Abstract:
A semiconductor integrated circuit includes a clock signal source for generating two-phase clock signals having spacing periods, a two-phase clock wiring for transmitting the two-phase clock signals to a plurality of internal circuits constructing the integrated circuit, and a waveform correction circuit having a plurality of MOS transistors of the same conductivity type connected between the two-phase clock wiring and a preset potential node and constructed to attain spacing periods of the two-phase clock signals. The waveform correction circuit corrects the blunted portions of the two-phase clock signals to stably attain spacing periods, and when it is distributed and arranged in portions far apart from the clock signal source, a problem of racing and the like can be effectively suppressed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-205423, filed Jul. 6, 2000, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a semiconductor integrated circuit and a layout design method thereof and more particularly to a waveform correction circuit used in a logic LSI, for example, for providing countermeasures against clock skew of two-phase clock wires. 
     Generally, in the logic LSI, the clock propagation delay of a clock wiring used for transmitting a clock signal supplied from a clock signal source to an internal circuit becomes larger as the load capacitance including the wiring capacitance and wiring resistance (which vary in proportion to the length of the wiring) and the terminal capacitance of an internal circuit element becomes larger. Therefore, a clock buffer circuit is inserted at a halfway portion of the wiring. In this case, in the two-phase clock wiring used for transmitting two-phase clock signals supplied from the two-phase clock signal source (clock driver circuit) to the internal circuit, a difference between the delay amounts of the two-phase clock signals becomes important. 
     FIG. 1 shows one example of a two-phase clock wiring system in the conventional logic LSI and internal circuits connected thereto. 
     In FIG. 1, reference numeral  10  denotes a two-phase clock wiring which includes a first clock wire  11  and second clock wire  12 . 
     A first latch circuit  15  determines or holds input data supplied thereto via a data signal line in response to the falling edge of a first clock signal φ 1  from the first clock wire  11 . 
     A second latch circuit  16  fetches data from an output node Q 1  of the first latch circuit  15  in response to the rising edge of a second clock signal φ 0  from the second clock wire  12  and outputs latched data from an output node Q 2 . 
     FIG. 2 shows one example of waveforms of the clock signals φ 1 , φ 0  of the two-phase clock wires  11 ,  12  shown in FIG.  1  and operation waveforms of the two cascade-connected latch circuits  15 ,  16 . FIG. 3 shows an enlarged portion of the waveforms of the two-phase clock signals φ 1 , φ 0  in FIG.  2 . 
     As shown in FIGS. 2 and 3, the two-phase clock signals φ 1 , φ 0  have waveforms each having a period “L” (low level) and a period “H” (high level) which are set in an approximately complementary relation, a period of the same level (spacing period) exists between the trailing edge of the high level portion of one of the signals and the leading edge of the high level portion of the other signal and a period of the same level (spacing period) also exists between the leading edge of the high level portion of the former signal and the trailing edge of the high level portion of the latter signal. In this example, a spacing period (between a broken line a-b and a broken line c-d) of “L” exists between the falling edge of φ 1  and the rising edge of φ 0  and a spacing period of “L” exists between the falling edge of φ 0  and the rising edge of φ 1 . Thus, the latch operation and the output operation of the two latch circuits  15 ,  16  which are cascade-connected as described before are correctly effected. 
     For example, as shown in FIG. 4, there occurs a possibility that a period which is originally set as the spacing period of “L” will become a racing period (between a broken line e-f and a broken line g-h) in which the signals are set at “H” due to a difference between the blunted or rounded portion of the waveform of φ 0  and the blunted or rounded portion of the waveform of φ 1  in some cases. As the cause of a difference between the blunted portion of the waveform of φ 0  and the blunted portion of the waveform of φ 1 , it is considered that the load of the first clock wire  11  and the load of the second clock wire  12  are made different by branching the first clock wire  11  on the input side of the first latch circuit  15  as indicated by broken lines in FIG.  1  and connecting the same to a different circuit  15   a,  for example. Further, the above difference may occur in a portion separated far apart from the clock generating source and connected thereto via a long clock wire in a semiconductor chip. 
     If the racing period thus occurs, the latch operation and the output operation of the two latch circuits  15 ,  16  which are cascade-connected as described before are not correctly effected in some cases. For example, if the waveform of φ 1  is blunted or rounded as indicated by broken lines in FIG. 2, data fetching timing in the first latch circuit  15  is deviated and the second latch circuit  16  will fetch erroneous latched data of the first latch circuit  15  and output erroneous data. The same operation occurs when the blunted or rounded portion of the waveform of φ 0  becomes different from the rounded portion of the waveform of φ 1 . 
     That is, in a case wherein the two-phase clock wires  11 ,  12  are long, the wiring lengths thereof are different from each other (the resistances thereof are different) or the numbers of circuits such as latch circuits respectively supplied with the clock signals φ 0  and φ 1  are different (the capacitances thereof are different), then a difference between the loads for the φ 0  and φ 1  larger than expected occurs, the balance therebetween cannot be maintained, the timing relation between the falling edge and the rising edge of φ 0  and φ 1  is reversed, a spacing period of the two-phase clock signals φ 0  and φ 1  cannot be attained in portions of the paths of the two-phase clock wires  11 ,  12  and a racing period occurs. As a result, a period in which the first latch circuit  15  supplied with φ 1  as the clock input and the second latch circuit  16  supplied with φ 0  as the clock input as described before are both turned ON occurs and the latch operation and the output operation of the two cascade-connected latch circuits  15 ,  16  are not correctly effected. 
     Therefore, in the prior art, in order to prevent the predictable occurrence of a racing period, two-phase clock signals having a relatively long spacing period are generated from the two-phase clock signal source, but when a computer aided design (CAD) apparatus is used for LSI layout design, attention which is so delicate and adequate as in a case of manual design by a designer is not always given and there occurs a possibility that such a racing period as described above occurs in the circuit portion of a real product in which the spacing period is required. 
     BRIEF SUMMARY OF THE INVENTION 
     A semiconductor integrated circuit according to a first aspect of this invention comprises a clock signal source configured to generate two-phase clock signals having spacing periods; a two-phase clock wiring configured to transmit the two-phase clock signals to a plurality of internal circuits constructing the integrated circuit; and a waveform correction circuit having a plurality of MOS transistors of the same conductivity type which are connected between the two-phase clock wiring and a preset potential node and constructed to attain spacing periods of the two-phase clock signals. 
     A layout design method of a semiconductor integrated circuit according to a second aspect of this invention comprises the steps of arranging a plurality of circuit cells; arranging wires including two-phase clock wires; and distributing and arranging a plurality of MOS transistors for waveform correction connected between the two-phase clock wires and a preset potential node to attain spacing periods of the two-phase clock signals in spaces other than areas in which the plurality of circuit cells of an integrated circuit chip and the wires are arranged. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a circuit diagram showing one example of a two-phase clock wiring system of the conventional logic LSI and internal circuits connected thereto; 
     FIG. 2 is a diagram showing one example of waveforms of two-phase clock signals of two-phase clock wires in FIG.  1  and operation waveforms of two cascade-connected latch circuits; 
     FIG. 3 is a diagram showing a portion of the waveforms of the two-phase clock signals in FIG. 2, for illustrating spacing periods; 
     FIG. 4 is a waveform diagram showing the state in which a racing period occurs due to a difference between blunted portions of the waveforms of the two-phase clock signals; 
     FIG. 5 is a circuit diagram showing one example of a two-phase clock wiring system and internal circuits connected thereto in a logic LSI according to a first embodiment of this invention; 
     FIG. 6 is a diagram showing one example of operation waveforms of two cascade-connected latch circuits in FIG. 5; 
     FIG. 7 is a timing chart for illustrating the operation of a waveform correction circuit by taking a case wherein the blunted degree of the waveform at the falling edge of φ 1  is larger than the blunted degree of the waveform at the rising edge of φ 0  as one example in which the blunted degrees of the waveforms of the two-phase clock signals at an H node and G node in FIG. 5 are different from each other; 
     FIG. 8 is a circuit diagram showing one example of the waveform correction circuit used in a low-active logic circuit in the first embodiment; 
     FIG. 9 is a circuit diagram showing one example of a two-phase clock wiring system and internal circuits connected thereto in a logic LSI according to a second embodiment of this invention; 
     FIG. 10 is a circuit diagram showing one example of a waveform correction circuit used in a low-active logic circuit in the second embodiment; and 
     FIGS. 11A to  11 C are schematic layout views (plan views) of an integrated circuit chip, for illustrating an integrated circuit layout design method according to a third embodiment of this invention in a stepwise fashion. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There will now be described embodiments of this invention with reference to the accompanying drawings. 
     First Embodiment 
     FIG. 5 shows one example of a two-phase clock wiring system and internal circuits connected thereto in a logic LSI according to a first embodiment of this invention. 
     In FIG. 5, reference numeral  10  denotes a two-phase clock wiring including a first clock wire  11  and second clock wire  12 . Reference numeral  13  denotes a first clock buffer circuit inserted into the first clock wire  11  and reference numeral  14  denotes a second clock buffer circuit inserted into the second clock wire  12 . 
     The clock buffer circuits  13  and  14  each include a PMOS transistor QP and NMOS transistor QN which are serially connected between a power supply node (Vdd node) and a ground node (GND node), the gates of the MOS transistors are commonly connected to an input node and the drains thereof are commonly connected to an output node. 
     The first latch circuit  15  determines or holds input data DATA in response to the falling edge of a first clock signal φ 1  supplied via the first clock wire  11 . 
     The second latch circuit  16  fetches data from an output node Q 1  of the first latch circuit  15  in response to the rising edge of a second clock signal φ 0  supplied via the first clock wire  12  and outputs latched data from an output node Q 2 . 
     A waveform correction circuit  17  is located on the preceding stage side with respect to the first latch circuit  15  and second latch circuit  16  and includes a plurality of transistors connected between the two-phase clock wires  11 ,  12  and the GND node to attain spacing periods of the two-phase clock signals φ 1 , φ 0 . 
     The waveform correction circuit in the first embodiment includes a first NMOS transistor N 1  whose drain-source path is connected between the first clock wire  11  and the GND node and a second NMOS transistor N 2  whose drain-source path is connected between the second clock wire  12  and the GND node and whose gate and drain are respectively connected to the drain and gate of the first transistor N 1 . 
     The operation of the circuit with the above construction is now explained. In this case, it is preferred that the desired spacing periods exist in the two-phase clock signal  10  input to the first clock buffer  13  and second clock buffer  14  from the clock signal source. That is, as explained with reference to FIG. 3, for example, it is desirable that a spacing period (between the broken line a-b and broken line c-d) of “L” exists between the falling edge of φ 1  and the rising edge of φ 0  and a spacing period of “L” exists between the falling edge of φ 0  and the rising edge of φ 1 . 
     However, if the circuits are located far apart from the clock signal source, the waveforms of the two-phase clock signals φ 1 , φ 0  are blunted or rounded as shown in FIG.  6 . FIG. 6 shows the operation waveforms of respective portions (DATA, φ 1 , φ 0  Q 2 ) of the two cascade-connected latch circuits  15 ,  16  in FIG.  5 . Even if the two-phase clock signals φ 1 , φ 0  are thus blunted, the latch circuit  16  can output a signal which is originally expected as shown by Q in response to an input signal DATA to the latch circuit  15  by use of the circuit construction of FIG.  5 . 
     FIG. 7 is a timing chart showing the waveforms of the two-phase clock signals φ 1 , φ 0  at the H node and G node of FIG. 5, for illustrating the operation of the waveform correction circuit  17 . In this case, an example wherein the blunted degree of the waveform of the falling edge of φ 1  is larger than the blunted degree of the waveform of the falling edge of φ 0  is shown as one example of cases wherein the blunted degrees of the waveforms output from the clock buffers  13 ,  14  are different from each other. 
     Now, attention is paid to a case wherein the clock signal input φ 1  to the first clock buffer circuit  13  changes from “H” to “L” at the first clock wire  11  and the clock signal input φ 0  to the second clock buffer circuit  14  changes from “L” to “H” at the second clock wire  12 . 
     In the process in which the clock signal output φ 0  of the second clock buffer circuit  14  changes from “L” to “H”, the transistor N 1  is turned ON when the clock signal exceeds the threshold voltage (I point) of the transistor N 1  (broken line i-j), and the falling edge of the clock signal output φ 1  from the first clock buffer circuit  13  falls at a higher speed as shown by the waveform indicated by a broken line. At this time point, since the PMOS transistor QP of the first clock buffer circuit  13  is already set in the OFF state, it does not obstruct the operation of the falling edge of φ 1  which falls at the higher speed, as previously described. 
     At this time, since the clock signal output φ 1  from the first clock buffer circuit  13  is still set at “H” and the transistor N 2  is not yet completely set into the OFF state in the system of the first clock wire  11 , the rising edge of the clock signal output φ 0  from the second clock buffer circuit  14  rises at a lower speed, as shown by the waveform indicated by a broken line. 
     In the process in which the clock signal output φ 1  from the first clock buffer circuit  13  changes from the “H” level to the “L” level via a K point of the threshold voltage of the circuit, the transistor N 2  is turned OFF when the threshold voltage (L point) of the transistor N 2  is exceeded (a broken line m-n), and the rising edge of the clock signal output φ 0  from the second clock buffer circuit  14  rises at a higher speed as indicated by a broken line after paint J. At this time point, since the NMOS transistor QN of the second clock buffer circuit  14  is already set in the OFF state, it does not obstruct the operation of the rising edge of φ 0  which rises at a higher speed, as previously described. 
     If the waveform correction process is effected as indicated by the broken lines, waveforms after passing through wave-shaping circuits contained in the first and second latch circuits or the like will become the originally expected waveforms having a spacing period as shown in the lower half portion of FIG.  7 . 
     Thus, the spacing period (between the broken line k-l and the broken line m-n) can be stably attained between the falling edge of the clock signal output φ 1  from the first clock buffer circuit  13  and the rising edge of the clock signal output φ 0  from the second clock buffer circuit  14  by use of the waveform correction circuit  17  and an occurrence of the racing period can be prevented. 
     If occurrence of the racing period can be thus prevented, the latch operation and output operation of the two cascade-connected latch circuits  15 ,  16  can be correctly effected, as previously described, and Q shown in FIG. 6 can be output. 
     By maintaining the balance between the characteristics of the NMOS transistors N 1  and N 2 , the same operation as described above can also be attained between the falling edge of the clock signal output φ 0  from the second clock buffer circuit  14  and the rising edge of the clock signal output φ 1  from the first clock buffer circuit  13  and an occurrence of the racing period can be prevented. 
     Therefore, for example, if the two-stage latch circuits  15 ,  16  are used on the master side and different two-stage latch circuits (not shown) are provided on the slave side, then the latch operation can be correctly performed in response to the falling edge of φ 0  and the output operation can be correctly performed in response to the rising edge of φ 1  with respect to the different two-stage latch circuits. 
     If the waveform correction circuit  17  is arranged in a portion such as the end portion of the two-phase clock wires  11 ,  12  in which the largest time constant is expected, the effect of the present invention becomes larger, but it is preferable to locate the waveform correction circuit on the preceding stage side with respect to the internal circuit (such as the latch circuits  15 ,  16 ) which requires the spacing periods of the two-phase clock signals φ 1 , φ 0 . In this case, it is possible to distribute and arrange the NMOS transistors N 1 , N 2  for waveform correction in a plurality of portions including a portion located on the preceding stage side of the internal circuit in the integrated circuit chip. 
     In FIG. 5, a case wherein the waveform correction circuit  17  constructed by a plurality of NMOS transistors is used in the high-active logic circuit to make the waveform of the falling edge of the clock signal sharp and make the waveform of the rising edge gentle is shown. However, in a case wherein the waveform of the rising edge of the clock signal is made sharp and the waveform of the falling edge is made gentle in the low-active logic circuit, it is possible to use a waveform correction circuit  17 ′ which corresponds to the waveform correction circuit  17  and is constructed by a plurality of PMOS transistors connected between the two-phase clock wires  11 ,  12  and the Vdd node as shown in FIG.  8 . 
     Second Embodiment 
     It is sometimes desired to fixedly keep both of the two-phase clock signals φ 1 , φ 0  at “H” in a portion of the internal circuit so as to reduce the power consumption in the integrated circuit chip. In this case, in the circuit of the first embodiment, the NMOS transistors N 1  and N 2  tend to lower the potentials of the two-phase clock wires  11 ,  12 , that is, to prevent φ 1 , φ 0  from being fixed at “H”. The second embodiment improves on this point. 
     FIG. 9 shows one example of a two-phase clock wiring system and internal circuits connected thereto in a logic LSI according to the second embodiment of this invention. 
     The circuit shown in FIG. 9 is similar to that of FIG. 5 except for the waveform correction circuit  17   a . Portions which are the same as those of FIG. 5 are denoted by the same reference numerals. 
     The waveform correction circuit  17   a  includes a third NMOS transistor N 3  whose drain-source path is connected in series with the NMOS transistor N 1  between the first clock wire  11  and the GND node and whose gate is supplied with an enable control signal EN to control the ON/OFF state thereof, and a fourth NMOS transistor N 4  whose drain-source path is connected in series with the NMOS transistor N 2  between the second clock wire  12  and the GND node and whose gate is supplied with the enable control signal EN to control the ON/OFF state thereof, in addition to the waveform correction circuit  17  of the first embodiment. 
     With the above construction, when the signal EN is set at “H”, the NMOS transistors N 3 , N 4  are set in the ON state and the NMOS transistors N 1  and N 2  can perform the same operation as in the first embodiment. 
     On the other hand, if the signal EN is set at “L” when it is required to fix φ 1 , φ 0  at “H”, the NMOS transistors N 3 , N 4  are set in the OFF state to cut off the power supplies on the source sides of the NMOS transistors N 1  and N 2 , and therefore, it becomes possible to prevent the NMOS transistors N 1  and N 2  from lowering the potentials of the two-phase clock wires  11 ,  12 . 
     In the second embodiment, a case wherein the waveform correction circuit  17   a  constructed by a plurality of NMOS transistors is used in the high-active logic circuit to make the waveform of the falling edge of the clock signal sharp and make the waveform of the rising edge gentle is shown. However, in a case wherein the waveform of the rising edge of the clock signal is made sharp and the waveform of the falling edge is made gentle in the low-active logic circuit, it is possible to use a waveform correction circuit  17   a ′ which corresponds to the waveform correction circuit  17   a  and is constructed by a plurality of PMOS transistors connected between the two-phase clock wires  11 ,  12  and the Vdd node as shown in FIG.  10 . 
     Third Embodiment 
     In the third embodiment, a layout design method for distributing and arranging a group of MOS transistors for waveform correction in a plurality of portions in the integrated circuit chip is explained. 
     FIGS. 11A to  11 C are schematic layout views of a chip, for illustrating the integrated circuit layout design method according to the third embodiment of this invention in a stepwise fashion. 
     In the process of the layout design of a semi-conductor integrated circuit having two-phase clock wires for transmitting two-phase clock signals having spacing periods to internal circuits of the integrated circuit, first, a plurality of cells  21  which are provided as circuit elements are arranged according to the design rules (FIG. 11A) and then wires  23  including the two-phase clock wires are arranged (FIG.  11 B). Generally, the above steps are automatically performed by use of a computer, but a plurality of space areas in which neither the cells  21  nor the wires  23  are arranged exist. 
     As the next step, waveform correction circuits  25  which are connected between the two-phase clock wires contained in the wires  23  and a GND line contained in the wires  23  to stably attain spacing periods of the two-phase clock signals are distributed and arranged in the space areas which are located far apart from a clock signal source  21   c  (FIG.  11 C). An increase in the chip area (size) can be suppressed to a minimum by utilizing the space areas. 
     As described above, according to this invention, it is possible to provide a semi-conductor integrated circuit and a layout design method thereof in which the possibility of occurrence of racing in a circuit portion in which spacing periods of the two-phase clock signals are required can be prevented and the operation of two-stage latch circuits supplied with the two-phase clock signals can be correctly performed. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.