disconnected or failure in wiring through incomplete VIA. 
     The delay failure test in the actual operating frequency is performed by the clock signal CB (or clock signal C) shown in  FIG. 26 . However, as described above, the clock signal CB of Neg-type F/F needs to have the shape shown in  FIG. 24 . If the clock signal CB shown in  FIG. 26  is input, the circuit does not properly operate. Accordingly, also for the clock signal CB, an inverter needs to be provided to invert the clock signal CB. 
     However, the delay failure in the actual operating frequency needs to be performed under the actual operation circumstances, and it is impossible to perform the test while keeping the inverter provided since the inverter is not used for the original actual operation. Specifically, when the scan test circuit is formed using the F/F of LSSD type, it is absolutely necessary to provide an inverter to invert the scan dedicated clock signal and the clock signal when the Neg-type F/F and the Pos-type F/F are connected. However, when the inverter is provided, it is impossible to perform the delay test in the actual operating frequency. 
     In a scan test of a recent semiconductor integrated circuit, a flip-flop of MUXSCAN type has been widely used that operates by the same clock signal in both of the Neg-type F/F and the Pos-type F/F (see  FIG. 2  and the like in Japanese Unexamined Patent Application Publication No. 2007-127602). Further, while the F/F of LSSD type normally includes three latches, the F/F of MUXSCAN type can be formed of two latches. Thus, the F/F of MUXSCAN type has an advantage over the F/F of LSSD type in that the gate size can be reduced. 
     Next, the F/F of MUXSCAN type will be described.  FIG. 27  is a circuit view showing an F/F of Pos-type which is an F/F of MUXSCAN type. As shown in  FIG. 27 , a MUXSCAN  500  includes a master latch  510  which is a Low level latch, a slave latch  511  which is a High level latch, and a selector  520 . 
     The selector  520  receives a data signal and scan shift data. Then, the selector  520  selects a scan shift data input (scan in) SIN when a scan mode control signal SMC is in the High level, and selects the data input signal D in the normal operation when the SMC is in the Low level. 
     A data input terminal of the master latch  510  is connected to the output of the selector  520 , and a latch signal terminal of the master latch  510  receives a clock signal CLK. 
     A data input terminal of the slave latch  511  is connected to a data output terminal of the master latch  510 , and a latch signal terminal of the slave latch  511  receives the clock signal CLK. 
       FIG. 28  is a circuit view showing an F/F of Neg-type which is an F/F of MUXSCAN type. As shown in  FIG. 28 , a MUXSCAN  600  includes a master latch  610  which is a High level latch, a slave latch  611  which is a Low level latch, and a selector  620 . 
     The selector  620  receives a data signal and scan shift data. Then, the selector  620  selects the scan shift data input SIN when a scan mode control signal SMC is in the High level, and selects the data input signal D in the normal operation when the SMC is in the Low level. 
     A data input terminal of the master latch  610  is connected to the output of the selector  620 , and a latch signal terminal of the master latch  610  receives a clock signal CLKB. 
     A data input terminal of the slave latch  611  is connected to a data output terminal of the master latch  610 , and a latch signal terminal of the slave latch  611  receives the clock signal CLKB. 
     Next, an operation of the F/F of MUXSCAN type will be described.  FIG. 29  is a timing chart showing an operation of a scan F/F of MUXSCAN type (Pos type).  FIG. 29  shows changes in the levels of the clock signal CLK, the scan mode control signal SMC, the data input signal D, the scan shift data input SIN, the input M of the master latch, the input S of the slave latch, and the data output Q in the scan F/F of MUXSCAN type (Pos type) in the periods of a scan shift cycle SS (clock signals C 1 , C 2 , and C 3 ) and a scan capture cycle SC (clock signals C 4  and C 5 ). 
     In the scan shift cycle SS, the master latch  510  captures data from the scan shift data input SIN according to a falling of the clock signal CLK, the slave latch  511  captures a data signal according to a falling of the clock signal CLK, and outputs the data signal to the data output Q at the next rising. In the scan capture cycle SC, the data from the data input is captured at the rising timing of the clock signal CLK, and the data is output to the data output Q. 
       FIG. 30  is a timing chart showing an operation of a scan F/F of MUXSCAN type (Neg type).  FIG. 30  shows changes in the levels of the clock signal CLK, the scan mode control signal SMC, the data input signal D, the scan shift data input SIN, the input M of the master latch, the input S of the slave latch, and the data output Q in the scan F/F of MUXSCAN type (Neg type) in the periods of a scan shift cycle SS (clock signals C 1 , C 2 , and C 3 ) and a scan capture cycle SC (clock signals C 4  and C 5 ). 
     In the scan shift cycle SS, the master latch  610  captures data from the scan shift data input SIN according to a falling of the clock signal CLK, the slave latch  611  captures a data signal according to a rising of the clock signal CLK, and outputs the data signal to the data output Q at the next rising. In the scan capture cycle SC, the data from the data input is captured according to a falling of the clock signal CLK, and the data is output to the data output Q. 
       FIG. 31  is a view showing a circuit configuration in which F/Fs of MUXSCAN type (Pos type) are connected. The data from the previous scan flip-flop circuit  1010  is connected to a scan shift data input terminal SIN of a MUXSCAN  500   a  in the left side of  FIG. 31 , and data output Q is connected to a scan shift data input terminal SIN of a subsequent MUXSCAN  500   b . The output from the combination circuit  1000  is connected to the data input terminal. 
       FIG. 32  is a timing chart showing an operation of the F/Fs of MUXSCAN type (Pos type) shown in  FIG. 31 . The MUXSCAN  500   a  and the MUXSCAN  500   b  receive the same clock signal. Since the data output terminal Q of the MUXSCAN  500   a  is directly connected to the scan shift data input terminal SIN of the MUXSCAN  500   b , the delay is substantially 0. Since the timing for capturing data by the slave latch  511  in the MUXSCAN  500   a  and the timing for holding data by the master latch  510  in the MUXSCAN  500   b  are in the same edge, this causes occurrence of hold violations and leaking of data. Accordingly, it is required to provide a delay element for securing the hold time in a scan shift data line between the F/F of MUXSCAN type (Pos type) and the F/F of MUXSCAN type (Pos type). 
       FIG. 33  is a view showing one example of forming a scan chain by F/Fs of MUXSCAN type. When the Pos-type F/F and the Neg-type F/F are mixed, the Pos-type F/Fs  500  and the Neg-type F/Fs  600  are connected. Accordingly, as shown in  FIG. 33 , a delay element  1  is required between each of the Pos-type F/Fs  500  and the Neg-type F/Fs  600 . 
     As described above, in recent years, flip-flops of MUXSCAN type have been widely employed as a technique for performing a scan test in the semiconductor integrated circuit in order to perform the delay failure test in the actual operating frequency. However, the shift line of the scan is connected in the shift register structure, and the input clock signal of the previous flip-flop and the input clock signal of the subsequent flip-flop are the same in the shift register structure. Thus, hold time is not sufficiently secured. This causes insufficiency of hold time due to manufacturing variations or clock skew at the time of design. Accordingly, as shown in  FIG. 33 , it is required to provide the delay elements to secure the hold time. 
     In summary, since the delay element (buffer) to secure the hold time is required to be provided in all areas between flip-flops operating in the same edge in MUXSCAN, the process to arrange buffers for performing hold compensation is required in a later process. Further, since the buffers are arranged in the later process, it is impossible to provide buffers in target positions when there is no vacant space, which causes deterioration of wiring. Accordingly, the use of the F/F of MUXSCAN type may cause an increase in TAT (Turn Around Time) in the layout process, deterioration of wiring, and an increase in the gate size. In particular, the LSI has recently been increasing in size, which causes a great increase in the gate size and deterioration of wiring. 
     SUMMARY 
     As described above, desired in the technique of the scan test in the semiconductor integrated circuit is a scan test technique which is a small-sized test circuit with hold free and can perform test in the actual operating frequency. 
     A first exemplary aspect of the present invention is a flip-flop circuit including: a master latch that selectively receives a data signal or a scan data signal; and a slave latch that receives an output signal of the master latch, in which one of the master latch and the slave latch is a high-level latch, and another one of the master latch and the slave latch is a low-level latch, the high-level latch being a circuit that captures a signal in a high level of a clock signal and holds data of the signal in a low level, the low-level latch being a circuit that captures a signal in a low level of a clock signal and holds data of the signal in a high level, in a scan shift operation, the master latch captures the scan data signal in response to a first clock signal and the slave latch captures the output data signal in response to a second clock signal different from the first clock signal, and in another operation, the master latch captures the data signal in response to a normal clock signal and the slave latch captures the output data signal in response to the normal clock signal. 
     A second exemplary aspect of the present invention is a scan test circuit including: a Positive-type scan flip-flop (Pos-type F/F) that operates in synchronization with a rising edge of a clock signal and a Negative-type scan flip-flop (Neg-type F/F) that operates in synchronization with a falling edge of the clock signal, in which each of the Pos-type F/F and the Neg-type F/F comprises a master latch and a slave latch, one of the master latch and the slave latch being a high-level latch, and the other one of the master latch and the slave latch being a low-level latch, the high-level latch being a circuit that captures a signal in a high level of a clock signal and holds data of the signal in a low level, the low-level latch being a circuit that captures a signal in a low level of a clock signal and holds data of the signal in a high level, the Pas-type F/F and the Neg-type F/F form a scan chain in a mixed manner, the Pos-type F/F captures scan data signal in response to a first clock signal to output the scan data signal in response to a second clock signal different from the first clock signal in a scan shift operation, and the Neg-type F/F captures the scan data signal in response to the second clock signal to output the scan data signal in response to the first clock signal in the scan shift operation. 
     A third exemplary aspect of the present invention is a method of controlling a scan test circuit that includes a Positive-type scan flip-flop (Pos-type F/F) and a Negative-type scan flip-flop (Neg-type F/F), each of the Pos-type F/F and the Neg-type F/F including a master latch and a slave latch, one of the master latch and the slave latch being a high-level latch and the other one of the master latch and the slave latch being a low-level latch, the Pos-type F/F operating in synchronization with a rising edge of a clock signal and the Neg-type F/F operating in synchronization with a falling edge of the clock signal forming a scan chain in a mixed manner, the high-level latch being a circuit that captures a signal in a high level of a clock signal and holds data of the signal in a low level, the low-level latch being a circuit that captures a signal in a low level of a clock signal and holds data of the signal in a high level, the method of controlling the scan test circuit including: inputting a first clock signal and a second clock signal different from the first clock signal to the master latch and the salve latch of Pos-type F/F, respectively, in a scan shift operation, and inputting the second clock signal and the first clock signal to the master latch and the slave latch of Neg-type F/F, respectively, in the scan shift operation. 
     In the present invention, two kinds of scan shift clock signals that control the operation of the latches in the shift operation are input to the scan flip-flop including two latches. Accordingly, even when the scan flip-flop that operates in synchronization with the rising edge of lock and the scan flip-flop that operates in synchronization with the falling edge are connected, the shift operation can be accurately performed. Further, since both of the scan flip-flops can be controlled with the same scan shift clock signal, the delay test in the actual operating frequency can be performed. 
     According to the present invention, it is possible to provide a scan-flip-flop which is a small-sized test circuit with hold free and can perform test in an actual operating frequency, a scan test circuit using the same, and a method of controlling the scan test circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, advantages and features will be more apparent from the following description of certain embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit view showing a Pos-type F/F  100  according to a first embodiment of the present invention; 
         FIG. 2  is a circuit view showing a Neg-type F/F  200  according to the first embodiment of the present invention; 
         FIG. 3  is a flowchart showing an operation of the Pos-type F/F  100  according to the first embodiment of the present invention in a waveform type  1 ; 
         FIG. 4  is a flowchart showing an operation of the Pos-type F/F  100  according to the first embodiment of the present invention in a waveform type  2 ; 
         FIG. 5  is a flowchart showing an operation of the Neg-type F/F  200  according to the first embodiment of the present invention in the waveform type  2 ; 
         FIG. 6  is a flowchart showing an operation of the Neg-type F/F  200  according to the first embodiment of the present invention in the waveform type  1 ; 
         FIG. 7  is a view showing a scan chain using the Pos-type F/Fs  100  and the Neg-type F/Fs  200  according to the first embodiment of the present invention; 
         FIG. 8  is a view showing a scan test circuit when a data output Q of the Neg-type F/F  200  and a scan shift data input SIN of the Pos-type F/F  100  are connected according to the first embodiment of the present invention; 
         FIG. 9  is a view showing a scan test circuit when a data output Q of the Pos-type F/F  100  and a scan shift data input SIN of the Neg-type F/F  200  are connected according to the first embodiment of the present invention; 
         FIG. 10  is a timing chart showing an operation of the scan test circuit after hold compensation shown in  FIG. 8  in the waveform type  1 ; 
         FIG. 11  is a timing chart showing an operation of the scan test circuit after hold compensation shown in  FIG. 8  in the waveform type  2 ; 
         FIG. 12  is a timing chart showing an operation of the scan test circuit after hold compensation shown in  FIG. 8  in the waveform type  1 ; 
         FIG. 13  is a timing chart showing an operation of the scan test circuit after hold compensation shown in  FIG. 9  in the waveform type  2 ; 
         FIG. 14  is a circuit view showing a Pos-type F/F  100  according to a second embodiment of the present invention; 
         FIG. 15  is a circuit view showing a Neg-type F/F  200  according to the second embodiment of the present invention; 
         FIG. 16  is a view showing a scan test circuit when the Pos-type F/F and the Neg-type F/F shown in  FIGS. 14 and 15  are connected by a scan chain; 
         FIG. 17  is a timing chart showing an operation of the scan test circuit according to the second embodiment of the present invention; 
         FIG. 18  is a view showing a representative LSSD latch circuit disclosed in a cited literature; 
         FIG. 19  is a circuit view showing a related Pos-type F/F circuit; 
         FIG. 20  is a circuit view showing an example of detail of a latch circuit  302  of the Pos-type F/F  300  shown in  FIG. 19 ; 
         FIG. 21  is a timing chart showing an operation of a transition delay test using the Pos-type F/F  300  shown in  FIG. 19 ; 
         FIG. 22  is a circuit view showing a related Neg-type F/F circuit; 
         FIG. 23  is a circuit view showing an example of detail of a latch circuit  402  of the Neg-type F/F  400  shown in  FIG. 22 ; 
         FIG. 24  is a timing chart showing an operation of the transition delay test using the Neg-type F/F  400  shown in  FIG. 22 ; 
         FIG. 25  is a view showing a configuration of a scan test circuit in a circuit where a Neg-type F/F and a Pos-type F/F are connected; 
         FIG. 26  is a timing chart showing a scan shift operation in the related scan test circuit shown in  FIG. 25 ; 
         FIG. 27  is a circuit view showing an F/F of Pos-type which is an F/F of MUXSCAN type; 
         FIG. 28  is a circuit view showing an F/F of Neg-type which is an F/F of MUXSCAN type; 
         FIG. 29  is a timing chart showing an operation of the F/F of MUXSCAN type (Pos type); 
         FIG. 30  is a timing chart showing an operation of the F/F of MUXSCAN type (Neg type); 
         FIG. 31  is a view showing a circuit configuration in which F/Fs of MUXSCAN type (Pos type) are connected; 
         FIG. 32  is a timing chart showing an operation of the F/Fs of MUXSCAN type (Pos type) shown in  FIG. 31 ; and 
         FIG. 33  is a view showing an example in which a scan chain is formed of FFs of MUSCAN type. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings. In these embodiments, the present invention is applied to a scan F/F including two latches of a master latch and a slave latch, the scan F/F being capable of securing hold time, and a scan test circuit using the scan F/F. 
     In these embodiments, two kinds of scan shift clock signals are input to the scan F/F, thereby making operation clock edges of the slave latch and the master latch different between F/Fs operating in the same edge. In this way, a scan F/F which does not require hold compensation (hold free) can be provided. 
     First Embodiment of the Present Invention 
       FIG. 1  is a circuit view showing a Pos-type F/F  100  according to a first embodiment of the present invention. As shown in  FIG. 1 , the Pos-type F/F  100  includes a master latch  110  which is a Low level latch, a slave latch  111  which is a High level latch, and selectors  120 ,  130 , and  140 . 
     The selector  120  receives a data signal and scan shift data (a scan shift data signal). The selector  120  selects a scan shift data input SIN when a scan mode control signal SMC is in a High level, and selects a data input D in a normal operation when the SMC is in a Low level. 
     The selector  140  receives a scan shift clock signal SCLK 1  and a normal clock signal CLK. The selector  140  selects the scan shift clock signal SCLK 1  when the scan mode control signal SMC is in the High level, and selects the normal clock signal CLK when the SMC is in the Low level. The selector  140  then outputs the selected signal as a clock signal CLK_M. 
     The selector  130  receives a scan shift clock signal SCLK 2  and the normal clock signal CLK. The selector  130  selects the scan shift clock signal SCLK 2  when the scan mode control signal SMC is in the High level, and selects the normal clock signal CLK when the SMC is in the Low level. The selector  130  then outputs the selected signal as a clock signal CLK_S. 
     A data input terminal of the master latch  110  receives the output from the selector  120 , and a latch signal terminal of the master latch  110  receives the output from the selector  140  (clock signal CLK_M). 
     A data input terminal of the slave latch  111  receives the data output of the master latch  110 , and a latch signal terminal of the slave latch  111  receives the output from the selector  130  (clock signal CLK_S). 
       FIG. 2  is a circuit view showing a Neg-type F/F  200  according to the first embodiment of the present invention. As shown in  FIG. 2 , the Neg-type F/F  200  includes a master latch  210  which is a High level latch, a slave latch  211  which is a Low level latch, and selectors  220 ,  230 , and  240 . 
     The selector  220  receives a data signal and scan shift data (scan shift data signal). The selector  220  selects a scan shift data input SIN when a scan mode control signal SMC is in the High level, and selects a data input D in a normal operation when the SMC is in the Low level. 
     The selector  240  receives a scan shift clock signal SCLK 2 B and a normal clock signal CLK. The selector  240  selects the scan shift clock signal SCLK 2 B when the scan mode control signal SMC is in the High level, and selects the normal clock signal CLK when the SMC is in the Low level. The selector  240  then outputs the selected signal as a clock signal CLK_M. 
     The selector  230  receives a scan shift clock signal SCLK 1 B and the normal clock signal CLK. The selector  230  selects the scan shift clock signal SCLK 1 B when the scan mode control signal SMC is in the High level, and selects the normal clock signal CLK when the SMC is in the Low level. The selector  230  then outputs the selected signal as a clock signal CLK_S. 
     A data input terminal of the master latch  210  receives the output from the selector  220 , and a latch signal terminal of the master latch  210  receives the output from the selector  240  (clock signal CLK_M). 
     A data input terminal of the slave latch  211  receives data output from the master latch  210 , and a latch signal terminal of the slave latch  211  receives the output from the selector  230  (clock signal CLK_S). 
       FIGS. 3 and 4  are flowcharts each showing an operation of the Pos-type F/F  100 . In  FIGS. 3 and 4 , waveform patterns of the scan clock signals SCLK 1  and SCLK 2  are different. In this specification, for the sake of convenience, the waveform illustrated in  FIG. 3  is denoted by a waveform type  1 , and the waveform illustrated in  FIG. 4  is denoted by a waveform type  2 . Further,  FIGS. 5 and 6  are flowcharts each showing an operation of the Neg-type F/F  200 .  FIGS. 5 and 6  show operations when the waveform type  2  and the waveform type  1  are input, respectively. 
     As shown in  FIGS. 3 to 6 , the waveform types  1  and  2  have such waveform patterns that, the scan shift clock signal SCLK 2  (SCLK 2 B) as a second clock signal is in the low level when the scan shift clock signal SCLK 1  (SCLK 1 B) as a first clock signal is in the low level; and the scan shift clock signal SCLK 1  (SCLK 1 B) is in the high level when the scan shift clock signal SCLK 2  (SCLK 2 B) is in the high level. 
     In the waveform type  1 , as shown in  FIGS. 3 and 6 , the rising edges of the scan shift clock signal SCLK 1  (SCLK 1 B) are located before the rising edges of the scan shift clock signal SCLK 2  (SCLK 2 B), and the falling edges of the scan shift clock signal SCLK 1  (SCLK 1 B) are located after the falling edges of the scan shift clock signal SCLK 2  (SCLK 2 B). 
     Further, in the waveform type  2 , as shown in  FIGS. 4 and 5 , the falling edges of the scan shift clock signal SCLK 2  (SCLK 2 B) are located before the falling edges of the scan shift clock signal SCLK 1  (SCLK 1 B), and the rising edges of the scan shift clock signal SCLK 2  (SCLK 2 B) are located after the rising edges of the scan shift clock signal SCLK 1  (SCLK 1 B). 
       FIGS. 3 and 4  show the normal clock signal CLK, the scan clock signals SCLK 1  and SCLK 2 , the scan mode control signal SMC, an output signal D from a user circuit, the scan shift data input SIN from the previous scan flip-flop, an input M of the master latch, an input S of the slave latch, and a data output Q in the Pos-type F/F  100 . 
     Further,  FIGS. 5 and 6  show the normal clock signal CLKB, the scan clock signals SCLK 1 B and SCLK 2 B, the scan mode control signal SMC, an output signal (data input) D from a user circuit, the scan shift data input SIN from the previous scan flip-flop, an input M of the master latch, an input S of the slave latch, and a data output Q in the Neg-type F/F  200 . 
     Furthermore,  FIGS. 3 to 6  show the changes in the levels of these signals in the periods of a scan shift cycle SS and a scan capture cycle SC. 
     It is assumed that the scan shift cycle SS includes clock cycles C 1 , C 2 , and C 3 , and the scan capture cycle SC includes C 4  and C 5 . In this case, the scan shift cycle SS indicates the last three cycle periods of the scan shift operation, and the scan capture cycle SC indicates the scan capture operation period. 
     First, the scan shift operation will be described using the clock cycles C 1  and C 2 . The scan mode control signal SMC becomes High level during the period of the scan shift cycle SS, becomes Low level at time T 30 , and keeps Low level during the period of the scan capture cycle SC. 
     In  FIGS. 3 and 4 , at time T 12  (T 42 ), since the scan clock signal SCLK 1  is fallen, the master latch  110  of Pos-type F/F captures test data (data input) D 2  from the scan data input terminal SIN from the previous F/F. 
     At time T 14  (T 44 ), since the shift clock signal SCLK 2  is raised, the slave latch  111  of Pos-type F/F captures test data D 2  from the master latch  110  of Pos-type F/F, and outputs the test data D 2  to the data output terminal Q. 
     At time T 15  (T 45 ), since the scan clock signal SCLK 2  is fallen, the slave latch  111  of Pos-type F/F holds the test data D 2  from the master latch  110  of Pos-type F/F. 
     In  FIG. 5 , at time T 40 , since the scan clock signal SCLK 2 B is raised, the master latch  210  of Neg-type F/F captures the test data D 2  from the scan shift data input terminal SIN. 
     In  FIG. 6 , at time T 10 , since the scan clock signal SCLK 2 B is raised, the master latch  210  of Neg-type F/F captures the test data D 1  from the scan shift data input terminal SIN. 
     At time T 41  (T 11 ), since the scan clock signal SCLK 2 B is fallen, the master latch  210  of Neg-type F/F holds the test data D 2  (D 1 ) from the scan shift data input terminal SIN. 
     At time T 42  (T 12 ), since the scan clock signal SCLK 1 B is fallen, the slave latch  211  of Neg-type F/F captures the test data D 1  from the master latch  210  of Neg-type F/F and outputs the test data D 2  (D 1 ) to the data output terminal Q. 
     At time T 43  (T 13 ), since the scan clock signal SCLK 1 B is raised, the slave latch  211  of Neg-type F/F holds the test data D 2  (D 1 ) from the master latch  210  of Neg-type F/F. 
     Next, the capture operation will be described using clock cycles C 4  and C 5 . After time T 30 , since the scan mode control signal SMC is in the Low level, the normal clock signal is supplied to the master latch and the slave latch in any scan F/F. 
     In  FIG. 6 , at time T 20 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures capture data cap 1  from the user circuit. Further, in  FIG. 3 , at time T 20 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/F holds capture data cap 1  from the user circuit. 
     In  FIG. 6 , at time T 21 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures output data cap 1  from the master latch  210  of Neg-type F/F. Further, at time T 21 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds capture data cap 1  from the user circuit. 
     Further, in  FIG. 3 , at time T 21 , since the normal clock signal CLK is fallen, the master latch  110  of Pos-type F/F captures the capture data cap 1  from the user circuit. Further, at time T 21 , since the normal clock signal CLK is fallen, the slave latch  111  of Pos-type F/F holds the capture data cap 1  from the master latch of Pos-type F/F. 
     Next, in  FIG. 6 , at time T 22 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures capture data cap 2  from the user circuit. 
     Further, in  FIG. 3 , at time T 22 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/F holds the capture data cap 2  from the user circuit. Further, at time T 22 , since the normal clock signal CLK is raised, the slave latch  111  of Pos-type F/F captures the capture data cap 2  from the master latch  110  of Pos-type F/F. 
     In  FIG. 6 , at time T 23 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures the output data cap 2  from the master latch  210  of Neg-type F/F. Further, at time T 23 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds the capture data cap 2  from the user circuit. 
     Further, in  FIG. 3 , at time T 23 , since the normal clock signal CLK is fallen, the slave latch  111  of Pos-type F/F holds the capture data cap 2  from the master latch of Pos-type F/F. 
     &lt;*1 START&gt; 
     In  FIG. 5 , at time T 20 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds capture data cap 1  from the user circuit. Further, at time T 20 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures output data cap 1  from the master latch  210  of Neg-type F/F. 
     Further, in  FIG. 4 , at time T 20 , since the normal clock signal CLK is fallen, the master latch  110  of Pos-type F/F captures the capture data cap 1  from the user circuit. Further, at time T 20 , since the normal clock signal CLK is fallen, the slave latch  111  of Pos-type F/F holds the capture data cap 1  from the master latch of Pos-type F/F. 
     In  FIG. 5 , at time T 21 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures capture data cap 2  from the user circuit. Further, in  FIG. 4 , at time T 21 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/F holds capture data cap 1  from the user circuit. 
     In  FIG. 5 , at time T 22 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds the capture data cap 2  from the user circuit. Further, at time T 22 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures the output data cap 2  from the master latch  210  of Neg-type F/F. 
     Further, in  FIG. 4 , at time T 22 , since the normal clock signal CLK is fallen, the slave latch  111  of Pos-type F/F holds the capture data cap 1  from the master latch of Pos-type F/F. 
     Next, in  FIG. 5 , at time T 23 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures capture data cap 2  from the user circuit. 
     Further, in  FIG. 4 , at time T 23 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/F holds the capture data cap 2  from the user circuit. Further, at time T 23 , since the normal clock signal CLK is raised, the slave latch  111  of Pos-type F/F captures the capture data cap 2  from the master latch  110  of Pos-type F/F. 
     &lt;*1 END&gt; 
     Next, the scan test circuit using the Pos-type F/F  100  and the Neg-type F/F  200  according to the first embodiment will be described.  FIG. 7  is a view showing a scan chain using the Pos-type F/F  100  and the Neg-type F/F  200 . In the LSI, the Pos-type F/F  100  and the Neg-type F/F  200  are used in a mixed manner in order to achieve the function and the operation. Accordingly, the Pos-type F/F  100  and the Neg-type F/F  200  are mixed in the scan chain using the Pos-type F/Fs  100  and the Neg-type F/Fs  200 . In this case, normally, the Pos-type F/Fs  100  are connected each other, and the Neg-type F/Fs  200  are connected each other to form the scan chain. This is because the connection of the Pos-type F/F  100  and the Neg-type F/F  200  causes operation at double speed of the normal clock signal. However, even when the Pos-type F/Fs  100  are connected each other or the Neg-type F/Fs  200  are connected each other, a pass  10  that connects the Pos-type F/F  100  and the Neg-type F/F  200  is generated at least in one position. 
     In the scan test circuit according to the first embodiment, a buffer (delay element) for performing hold compensation is required to be arranged in the pass  10  that connects the Pos-type F/F  100  and the Neg-type F/F  200 . However, unlike  FIG. 33 , there is no need to provide a large number of hold compensation buffers as in the related scan test circuit. 
       FIG. 8  is a scan test circuit when the data output Q of the Neg-type F/F  200  and the scan shift data input SIN of the Pos-type F/F  100  are connected, and  FIG. 9  shows a scan test circuit when the data output Q of the Pos-type F/F  100  and the scan shift data input SIN of the Neg-type F/F  200  are connected. As shown in  FIGS. 8 and 9 , a delay element for hold compensation is arranged in the pass  10  between the both F/Fs. When the hold compensation is not performed, as shown in  FIG. 32 , the subsequent F/F also outputs data at the timing at which the previous F/F outputs data. To address with this problem, a buffer that delays the scan shift data input SIN by a predetermined period is arranged for hold compensation. 
     &lt;*2 START&gt; 
     As shown in  FIG. 8 , the data input terminal D of the Neg-type F/F  200  receives input data from a user circuit  1000 , the scan shift data input terminal SIN of the Neg-type F/F  200  receives data output from the data output terminal Q of a scan F/F  1010  which is previously provided, the scan mode control input terminal SMC of the Neg-type F/F  200  receives the scan mode control signal SMC, the clock terminal CLKB of the Neg-type F/F  200  receives the clock signal CLK in the normal operation, the scan clock terminal SCLK 1 B of the Neg-type F/F  200  receives the scan clock signal SCLK 1 , the scan clock terminal SCLK 2 B of the Neg-type F/F  200  receives the scan clock signal SCLK 2 , and the output Q of the Neg-type F/F  200  is supplied to the user circuit  1001  and to the scan shift data input SIN of the Pos-type F/F  100 . 
     The data input terminal D of the Pos-type F/F  100  receives input data from the user circuit  1001 , the scan shift data input terminal SIN of the Pos-type F/F  100  receives data output from the data output terminal Q of the Neg-type F/F  200 , the scan mode control input terminal SMC of the Pos-type F/F  100  receives the scan mode control signal SMC as is similar to the scan mode control input terminal SMC of the Neg-type F/F  200 , the clock terminal CLK of the Pos-type F/F  100  receives the clock signal CLK in the normal operation as is similar to the clock terminal CLKB of the Neg-type F/F  200 , the scan clock terminal SCLK 1  of the Pos-type F/F  100  receives the scan clock signal SCLK 1  as is similar to the scan clock terminal SCLK 1 B of the Neg-type F/F  200 , the scan clock terminal SCLK 2  of the Pos-type F/F  100  receives the scan clock signal SCLK 2  as is similar to the scan clock terminal SCLK 2 B of the Neg-type F/F  200 , and the output Q of the Pos-type F/F  100  is supplied to a user circuit and to a scan shift data input terminal SIN of a scan F/F provided in the subsequent stage. 
     The scan test circuit shown in  FIG. 9  is similar to that shown in  FIG. 8  described above. Specifically, as shown in  FIG. 9 , the data input terminal D of the Pos-type F/F  100  receives input data from a user circuit  1001 , the scan shift data input terminal SIN of the Pos-type F/F  100  receives data output from the data output terminal Q of the scan F/F  1010 , the scan mode control input terminal SMC of the Pos-type F/F 100  receives the scan mode control signal SMC, the clock terminal CLK of the Pos-type F/F  100  receives the clock signal CLK in the normal operation, the scan clock terminal SCLK 1  of the Pos-type F/F  100  receives the scan clock signal SCLK 1 , the scan clock terminal SCLK 2  of the Pos-type F/F  100  receives the scan clock signal SCLK 2 , and the output Q of the Pos-type F/F  100  is supplied to the user circuit  1000  and to the scan shift data input SIN of the Neg-type F/F  200 . 
     The data input terminal D of the Neg-type F/F  200  receives input data from the user circuit  1000 , the scan shift data input terminal SIN of the Neg-type F/F  200  receives data output from the data output terminal Q of the Pos-type F/F  100 , the scan mode control input terminal SMC of the Neg-type F/F  200  receives the scan mode control signal SMC, the clock terminal CLKB of the Neg-type F/F  200  receives the clock signal CLK in the normal operation, the scan clock terminal SCLK 1 B of the Neg-type F/F  200  receives the scan clock signal SCLK 1 , the scan clock terminal SCLK 2 B of the Neg-type F/F  200  receives the scan clock signal SCLK 2 , and the output Q of the Neg-type F/F  200  is supplied to a user circuit and to a scan shift data input terminal SIN of a scan F/F provided in the subsequent stage. 
     &lt;*2 END&gt; 
       FIGS. 10 and 11  are timing charts showing the operation of the scan test circuit after hold compensation shown in  FIG. 8  in the waveform types  1  and  2 , respectively.  FIGS. 12 and 13  are timing charts showing the operation of the scan test circuit after hold compensation shown in  FIG. 9  in the waveform types  1  and  2 , respectively. As shown in  FIGS. 10 to 13 , the scan shift data input SIN in the lower view is delayed compared to the scan shift data input SIN in the upper view by a predetermined period. For example, in  FIG. 10 , data D 1  is output at time T 50  in the Neg-type F/F  200  in the upper view, whereas the data output D 1  is output at time T 52  in the Pos-type F/F  100  in the lower view. 
     The operations are similar to those shown in  FIGS. 3 to 6  described above. The master latch  210  of the Neg-type F/F  200  latches data at the rising timing of the scan shift clock signal SCLK 2 B (time T 52 ), and the slave latch  211  latches data at the falling timing of the scan shift clock signal SCLK 1 B (time T 54 ). The master latch  110  of the Pos-type F/F  100  latches data at the falling timing of the scan shift clock signal SCLK 1  (time T 54 ), and the slave latch  111  latches data at the rising timing of the scan shift clock signal SCLK 2  (time T 52 ). 
     &lt;*3 START&gt; 
     Next, the capture operation will be described with reference to clock cycles C 4  and C 5  with reference to  FIG. 10 . After time T 30 , since the scan mode control signal SMC is in the Low level, the normal clock signal is supplied to the master latch and the slave latch in any scan F/F. 
     At time T 20 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures the capture data cap 1  from the user circuit  1000 . Further, at time T 20 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/holds capture data capA from the user circuit  1001 . 
     At time T 21 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures output data cap 1  from the master latch  210  of Neg-type F/F. Further, at time T 21 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds capture data cap 1  from the user circuit  1000 . 
     Further, at time T 21 , the master latch  110  of Pos-type F/F captures capture data capB from the user circuit  1001 . Further, at time T 21 , the slave latch  111  of Pos-type F/F holds capture data capA from the master latch  110  of Pos-type F/F. 
     Next, at time T 22 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures capture data cap 2  from the user circuit  1000 . 
     Further, at time T 22 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/F holds capture data capB from the user circuit  1001 . Further, at time T 22 , since the normal clock signal CLK is raised, the slave latch  111  of Pos-type F/F captures capture data capB from the master latch  110  of Pos-type F/F. 
     At time T 23 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures output data cap 2  from the master latch  210  of Neg-type F/F. Further, at time T 23 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds capture data cap 2  from the user circuit  1000 . 
     &lt;*3 END&gt; 
     The waveform type  2  shown in  FIG. 11  is similar to that shown in  FIG. 10 . While the data output Q (D 1 ) of the Neg-type F/F  200  is time T 60 , the data output Q of the Pos-type F/F  100  is time T 63 . The operation is similar to that described above. The master latch  210  of the Neg-type F/F  200  latches data at the rising timing of the scan shift clock signal SCLK 2 B (time T 60 ), and the slave latch  211  latches data at the falling timing of the scan shift clock signal SCLK 1 B (time T 62 ). The master latch  110  of the Pos-type F/F  100  latches data at the falling timing of the scan shift clock signal SCLK 1  (time T 62  and T 65 ), and the slave latch  111  latches data at the rising timing of the scan shift clock signal SCLK 2  (time T 60  and T 63 ). 
       FIGS. 12 and 13  show a case in which the connection is made from the Pos-type F/F  100  to the Neg-type F/F  200 , and the basic operations are similar to those shown in  FIGS. 10 and 11 . 
     In the first embodiment, two kinds of scan shift clock signals are input. Accordingly, in the scan operation (the scan mode control signal SMC is in the High level), the Pos-type F/F  100  supplies the scan shift clock signal SCLK 1  to the master latch  110  and supplies the scan clock signal SCLK 2  to the slave latch  111 , and the Neg-type F/F  200  supplies the scan clock signal SCLK 2 B to the master latch  210  and supplies the scan shift clock signal SCLK 1 B to the slave latch  211 . In the normal operation (the scan mode control signal SMC is in the Low level), the clock normal signal CLK (CLKB) is supplied to both of the master latch  110  and the slave latch  111  of the Pos-type F/F  100  and the master latch  210  and the slave latch  211  of the Neg-type F/F  200 . 
     As to the clock signal input in the scan test operation, the rising edges of the scan shift clock signal SCLK 1  (SCLK 1 B) are located before the rising edges of the scan shift clock signal SCLK 2  (SCLK 2 B) and the falling edges are located after the falling edges of the scan shift clock signal SCLK 2  (SCLK 2 B) (waveform type  1 ), or the falling edges of the scan shift clock signal SCLK 1  (SCLK 1 B) are located before the falling edges of the scan shift clock signal SCLK 2  (SCLK 2 B) and the rising edges are located after the rising edges of the scan shift clock signal SCLK 2  (SCLK 2 B) (waveform type  2 ). Specifically, when the transition delay failure test between circuits in which the Neg-type F/Fs and the Pos-type F/Fs are connected is executed by using such a scan shift clock signal that the scan shift clock signal SCLK 2  (SCLK 2 B) is in the Low level when the scan shift clock signal SCLK (SCLK 1 B) is in the Low level, and the scan shift clock signal SCLK (SCLK 1 B) is in the High level when the scan shift clock signal SCLK 2  (SCLK 2 B) is in the High level, the test data D 1  input to the scan shift data input SIN of the Neg-type F/F  200  in one clock cycle period C 1  is not output from the data output terminal Q of the Pos-type F/F  100  but is output in the next clock cycle period C 2 . In this way, it is possible to perform the correct shift operation and to perform the transition delay failure test. 
     Further, the Pos-type F/F  100  and the Neg-type F/F  200  operate by the same scan shift clock signals SCLK 1  and SCLK 2  (=SCLK 1 B, SCLK 2 B). In summary, there is no need to invert the clock signals unlike the flip-flop of LSSD type. Accordingly, it is possible to perform the delay failure test in the actual operating frequency. 
     Furthermore, while the latch circuit of the flip-flop of LSSD type requires three latch circuits of a latch circuit  301 , a latch circuit  302 , and a latch circuit  303  as shown in  FIG. 19 , the first embodiment only requires two latch circuits of the master latch circuit  110  and the slave latch circuit  111 , thereby decreasing the gate size. 
     Second Embodiment of the Present Invention 
     Next, a second embodiment of the present invention will be described. In the first embodiment described above, two kinds of scan shift clock signals are input to the scan F/F. On the other hand, in the second embodiment, only one kind of scan shift clock signal is input instead of two kinds of scan shift clock signals, and the other signal is substituted by a normal clock signal. Accordingly, only one kind of scan shift clock signal is input, which requires only one selector that selects the signal. 
       FIG. 14  is a circuit view showing a Pos-type F/F  100  according to the second embodiment of the present invention. As shown in  FIG. 14 , the Pos-type F/F  100  includes a master latch  110  which is a Low level latch, a slave latch  111  which is a High level latch, and selectors  120  and  130 , as is similar to the first embodiment. In short, there is no need to provide a selector  140  shown in  FIG. 1 . 
     The selector  120  receives a data signal and scan shift data. The selector  120  selects a scan shift data input SIN when a scan mode control signal SMC is in a High level, and selects a data input D in the normal operation when the SMC is in a Low level. 
     The selector  130  receives a scan shift clock signal SCLK and a normal clock signal CLK. The selector  130  selects the scan shift clock signal SCLK when the scan mode control signal SMC is in the High level, and selects the normal clock signal CLK when the SMC is in the Low level. The selector  130  then outputs the selected signal as a clock signal CLK_S. 
     A data input terminal of the master latch  110  receives the output of the selector  120 , and a latch signal terminal of the master latch  110  receives the normal clock signal CLK. 
     A data input terminal of the slave latch  111  receives the data output of the master latch  110 , and a latch signal terminal of the slave latch  111  receives the output of the selector  130  (clock signal CLK_S). 
       FIG. 15  is a circuit view showing a Neg-type F/F  200  according to the second embodiment. As shown in  FIG. 15 , the Neg-type F/F  200  includes a master latch  210  which is a High level latch, a slave latch  211  which is a Low level latch, and selectors  220  and  230 , as is similar to the first embodiment. In short, there is no need to provide the selector  240  shown in  FIG. 2 . 
     The selector  220  receives a data signal and scan shift data. The selector  220  selects a scan shift data input SIN when a scan mode control signal SMC is in the High level, and selects a data input D in the normal operation when the SMC is in the Low level. 
     The selector  230  receives a scan shift clock signal SCLKB and a normal clock signal CLKB. The selector  230  selects the scan shift clock signal SCLKB when the scan mode control signal SMC is in the High level, and selects the normal clock signal CLKB when the SMC is in the Low level. The selector  230  then outputs the selected signal as a clock signal CLK_M. 
     A data input terminal of the master latch  210  receives the output from the selector  220 , and a latch signal terminal of the master latch  210  receives the output (clock signal CLK_M) of the selector  230 . 
     A data input terminal of the slave latch  211  receives the data output of the master latch  210 , and a latch signal terminal of the slave latch  211  receives the normal clock signal CLKB. 
       FIG. 16  is a view showing a scan test circuit when the Pos-type F/F shown in  FIG. 14  and the Neg-type F/F shown in  FIG. 15  are connected by a scan chain. As shown in  FIG. 16 , in the scan test circuit, the Neg-type F/F  200  is provided in the previous stage of the Pos-type F/F  100 . 
     The data input terminal D of the Neg-type F/F  200  receives input data from the user circuit  1000 , the scan shift data input terminal SIN of the Neg-type F/F  200  receives the output from the data output terminal Q of the scan F/F  1010  which is previously provided, the scan mode control input terminal SMC of the Neg-type F/F  200  receives the scan mode control signal SMC, the clock terminal CLKB of the Neg-type F/F  200  receives the clock signal CLK in the normal operation, the scan clock terminal SCLKB of the Neg-type F/F  200  receives the scan clock signal SCLK, and the output Q of the Neg-type F/F  200  is supplied to the user circuit  1001  and to the scan shift data input SIN of the Pos-type F/F  100 . 
     The data input terminal D of the Pos-type F/F  100  receives input data from the user circuit  1001 , the scan shift data input terminal SIN of the Pos-type F/F  100  receives data output from the data output terminal Q of the Neg-type F/F  200 , the scan mode control input terminal SMC of the Pos-type F/F  100  receives the scan mode control signal SMC as is similar to the scan mode control input terminal SMC of the Neg-type F/F  200 , the clock terminal CLK of the Pos-type F/F  100  receives the clock signal CLK in the normal operation as is similar to the clock terminal CLKB of the Neg-type F/F  200 , the scan clock terminal SCLK of the Pos-type F/F  100  receives the scan clock signal SCLK as is similar to the scan clock terminal SCLKB of the Neg-type F/F  200 , and the output Q of the Pos-type F/F  100  is supplied to a user circuit and to a scan shift data input terminal SIN of a scan F/F provided in the subsequent stage. 
       FIG. 17  is a timing chart showing an operation of the scan test circuit shown in  FIG. 16 . An operation of the scan test circuit shown in  FIG. 17  is similar to the operation of the scan test circuit shown in  FIGS. 12 and 13  in the first embodiment except clock signals that are used. Further, in the second embodiment as well, when the Pos-type F/F  100  and the Neg-type F/F  200  are connected, a delay circuit is arranged in a data line of the scan shift data input SIN to perform hold compensation. 
       FIG. 17  shows the normal clock signal CLKB, the scan clock signal SCLKB, the scan mode control signal SMC, the output signal D from the user circuit  1000 , the scan shift data input SIN from the previous scan flip-flop  1010 , the input M of the master latch, the input S of the slave latch, and the data output Q in the Neg-type F/F  200 . Further,  FIG. 17  shows the normal clock signal CLK, the scan clock signal SCLK, the scan mode control signal SMC, the output signal D from the user circuit  1001 , the scan shift data input SIN from the previous scan flip-flop, the input M of the master latch, the input S of the slave latch, and the data output Q in the Pos-type F/F  100 .  FIG. 17  shows changes in the levels of these signals in the periods of a scan shift cycle SS and a scan capture cycle SC. 
     It is assumed here that the scan shift cycle SS includes clock cycles C 1 , C 2 , and C 3 , and the scan capture cycle SC includes C 4  and C 5 . In this case, the scan shift cycle SS indicates the last three cycle periods of the scan shift operation, and the scan capture cycle SC indicates the scan capture operation period. 
     First, the scan shift operation will be described using clock cycles C 1  and C 2 . The scan mode control signal SMC becomes the High level in the period of the scan shift cycle SS, becomes the Low level at time T 30 , and keeps the Low level in the period of the scan capture cycle SC. 
     At time T 10 , since the scan clock signal SCLKB is raised, the master latch  210  of Neg-type F/F captures test data D 1  from the scan shift data input terminal SIN. 
     At time T 11 , since the scan clock signal SCLKB is fallen, the master latch  210  of Neg-type F/F holds the test data D 1  from the scan shift data input terminal SIN. 
     At time T 12 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures the test data D 1  from the master latch  210  of Neg-type F/F and outputs the test data D 1  to the data output terminal Q. Further, at time T 12 , since the normal clock signal CLK is fallen, the master latch  110  of Pos-type F/F captures the test data D 1  from the Neg-type F/F. 
     At time T 13 , since the normal clock signal CLKB is raised, the slave latch  211  of Neg-type F/F holds the test data D 1  from the master latch  210  of Neg-type F/F. 
     At time T 14 , since the shift clock signal SCLK is raised, the slave latch  111  of Pos-type F/F captures the test data D 1  from the slave latch  110  of Pos-type F/F, and outputs the test data D 1  to the data output terminal Q. 
     At time T 15 , since the scan clock signal SCLK is fallen, the slave latch  111  of Pos-type F/F holds the test data D 1  from the slave latch  110  of Pos-type F/F. 
     Next, the capture operation will be described with reference to clock cycles C 4  and C 5 . After time T 30 , since the scan mode control signal SMC is in the Low level, the normal clock signal is supplied to the master latch and the slave latch in any scan F/F. 
     At time T 20 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures the capture data cap 1  from the user circuit  1000 . Further, at time T 20 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/holds capture data capA from the user circuit  1001 . 
     At time T 21 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures output data cap 1  from the master latch  210  of Neg-type F/F. Further, at time T 21 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds capture data cap 1  from the user circuit  1000 . 
     Further, at time T 21 , the master latch  110  of Pos-type F/F captures capture data capB from the user circuit  1001 . Further, at time T 21 , the slave latch  111  of Pos-type F/F holds capture data capA from the master latch  110  of Pos-type F/F. 
     Next, at time T 22 , since the normal clock signal CLKB is raised, the master latch  210  of Neg-type F/F captures capture data cap 2  from the user circuit  1000 . 
     Further, at time T 22 , since the normal clock signal CLK is raised, the master latch  110  of Pos-type F/F holds capture data capB from the user circuit  1001 . Further, at time T 22 , since the normal clock signal CLK is raised, the slave latch  111  of Pos-type F/F captures capture data capB from the master latch  110  of Pos-type F/F. 
     At time T 23 , since the normal clock signal CLKB is fallen, the slave latch  211  of Neg-type F/F captures output data cap 2  from the master latch  210  of Neg-type F/F. Further, at time T 23 , since the normal clock signal CLKB is fallen, the master latch  210  of Neg-type F/F holds capture data cap 2  from the user circuit  1000 . 
     Further, at time T 23 , the slave latch  111  of Pos-type F/F holds capture data capB from the master latch  210  of Pos-type F/F. 
     The second embodiment achieves the similar effect as that in the first embodiment. In summary, it is possible to perform the correct shift operation and to perform the transition delay failure test, and there is no need to invert clock signals unlike the related arts. Accordingly, the delay failure test can be performed in the actual operating frequency. Further, only two latches of the master latch circuit  110  and the slave latch circuit  111  are required, which makes it possible to reduce the gate size. Further, in the second embodiment, since the number of clock signals is reduced from three to two, the layout area can further be reduced. 
     The first and second embodiments can be combined as desirable by one of ordinary skill in the art. 
     While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention can be practiced with various modifications within the spirit and scope of the appended claims and the invention is not limited to the examples described above. 
     Further, the scope of the claims is not limited by the embodiments described above. 
     Furthermore, it is noted that, Applicant&#39;s intent is to encompass equivalents of all claim elements, even if amended later during prosecution.