Patent Publication Number: US-7584393-B2

Title: Scan test circuit and method of arranging the same

Description:
CROSS-REFERENCE OF RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-167278, filed on Jun. 7, 2005, the entire contents of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   This invention generally relates to feasible function-test design for a semiconductor integrated circuit and, in particular, to scan-test circuit applied to a scan-test method and a method of arranging the same. 
   BACKGROUND OF THE INVENTION 
   Since a system LSI device has a lot of system function devices formed on a same single chip and an SoC (system-on-chip) device has memories, logic circuits and analog circuits integrated in a chip, such a system LSI device and an SoC device have been used in mobile intelligence apparatus and personal computers as information handling equipment, etc. have been recently developed to have high-degree performances and versatile functions. For large scale and high-speed system LSI devices and SoC devices, a technology called the design for testability (DFT), such as a scan-test method and a built-in self-test (BIST) method, is used to prevent test costs, etc. from increasing. In the scan-test method, flip-flop circuits are substituted for scan flip-flop circuits. When the scan flip-flop circuits are used, their values can be set from the outside and such values can be read out from outer input-output terminals of the scan flip-flop circuits. As a result, test patterns can be made easily by means of an automatic test pattern generator (ATPG) as disclosed in Japanese Patent Publication 2002-329784, for instance (see particularly descriptions on page 5 and FIG. 2). 
   A scan-test circuit used in the scan-test method receives a scan-shift enable signal as a scan control signal. When the scan-shift enable signal is supplied through pipe-lined architecture, additional flip-flop circuits are inserted in the circuits to distribute scan enable signals to loads and to synchronize the same with each other. Such arrangements are so troublesome for timing adjustments and layout design that the arrangements take unexpected time for design and are difficult for optimization. 
   The present invention provides a scan test circuit with easy optimization for timing adjustments and layout design and a method of arranging the same. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention is directed to a scan test circuit including a clock control circuit, a clock buffer circuit section, a replaced cell and a scan circuit. The clock buffer circuit section has clock buffer circuits cascade-connected to drive a clock signal supplied from the clock control circuit and forms a clock tree circuit. The replaced cell is connected to the clock buffer circuit section and is set in place of a stage of the clock buffer circuits of the buffer circuit section. The replaced cell receives a scan shift enable signal supplied from the scan control circuit and a clock signal supplied from the clock circuits, and outputs the clock signal and the scan shift enable signal synchronized with the clock signal. The scan circuit receives the scan shift enable signal synchronized with the clock signal output from the replaced cell, the clock signal and a scan input signal, and outputs signals for scan tests. 
   Another aspect of the present invention is directed to a method of arranging a scan test circuit which connects among a clock control circuit, a scan control circuit, a clock buffer circuit section, a replaced cell and a scan circuit. The clock buffer circuit section has clock buffer circuits cascade-connected in the form of a clock tree circuit to drive a clock signal supplied from the clock control circuit. The replaced cell is connected to the clock buffer circuit section and is set in place of a stage of the clock tree circuit of the clock buffer circuits in the buffer section. The replaced cell receives a scan shift enable signal supplied from the scan control circuit, and a clock signal supplied from the clock buffer circuit section and provides the scan circuit with the scan shift enable signal synchronized with the clock signal. The scan circuit outputs signals for scan tests. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of its attendant advantages will be readily obtained as the same becomes better understood by reference to the following detailed descriptions when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of a replaced cell in accordance with a first embodiment of the present invention; 
       FIG. 2  is a flow chart of a method of designing a semiconductor integrated circuit as a system LSI in accordance with the first embodiment of the present invention; 
       FIG. 3  is a flow chart of a method of arranging the replaced cell in accordance with the first embodiment of the present invention; 
       FIG. 4  is a block diagram of a semiconductor integrated circuit provided with the replaced cell in accordance with the first embodiment of the present invention; 
       FIG. 5  is a block diagram of a scan flip-flop circuit in accordance with the first embodiment of the present invention; 
       FIG. 6  is a timing chart of a scan operation in accordance with the first embodiment of the present invention; 
       FIG. 7  is a block diagram of a replaced cell in accordance with a second embodiment of the present invention; and 
       FIG. 8  is a block diagram of a replaced cell in accordance with a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be explained below with reference to the attached drawings. It should be noted that the present invention is not limited to the embodiments but covers their equivalents. Throughout the attached drawings, similar or same reference numerals show similar, equivalent or same components. The drawings, however, are shown schematically for the purpose of explanation so that their components are not necessarily the same in shape or dimension as actual ones. In other words, concrete shapes or dimensions of the components should be considered as described in these specifications, not in view of the ones shown in the drawings. Further, some components shown in the drawings may be different in dimension or ratio from each other. 
   FIRST EMBODIMENT 
   A first embodiment of a scan test circuit and a method of arranging the same in accordance with the present invention will be described below with reference to the attached drawings.  FIG. 1  is a block diagram of a replaced cell provided to make timing adjustments and optimization of layout design feasible. In this embodiment, a multiplexed scan flip-flop circuit is used for a scan circuit that receives an output signal from the replaced cell. 
   As shown in  FIG. 1 , replaced cell CELL 1  includes clock buffer circuit CB 1  and flip-flop circuit FF 1 . Replaced cell CELL 1  is set in place of a final stage clock buffer circuit neighboring on a side of the scan circuit between the scan circuit and cascade-connected clock buffer circuits (called a clock buffer circuit section) to make a clock tree circuit. Arrangements and structures of the clock buffer circuits and the scan circuit will be explained later in detail. 
   Clock buffer circuit CB 1  has the same structure as one of the clock buffer circuits at the final stage that has not been replaced yet, receives input clock signal CLKI, is driven by clock signal CLKI, and provides clock signal CLKO to the scan circuit and the like, such as the scan flip-flop circuit. Flip-flop circuit FF 1  receives scan shift enable signal SSEI input at port D, latches scan shift enable signal SSEI at a falling-down time of clock CLKI, and supplies port Q with the latest scan shift enable signal SSEO updated and stored in flip-flop circuit FF 1 . Thus, output scan shift enable signal SSEO is synchronized with output clock signal CLKO. 
   Next, a method of designing a semiconductor integrated circuit will be described below with reference to  FIGS. 2 and 3 .  FIG. 2  is a flow chart of a method of designing a semiconductor integrated circuit as a system LSI by using an electronic design automation (EDA) tool.  FIG. 3  is a flow chart of a method of arranging a replaced cell. A semiconductor integrated circuit can be designed by using various other design methods than the one shown in  FIG. 2 . Thus, the embodiment is not limited to the same as shown in  FIG. 2 . 
   As shown in  FIG. 2 , the hardware description language (HDL) is used to synthesize register transfer level (RTL) descriptions from operating descriptions close to software programs based on system designed information (step S 1 ). The RTL descriptions are then divided in detail so that gate level logic circuits are synthesized (step S 2 ). Subsequently, a logic simulator is used to carry out equivalence verification as to whether logic functions are correct (step S 3 ). Connection related data (net list) of macro-cells set in the logic circuits are then checked (step S 4 ). 
   Estimates are then carried out for an LSI size, power consumption, a chip area, a package size (floor plan), etc. (step S 5 ). Each macro-cell and the like are disposed and connected in the LSI chip while operating timings are optimized (step  6 ). 
   Next, clock tree synthesis (CTS) is tested on a trial basis and is estimated (step S 7 ). Concretely, as shown in  FIG. 3 , a clock tree circuit is logically generated and clock buffer circuits are then disposed and connected (step S 71 ). The clock buffer circuit (final stage clock buffer circuit) neighboring on the side of the scan flip-flop circuit between the cascade connected clock buffer circuits and the scan flip-flop circuit is exchanged by replaced cell CELL 1  shown in  FIG. 1  (step S 72 ). The clock buffer circuits and the replaced cell are then disposed and connected to each other (step S 73 ). The replaced cell and the flip-flop circuits are also disposed and connected to each other (step  74 ). 
   Next, it is confirmed whether connections or wirings among each macro-cell, the clock buffer circuits to form the clock tree circuit, the scan flip-flop circuits, the replaced cell, etc. are proper (step S 8 ). Analysis (step  9 ) as to whether timings are set in a predetermined range is executed by means of a static timing analyzer or the like. If the timings are out of the range, the steps from the floor plan (step S 5 ) to timing analysis (step  9 ) are repeated all over again. 
   Subsequently, a logic simulator is used to carry out equivalence verification as to whether logic functions are correct (step S 10 ). Layout verification tools of a design rule checker (DRC), a layout-versus-schematic (LVS), Ant, and the like are used to verify whether layout data are proper (step S 11 ). Designed data are transformed into CAD layout data in the form of graphic data system (GDS) II (step S 12 ). 
   Further, with reference to  FIGS. 4 and 5 , a semiconductor integrated circuit as a designed system LSI will be described below.  FIGS. 4 and 5  are block diagrams of a semiconductor integrated circuit provided with replaced cells, and a scan flip-flop circuit, respectively. As those skilled in the semiconductor art can understand, such a semiconductor integrated circuit further includes input and output circuits and a memory not shown in  FIGS. 4 and 5  and their descriptions are omitted. 
   Semiconductor integrated circuit  1  is provided with scan control circuit  2 , clock control circuit  3 , logic circuit section  4 , buffer circuit B 1 , clock buffer circuits CB 1   a -CB 1   d , replaced cells CELL 1   a  and CELL 1   b  and scan flip-flop circuits SFF 1 -SFF 4 . 
   Scan control circuit  2  generates a scan control signal of scan shift enable signal SSE and supplies the same to buffer circuit B 1 . Clock control circuit  3  generates a control signal of clock signal CLK and supplies the same to clock buffer circuits CB 1   a  and CB 1   b.    
   Buffer circuit B 1  is provided between scan control circuit  2  and replaced cells CELL 1   a  and CELL 1   b , drives scan shift enable signal SSE, and supplies an output signal to replaced cells CELL 1   a  and CELL 1   b . A plurality of buffer circuits may be set in place of buffer circuit B 1  or buffer circuit B 1  may be deleted. 
   Clock control circuit  3  is connected to clock buffer circuits CB 1   a -CB 1   d  (i.e., a clock buffer circuit section), which forms clock tree circuit  5 . Clock buffer circuit CB 1   c  is connected to replaced cells CELL 1   a -CELL 1   b . First stages of clock buffer circuits CB 1   a -CB 1   b  receive clock signal CLK from clock control circuit  3 . Second stages of clock buffer circuits CB 1   c -CB 1   d  receive an output clock signal from clock buffer circuit CB 1   b . Clock buffer circuit CB 1   c  provides an output clock signal to replaced cells CELL 1   a -CELL 1   b . The clock tree circuit is not limited to such a two-stage structure but may be changed to a three-stage structure or more to comply with requirements for semiconductor integrated circuit  1 . 
   Replaced cell CELL 1   a  is provided between buffer circuit B 1  and scan flip-flop circuits SFF 1 -SFF 3 , receives scan shift enable signal SSE(I) from scan control circuit  2  at ports SSEI and the output clock signal from clock buffer circuit CB 1   c  at port CLKI, and supplies an output clock signal from port CLKO and output scan shift enable signal SSE(O) synchronized with clock signal CLK (O) from port SSEO. 
   Replaced cell CELL 1   b  is provided between buffer circuit B 1  and clock tree circuit  5  and scan flip-flop circuit SFF 4 , receives scan shift enable signal SSE(I) from scan control circuit  2  at ports SSEI and output clock signal CLK(I) from clock buffer circuit CB 1   c  at port CLKI, and supplies output clock signal CLK(O) from port CLKO and output scan shift enable signal SSE(O) synchronized with clock signal CLK(O) from port SSEO. 
   Scan flip-flop circuits SFF 1 -SFF 3  are provided between replaced cell CELL 1   a  and logic circuit section  4 , receive output clock signal CLK(O) and output scan shift enable signal SSE(O) from replaced cell CELL 1   a  at input ports and ports SSEa of each of scan flip-flop circuits SFF 1 -SFF 3  and produce processed-output signals from ports Q and SO of each of scan flip-flop circuits SFF 1 -SFF 3 . 
   Logic circuit section  4  connected between scan flip-flop circuits SFF 1 -SFF 3  and scan flip-flop circuit SFF 4  includes various logic circuits required for a system LSI, cores of processors, scan registers, etc. Logic circuit section  4  receives the output signals from ports Q of scan flip-flop circuit SFF 1 -SFF 3 , executes various logic functions for a system operation at ordinary time and scan operations at scan test to produce logic function signals and scan operation signals. When a circuit size of logic circuit section  4  is so large that the exclusive application of a scan test method to the system LSI causes data volume increase and the data volume is bigger than a memory capacity of a LSI tester, it is preferable to provide the system LSI with a logic BIST function. 
   Scan flip-flop circuit SFF 4  receives the output signal from logic circuit section  4  at port D, clock signal CLK(O) from replaced cell CELL 1   b  and scan shift enable signal SSE(O) synchronized with clock CLK(O) from replaced cell CELL 1   b  at port SSEa, and produces the processed output signals from port Q or SO. 
   Here, since the scan shift enable signals are distributed to the loads, a suitable distribution of loads can be carried out independently of a place or a configuration of the clock tree circuit in the case where the replaced cell is set in place of the final stage of the clock tree circuit in comparison with the case where flip-flop circuits are inserted to carry out a synchronizing method. 
   As shown in  FIG. 5 , each of scan flip-flop circuits SFF 1 -SFF 4  functioning as a scan circuit is a multiplexed scan flip-flop circuit that is composed of multiplexer MUX 1  and flip-flop circuit FF 2  and that is controlled by multiplexer MUX 1 . 
   Multiplexer MUX 1  receives functional input signal DI and scan input signal SI, selects data DI or scan input signal SI in response to levels of scan shift enable signal SSEa and supplies an output signal to flip-flop circuit FF 2 . 
   Flip-flop circuit FF 2  receives the output signal from multiplexer MUX 1  at port D and latches the same at its rising-up time, stores the latched signal, updates it with the latest signal, and produces output signal OUT. 
   Thus, when multiplexer MUX 1  selects functional input signal DI, flip-flop circuit FF 2  stores data, updates the same with the latest data, and produces a functional signal as output signal OUT. When multiplexer MUX 1  selects scan input signal SI, however, flip-flop circuit FF 2  stores data, updates the same with the latest data, and produces a scan output signal as output signal OUT. 
   A scan operation will now be explained with reference to a timing chart shown in  FIG. 6 . The chart indicates a scan test to detect a delay fault of the logic circuit section in the case where scan flip-flop circuit SFF 1  and SFF 4  are selected as a launch flip-flop circuit and a capture flip-flop circuit, respectively. 
   As shown in  FIG. 6 , scan enable signal SSE(I) input to port SSEI of replaced cell CELL 1   a  shits from a “High” level to a “Low” level during one cycle before a last shift launch clock. Scan shift enable signal SSE(O) output from port SSEO of replaced cell CELL 1   a  maintains a “High” level for a period of time until the last shift launch clock falls down. Since scan shift enable signals SSE(O) output from port SSEO of replaced cell CELL 1   a  and CELL 1   b  are supplied to ports SSEa of scan flip-flop circuits SFF 1 -SFF 3  and scan flip-flop circuit SFF 4  during such a period of time, respectively, ports SSEa of scan flip-flop circuits SFF 1  and SFF 4  are in a “High” level scan shift state. 
   When the last shift launch clock is input, scan shift enable signal SSE(O) supplied from port SSEO of replaced cell CELL 1   a  is shifted to a “Low” level at a rising-up time of the last shift launch clock. At this time, ports SSEa of scan flip-flop circuits SFF 1 -SFF 4  to which scan shift enable signal SSE(O) is supplied from port SSEO of replaced cell CELL 1   a  becomes a “Low” level. 
   In the scan test to detect a delay fault of the logic circuit section, when the last shift launch clock is supplied to scan flip-flop circuit SFF 1  and scan shift enable signal SSE(O) output from port SSEO of replaced cell CELL 1   a  is a “High” level, scan flip-flop circuit SFF 1  outputs last shift data SFF 1 (Q) as launch data from port Q. Here, the launch data are scan-in data used for the scan test. 
   Next, logic circuit section  4  receives the last shift launch data output from scan flip-flop circuit SFF 1  so that scan registers provided in logic circuit section  4  carry out the scan test in response to the last shift launch data. Scan flip-flop circuit SFF 4  is then selected in response to scan shift enable signal SSE(O) output from port SSEO of replaced cell CELL 1   b  that has been shifted to a “Low” level before a capture clock signal arrives. Thus, scan flip-flop circuit SFF 4  receives scan test information of capture data SFF 4 (D) supplied from logic circuit section  4  to port D and provides an output from port SO of scan flip-flop circuit SFF 4 . 
   As set forth above, the scan test circuit of the semiconductor integrated circuit in accordance with the embodiment and the method of arranging the same include the clock buffer circuits cascade-connected to form clock tree circuit  5  and replaced cells CELL 1   a  and CELL 1   b  set in place of the last stage clock buffer circuit neighboring the scan circuit side between clock tree circuit  5  and the scan circuit. Clock signal CLK(O) output from clock buffer circuit CB 1  is synchronized with scan shift enable signal SSE(O) output from flip-flop circuit FF 1 . 
   Thus, when scan shift signal SSE is provided through pipe-line architecture, scan shift enable signal SSE is distributed to loads, so that troublesome operations are not required, the degree of freedom for layout design is improved and optimization of timing adjustment and layout design can be easily achieved in comparison with a conventional method of inserting flip-flop circuits and the like for synchronization. The scan test circuit of the invention fulfils high-speed operations and optimum timing adjustments for a large circuit size system LSI or SoC device. 
   Further, although the scan test is carried out the clock signal output from clock control circuit  3 , a system clock that is generated by phase-locked-loop circuits in semiconductor integrated circuit  1  and that is faster than the clock signal can also be additionally used. The clock signal is used for writing data in the scan test circuit or reading data from the same while the high-speed system clock is used for the detection of a delay fault of the logic circuit section. In this case, a selector and the like are used for the selection of the clock signal or the system clock signal. 
   SECOND EMBODIMENT 
   Next, a scan test circuit in accordance with a second embodiment of the present invention will be described with reference to the attached drawings.  FIG. 7  is a block diagram of a replaced cell. A latch circuit is used for a replaced cell in this embodiment. 
   Identical reference symbols for components of the second embodiment indicate the same ones as those of the first embodiment so that different components will be primarily described below. 
   As shown in  FIG. 7 , replaced cell CELL 2  includes clock buffer circuit CB 1  and latch circuit LATCH 1 . Replaced cell CELL 2  is provided to replace the last clock buffer circuit neighboring the scan circuit side between the scan circuit and the clock buffer circuits cascade-connected to form the clock tree circuit. 
   Latch circuit LATCH 1  is simpler in structure than a master-slave type flip-flop circuit, for example. Latch circuit LATCH 1  receives scan shift enable signal SSE(I) at port D as a scan control signal used for a scan test method, latches scan shift enable signal SSE(I) at a rising-up time of clock signal CLK(I), and outputs the latest scan enable signal SSE(O) stored and updated from port Q to the scan circuit. Thus, output clock signal CLK(O) is synchronized with output scan enable signal SSE(O). 
   As set forth above, the scan test circuit in accordance with the second embodiment includes the clock buffer circuits cascade-connected to form clock tree circuit  5  and replaced cell CELL 2  composed of clock buffer circuit CB 1  and latch circuit LATCH 1  to replace the final stage buffer circuit neighboring on the scan circuit side between clock tree circuit  5  and the scan circuit. Clock signal CLK(O) output from clock buffer circuit CB 1  is synchronized with scan shift enable signal SSE(O) output from latch circuit LATCH 1 . 
   Thus, when scan shift signal SSE is provided through pipe-line architecture, scan shift enable signal SSE is distributed to loads, so that troublesome operations are not required, the degree of freedom for layout design is improved and optimization of timing adjustment and layout design can be easily achieved in comparison with a conventional method of inserting flip-flop circuits and the like for synchronization. The replaced cell circuit of the second embodiment is more simplified than that of the first embodiment to suppress increase of a system LSI size. 
   THIRD EMBODIMENT 
   Next, a scan test circuit in accordance with a third embodiment of the present invention will be described with reference to the attached drawings.  FIG. 8  is a block diagram of a replaced cell. A flip-flop circuit is used for a replaced cell in this embodiment. 
   Identical reference symbols for components of the third embodiment indicate the same ones as those of the first embodiment, so that different components will be primarily described below. 
   As shown in  FIG. 8 , replaced cell CELL 3  includes clock buffer circuit CB 1  and flip-flop circuit FF 3 . Replaced cell CELL 3  is provided to replace the last clock buffer circuit neighboring the scan circuit side between the scan circuit and the clock buffer circuits cascade-connected to form the clock tree circuit. 
   Flip-flop circuit FF 3  receives scan shift enable signal SSE(I) at port D as a scan control signal used for a scan test method, latches scan shift enable signal SSE(I) at a rising-up time of clock signal CLK(I), and outputs the latest scan enable signal SSE(O) stored and updated from port Q to the scan circuit. Thus, output clock signal CLK(O) is synchronized with output scan enable signal SSE(O). 
   As set forth above, the scan test circuit in accordance with the third embodiment includes the clock buffer circuits cascade-connected to form clock tree circuit  5  and replaced cell CELL 3  composed of clock buffer circuit CB 1  and flip-flop circuit FF 3  to replace the final stage buffer circuit neighboring on the scan circuit side between clock tree circuit  5  and the scan circuit. Clock signal CLK(O) output from clock buffer circuit CB 1  is synchronized with scan shift enable signal SSE(O) output from flip-flop circuit FF 3 . 
   Thus, when scan shift signal SSE is provided through pipe-line architecture, scan shift enable signal SSE is distributed to loads, so that troublesome operations are not required, the degree of freedom for layout design is improved and optimization of timing adjustment and layout design can be easily achieved in comparison with a conventional method of inserting flip-flop circuits and the like for synchronization. 
   The present invention is not limited to the embodiments described above, but various variations and modification may be made without departing from the scope of the present invention. 
   In the foregoing description, certain terms have been used for brevity, clearness and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are used for descriptive purposes herein and are intended to be broadly construed. Moreover, the embodiments of the improved construction illustrated and described herein are by way of example, and the scope of the invention is not limited to the exact details of construction. Having now described the invention, the construction, the operation and use of embodiments thereof, and the advantageous new and useful results obtained thereby, the new and useful construction, and reasonable equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.