Abstract:
A circuit simulator includes: a DC analysis section which analyses a static stable potential on a transmission circuit if a capacitor which blocks a DC current while allowing an AC current to pass therethrough is connected in series in the line of the transmission circuit; and an initial potential application section which applies, as an initial potential in the simulation, the stable potential obtained by the DC analysis section to an application position on the upstream side of the capacitor in the flow of the signal through the transmission circuit. The simulator also includes a circuit simulation section which performs the simulation of the transmission circuit under the initial potential applied by the initial potential application section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a circuit simulator for performing a simulation of signal transmission in a transmission circuit in which a drive circuit which transmits a signal and a receiver circuit which receives the signal are connected to each other by a line, and to a circuit simulation program storage medium on which a circuit simulation program for enabling a computer to operate as the circuit simulator is stored. 
         [0003]    2. Description of the Related Art 
         [0004]    In the field of circuit designing, a circuit simulator for performing a simulation of a designed circuit on a computer before the circuit is actually manufactured is being used widely (see, for example, Japanese Patent Laid-Open Nos. 2000-331043 and 2002-197132). In the process of designing and manufacturing a transmission circuit in which a drive circuit which transmits a signal and a receiver circuit which receives the signal are connected to each other by a line, it is important to check the state of transmission of the signal before actually manufacturing the circuit. The circuit simulator is also used to check the state of transmission in such a case. Transient analysis for analyzing temporal changes of a signal on a line is ordinarily performed in a simulation of such a transmission circuit. 
         [0005]    In some case of designing of a transmission circuit such as that described above, a technique is adopted to couple the driver circuit and the receiver circuit to each other by a line in which an AC coupling component such as a capacitor capable of blocking a DC current while allowing an AC current to pass therethrough is connected in series at an intermediate position in the line, for example, if a DC potential difference exists between the output end of the driver circuit and the input end of the receiver circuit. 
         [0006]      FIG. 10  is a diagram showing a circuit model representing an example of a transmission circuit using a capacitor as an AC coupling component. 
         [0007]    The transmission circuit represented by a circuit model  500  in  FIG. 10  is a differential signal transmitting type of transmission circuit suitable for high-speed signal transmission. 
         [0008]    The circuit model  500  shown in  FIG. 10  has a device model  510  (hereinafter referred to as “driver model”) representing a drive circuit which transmits a signal, and a device model  520  (hereinafter referred to as “receiver model”) representing a receiver circuit which receives the signal. The two device models  510  and  520  are coupled to each other by two line models  530  and  540  respectively representing two lines. One of the two lines will be referred to as a Pos line, and the other of the two lines will be referred to as a Neg line. The line model representing the Pos line will be referred to as Pos line model  530 , and the line model representing the Neg line will be referred to as Neg line model  540 . In the transmission circuit represented by the circuit model  500 , a signal is transmitted as a potential difference between the Pos line and the Neg line. The Pos line model  530  and the Neg line model  540  is a differential line model formed by also considering the coupling between the lines. 
         [0009]    In the transmission circuit represented by the circuit model  500 , a capacitor is connected in series as an AC coupling component in each of the Pos line and the Neg line. Accordingly, the Pos line model  530  is constituted by a pattern model  531  representing the pattern from the driver circuit to the capacitor, a device model  550  representing the capacitor (hereinafter referred to as “capacitor model”), and a pattern model  532  representing the pattern from the capacitor to the receiver circuit. Similarly, the Neg line model  540  is constituted by a pattern model  541  representing the pattern from the driver circuit to the capacitor, a capacitor model  550 , and a pattern model  542  representing the pattern from the capacitor to the receiver circuit. 
         [0010]    In this circuit model  500 , an electrical direct connection is established between each adjacent pair of device models. 
         [0011]    In the transmission circuit represented by the circuit model  500 , a signal transmitted by the driver circuit is sent to the receiver circuit on the lines while any DC current is blocked. 
         [0012]    In a transmission circuit having a capacitor as an AC coupling component for example, signal transmission is performed in a steady state in which the capacitor is sufficiently charged and stable in the charged state. However, if such a steady state is simulated by transient analysis, the time required for simulation by computation is considerably long. If analysis results are output before the transmission circuit enters the steady state, they indicate only a state during transition to the steady state, and differ from the actual signal transmission results. 
         [0013]      FIG. 11  is a diagram showing analysis results of transient analysis before the steady state. 
         [0014]    Part (A) of  FIG. 11  is a Waveform showing changes in the potential on the Pos line model  530  and the potential on the Neg line model  540  during signal transmission. Part (B) of  FIG. 11  is a Waveform showing changes in the differential potential, i.e., the difference between the potential on the Pos line model  530  and the potential on the Neg line model  540 . 
         [0015]    In the actual transmission circuit, signal transmission is performed in a stable state such that the maximum potential on the Pos line is substantially equal to that on the Neg line, and the minimum potential on the Pos line is substantially equal to that on the Neg line. As a result, the differential potential changes up and down about 0 V. 
         [0016]    The analysis results shown in  FIG. 11 , however, are from a halfway state before the circuit model  500  enters the steady state. Line P 1  indicating the change in the potential on the Pos line model  530  is on the high-potential side (the upper section in the Waveform) with respect to line N 1  indicating the change in the potential on the Neg line model  540 , as shown in part (A). Line D 1  indicating the change in differential potential changes up and down about a potential of 300 mV much higher than 0 V, as shown in part (B). 
         [0017]    With the lapse of the analysis time, the potential on the Pos line model  530 , the potential on the Neg line model  540  and the differential potential in the simulation become closer to the values in the actual signal transmission. However, a long analysis time is required to obtain the desirable results, as described. 
         [0018]    For transient analysis in a transmission circuit using an AC coupling component such as that described above, a method is conventionally adopted in which a suitable potential for enabling a circuit model representing the transmission circuit to reach a steady state in a reduced time period is given to the circuit model as an initial value in transient analysis in advance. However, a certain level of skill is required to obtain and give a suitable initial value. 
       SUMMARY OF THE INVENTION 
       [0019]    The present invention has been made in view of the above circumstances and provides a circuit simulator capable of easily executing a simulation of a transmission circuit in a reduced time period, and a circuit simulation program storage medium on which a circuit simulation program for enabling a computer to operate as such a circuit simulator is stored. 
         [0020]    The present invention provides a circuit simulator which performs a simulation of a transmission circuit in which a driver circuit which transmits a signal and a receiver circuit which receives the signal are coupled to each other by a line, the circuit simulator including: 
         [0021]    a DC analysis section which analyses a static stable potential on the line if an AC coupling component which blocks a DC current while allowing an AC current to pass therethrough is connected in series in the line; 
         [0022]    an initial potential application section which applies, as an initial potential in the simulation, the stable potential obtained by the DC analysis section to an application position on the driver circuit side from the AC coupling component in a flow of the signal through the transmission circuit; and 
         [0023]    a simulation section which performs the simulation of the transmission circuit under the initial potential applied by the initial potential application section. 
         [0024]    In the circuit simulator of the present invention, a simulation of a transmission circuit having an AC coupling component is executed in a state where the stable potential is applied, that is, in a state where the transmission circuit has entered a substantially steady state, and the state of the transmission circuit is promptly changed into a steady state in the simulation. Therefore, the time required for simulation of the transmission circuit can be reduced. Also, in the circuit simulator of the present invention, a stable potential for enabling the simulation in a reduced time period as described above is analytically obtained by the DC analysis section. Therefore, even an inexperienced person can easily perform the simulation of signal transmission. Analysis in the DC analysis section can be performed only for the purpose of obtaining a static stable potential. Therefore, restricted analysis in which a condition suitable for this purpose is set with respect to the transmission circuit may suffice. For this reason, a stable potential can be obtained in a time period much shorter than that required in transient analysis or the like on a transmission circuit under an arbitrary condition. Therefore, even if the analysis time in the DC analysis section is included, the simulation of the transmission circuit requires a shorter time. That is, the circuit simulator of the present invention is capable of easily executing a simulation of a transmission circuit while limiting the analysis time. 
         [0025]    In the circuit simulator of the present invention, it is preferable that “the initial potential application section applies the initial potential to the application position by inserting at the application position a temporary device which first outputs the initial potential, and which thereafter behaves as a simple conductor.” 
         [0026]    In the circuit simulator in this preferred form, the initial potential can be applied to the transmission circuit without making any change in the electrical characteristics of the transmission circuit during simulation. 
         [0027]    The circuit simulator of the present invention may be formed such that “the initial potential application section applies the initial potential to a predetermined position on the line set as the application position” or such that “the initial potential application section applies the initial potential to a position in the driver circuit set as the application position.” 
         [0028]    The circuit simulator in such a form can reliably set the line in the transmission circuit in a statically stable potential state. 
         [0029]    The present invention also provides a simulation program storage medium that stores a circuit simulation program which is incorporated in a computer, and which enables the computer to operate as a circuit simulator performing a simulation of a transmission circuit where a driver circuit that transmits a signal and a receiver circuit that receives the signal are coupled to each other by a line, the circuit simulation program implementing: 
         [0030]    a DC analysis section which analyses a static stable potential on the line if an AC coupling component which blocks a DC current while allowing an AC current to pass therethrough is connected in series in the line; 
         [0031]    an initial potential application section which applies, as an initial potential in the simulation, the stable potential obtained by the DC analysis section to an application position on the driver circuit side from the AC coupling component in a flow of the signal through the transmission circuit; and 
         [0032]    a circuit simulation section which performs the simulation of the transmission circuit under the initial potential applied by the initial potential application section. 
         [0033]    The circuit simulation program of the present invention ensures that the circuit simulator of the present invention is easily constructed on a computer. 
         [0034]    Only the basic form of the circuit simulation program storage medium of the present invention is described here. This simplification of description is only for avoiding redundancy. The circuit simulation program storage medium of the present invention includes various forms corresponding to the forms of the circuit simulator as well as the basic forms. 
         [0035]    The components including the DC analysis section constructed on a computer by the circuit simulation program of the present invention may be such that one component is formed by one program part; one component is formed by plural program parts; or plural components are formed by one program part. These components may be constructed so as to perform the corresponding functions by themselves or execute the functions by providing instructions to a different program or program parts incorporated in the computer. 
         [0036]    According to the present invention, as described above, a simulation of a transmission circuit can be easily executed while limiting the analysis time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is an external perspective view of a computer which operates as an embodiment of the circuit simulator of the present invention; 
           [0038]      FIG. 2  is a diagram showing the configuration of the hardware of the computer; 
           [0039]      FIG. 3  is a diagram schematically showing a CD-ROM which is an embodiment of the circuit simulation program storage medium of the present invention; 
           [0040]      FIG. 4  is a block diagram showing an embodiment of the circuit simulator of the present invention; 
           [0041]      FIG. 5  is a flowchart showing the flow of simulation in a circuit simulator  400  shown in  FIG. 4 ; 
           [0042]      FIG. 6  is a diagram showing a state in which a model of a temporary device based on DC analysis is inserted in a circuit model  500  shown in  FIG. 10 ; 
           [0043]      FIG. 7  is a diagram showing a state in which a temporary device model  560  is inserted in a driver model  510  in the circuit model  500 ; 
           [0044]      FIG. 8  is a diagram showing a state in which a temporary device model  560  is inserted in the driver model  510  in the circuit model  500 , and which is different from that shown in  FIG. 7 ; 
           [0045]      FIG. 9  is a diagram showing the results of transient analysis on the circuit model  500  shown in  FIG. 6 , in which the temporary device model  560  is inserted; 
           [0046]      FIG. 10  is a diagram showing a circuit model representing an example of a transmission circuit using a capacitor as an AC coupling component; and 
           [0047]      FIG. 11  is a diagram showing the analysis results of transient analysis before a steady state. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0048]    An embodiment of the present invention will be described with reference to the accompanying drawings. 
         [0049]      FIG. 1  is an external perspective view of a computer  100  which operates as an embodiment of the circuit simulator of the present invention.  FIG. 2  is a diagram showing the configuration of the computer  100 . 
         [0050]    As shown in  FIG. 1 , the computer  100  has, as its components seen in its appearance, a main unit  101 , an image display  102  which displays an image on a display screen  102   a  according to a command from the main unit  101 , a keyboard  103  which inputs various kinds of information to the main unit  101  according to key operations, and a mouse  104  which inputs to the main unit  101  a command, for example, according to an icon or the like indicated at an arbitrary position designated on the display screen  102   a . The main unit  101  has an FD loading port  101   a  for loading a flexible disk (FD) and a CD-ROM loading port  101   b  for loading a CD-ROM. 
         [0051]    In the main unit  101  are provided, as shown in  FIG. 2 , a CPU  1011  which executes various programs, a main memory  1012  into which a program stored in a hard disk unit  1013  is read and loaded for execution by the CPU  1011 , the hard disk unit  1013  in which various programs, data and the like are stored, an FD drive  1014  into which an FD  201  is loaded, and in which the loaded FD  201  is accessed, a CD-ROM drive  1015  into which a CD-ROM  202 , a CD-R or the like is loaded, and in which the loaded CD-ROM  202  or the like is accessed, and an I/O interface  1016  for exchange of signals with an external device (not shown). These various components, the image display  102 , the keyboard  103  and the mouse  104  are connected to each other through a bus  105 . 
         [0052]    The CD-ROM  202  is an embodiment of the circuit simulation program storage medium of the present invention on which a circuit simulation program for enabling the computer  100  to operate as an embodiment of the circuit simulator of the present invention is stored. The program stored on the CD-ROM  202  is uploaded by the computer  100  to be stored on the hard disk unit  1013 . The computer  100  operates as an embodiment of the circuit simulator of the present invention by executing the program. 
         [0053]    Description will next be made of an embodiment of the circuit simulation program of the present invention. 
         [0054]      FIG. 3  is a diagram schematically showing a CD-ROM which is an embodiment of the circuit simulation program storage medium of the present invention. 
         [0055]      FIG. 3  is a diagram schematically showing the CD-ROM  202  on which a circuit simulation program  300  corresponding to the circuit simulation program of the present invention is stored. 
         [0056]    The circuit simulation program  300  enables the computer  100  to operate as an embodiment of the circuit simulator of the present invention, and has a DC analysis section  310 , an initial potential application section  320  and a simulation section  330 . Details of these components of the circuit simulation program  300  will be described later. 
         [0057]      FIG. 4  is a block diagram showing an embodiment of the circuit simulator of the present invention. 
         [0058]    A circuit simulator  400  shown in  FIG. 4  as an embodiment of the circuit simulator of the present invention is formed when the simulation program  300  shown in  FIG. 3  is installed and executed on the computer  100  shown in  FIG. 1 . The circuit simulator  400  has a DC analysis section  410 , an initial potential application section  420  and a simulation section  430 . 
         [0059]    When the simulation program  300  shown in  FIG. 3  is installed on the computer  100  shown in  FIG. 1 , the DC analysis section  310 , the initial potential application section  320  and the simulation section  330  of the simulation program  300  form the DC analysis section  410 , the initial potential application section  420  and the simulation section  430 , respectively, of the circuit simulator  400  shown in  FIG. 4 . While these components of the circuit simulator  400  are formed by the combination of the hardware of the computer and an operating system (OS) and an application program executed on the computer, the components of the circuit simulation program  300  shown in  FIG. 3  are formed only by the application program. 
         [0060]    The DC analysis section  410 , the initial potential application section  420  and the simulation section  430  in the circuit simulator  400  respectively correspond to examples of the DC analysis section, the initial potential application section and the simulation section in the circuit simulator of the present invention. 
         [0061]    Description will be made of the components of the circuit simulator  400  shown in  FIG. 4  as well as of the components of the circuit simulation program  300  shown in  FIG. 3 . 
         [0062]    In the process of performing a simulation of a transmission circuit by the circuit simulator  400 , a circuit model representing the transmission circuit to be simulated is first input to the circuit simulator  400 . 
         [0063]    In the circuit simulator  400  shown in  FIG. 4 , a static stable potential is obtained by the DC analysis section  410  with respect to the input circuit model. The static stable potential is then given to the circuit model as the initial potential by the initial potential application section  420 , and a simulation is executed under the initial potential by the simulation section  430 . 
         [0064]    The flow of simulation in the circuit simulator  400  will be described in detail below. In the following description, a reference is made to the components shown in  FIG. 4  without particularly referring to the figure number. 
         [0065]      FIG. 5  is a flowchart showing the flow of simulation in the circuit simulator  400  shown in  FIG. 4 . 
         [0066]    The simulation shown in the flowchart is started when a circuit model such as shown in  FIG. 10  is input to the circuit simulator  400  and when an operator inputs a command through a command screen (not shown) to start the simulation. Determination is first made in the DC analysis section  410  when processing is started as to whether or not a capacitor is connected in series in the lines in the circuit model (step S 101 ). 
         [0067]    If it is determined in processing in step S 101  that no capacitor is connected in series in the lines (determination of No in step S 101 ), transient analysis in step S 105  is executed. After the completion of transient analysis, the process shown in  FIG. 5  ends. Description will be made later of transient analysis in step S 105 . 
         [0068]    If it is determined in processing in step S 101  that a capacitor is connected in series in the lines (determination of Yes in step S 101 ), processing described below is executed. 
         [0069]    The following description will be made by assuming that the circuit model  500  shown in  FIG. 10  is input to the circuit simulator  400 . Since a capacitor is connected in series in the lines in the circuit model  500 , it is determined by processing in step S 101  that a capacitor is connected in series in the lines. In the following description, a reference is also made to the components shown in  FIG. 10  without particularly referring to the figure number. 
         [0070]    In this case, DC analysis is executed by computing potentials in the circuit model  500  on the assumption that the transmission circuit is statically stabilized (step S 102 ) In this embodiment, static stable potentials on the input section are obtained with respect to the two lines in the receiver model  520  by this DC analysis. 
         [0071]    A model of a temporary device described below is made by the initial potential application section  420  on the basis of the stable potentials obtained in step S 102  (step S 103 ), and the temporary device model thus made is inserted in the circuit model  500  (step S 104 ). 
         [0072]      FIG. 6  is a diagram showing a state in which a model of a temporary device based on DC analysis is inserted in the circuit model  500  shown in  FIG. 10 . 
         [0073]    The temporary device model  560  shown in  FIG. 6  represents a temporary device which outputs the stable potentials to the Pos line model  530  and the Neg line model  540  at the beginning of the operation of the circuit model  500 , and which thereafter behaves as a simple conductor. This temporary device corresponds to the temporary device in accordance with the present invention. 
         [0074]    In step S 104  shown in  FIG. 5 , the initial potential application section  420  inserts this temporary model device  500  immediately before the capacitor models  550  in the Pos line model  530  and the Neg line model  540 , as shown in  FIG. 6 . The position immediately before the capacitor models  550  corresponds to an example of the application position in accordance with the present invention. 
         [0075]    By the insertion of this temporary device model  560  in transient analysis executed in step S 105 , the state of the circuit model  500  is promptly changed into a steady state in which the capacitor model  550  is sufficiently charged. 
         [0076]    In this embodiment, the place in which the temporary device model  560  is inserted is specified on the program as a position immediately before each of the capacitor models in the Pos line model and the Neg line model in the circuit model of the transmission circuit (circuit model  500  in this embodiment). However, the present invention is not limited to this. For example, the place in which the temporary device model  560  is inserted may be in the driver model in the circuit model of the transmission circuit, as described below. 
         [0077]    Description will be made of an example of a case where an internal portion of the driver model in the circuit model of the transmission circuit is specified on the program as the place in which the temporary device model  560  is inserted. 
         [0078]      FIG. 7  is a diagram showing a state in which the temporary device model  560  is inserted in the driver model  510  in the circuit model  500 . 
         [0079]    The driver model  510  is constituted by a die model  511  expressing electrical characteristics of the driver circuit and a package model  512  expressing the shape of the driver circuit. In the state shown in  FIG. 7 , the temporary device model  560  inserted in the driver model  510  is placed in a stage following the package model  512 . 
         [0080]      FIG. 8  is a diagram showing a state in which the temporary device model  560  is inserted in the driver model  510  in the circuit model  500 , and which is different from that shown in  FIG. 7 . 
         [0081]    In the state shown in  FIG. 8 , the temporary device model  560  inserted in the driver model  510  is placed between the die model  511  and the package model  512 . 
         [0082]    The internal portion of the driver model  510  shown in  FIG. 7  or  8  described above as a place in which the temporary device model  560  is inserted corresponds to an example of the application position in accordance with the present invention. 
         [0083]    A further description will be made by referring again to  FIG. 5 . 
         [0084]    After the processing in step S 104  shown in  FIG. 5  has been completed, transient analysis is executed on the circuit model  500  in which the temporary device model  560  is inserted (step S 105 ). 
         [0085]    In transient analysis of step S 105 , the state of the circuit model  500  is promptly changed into a steady state by the static stable potential output as an initial potential from the temporary device model  560  as described above, and a simulation in the steady state is executed. 
         [0086]      FIG. 9  is a diagram showing the analysis results of transient analysis on the circuit model  500  shown in  FIG. 6 , in which the temporary device model  560  is inserted. 
         [0087]    Part (A) of  FIG. 9  is a Waveform showing changes in the potential on the Pos line model  530  and the potential on the Neg line model  540  during signal transmission. Part (B) of  FIG. 9  is a Waveform showing changes in the differential potential, i.e., the difference between the potential on the Pos line model  530  and the potential on the Neg line model  540 . 
         [0088]    At an initial stage of transient analysis of the circuit model  500  in which the temporary device model  560  is inserted, the maximum potential on the Pos line model  530  is substantially equal to that on the Neg line model  540 , and the minimum potential on the Pos line model  530  is substantially equal to that on the Neg line model  540 , as in signal transmission in the actual transmission circuit in the steady state. Accordingly, line P 2  indicating the change in potential on the Pos line model  530  in the circuit model  500  is superposed generally on line N 2  indicating the change in potential on the Neg line model  540 , as seen in part (A) of  FIG. 9 . As a result, line D 2  indicating the differential potential changes up and down about 0 V, as shown in part (B) of  FIG. 9 . 
         [0089]    Thus, in transient analysis in step S 105  shown in  FIG. 5 , the analysis results matching the actual signal transmission can be obtained in a short time even on the transmission circuit in which a capacitor is used as an AC coupling component. 
         [0090]    After the completion of this transient analysis, the process shown in the flowchart of  FIG. 5  ends. 
         [0091]    In the circuit simulator  400  in this embodiment, as described above, a simulation of even a transmission circuit in which a capacitor is used as an AC coupling component can be performed in a short time since transient analysis on the transmission circuit is executed after promptly changing the state of the circuit model of the transmission circuit into a steady state. The stable potential for changing the state of the circuit model into a steady state is analytically computed by DC analysis in which potentials in the circuit model  500  are computed on the assumption that the transmission circuit is statically stable. Therefore, even an inexperienced person can easily perform the simulation of signal transmission, and a shorter simulation time suffices for the simulation even if the time for such DC analysis is included. That is, the circuit simulator  400  in this embodiment is capable of easily executing a simulation while limiting the analysis time. 
         [0092]    A transmission circuit which transmits a differential signal has been described as an example of the transmission circuit to be analyzed with the circuit simulator of the present invention. However, the present invention is not limited to analysis of such a transmission circuit. For example, a circuit in which a signal is transmitted over a single line may be analyzed by the circuit simulator of the present invention. 
         [0093]    Also, while the transmission circuit described as an example of the transmission circuit to be analyzed with the circuit simulator of the present invention is a transmission circuit in one system, the present invention is not limited to analysis of such a transmission circuit. According to the present invention, a network circuit or the like formed of transmission circuits in plural systems may be analyzed. In the case of simulating such a network circuit, a simulation of the transmission circuit in each system is individually executed.