Patent Abstract:
This invention provides a method for determining leakage current in a CMOS circuit having several devices. It includes the steps of reading a netlist which describes the circuit and includes information on both these devices in the circuit and how these devices are interconnected. Next, an input signal state data file is generated which provides all of the possible input states for the circuit. A determination is made of which devices in the circuit are in an OFF state for each of the input signal states provided. Then the leakage current for each of these devices in the OFF state is computed for each of the input signal states.

Full Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates in general to a method and computer program product for determining circuit leakage current. In particular, the present invention relates to a method and computer program product for determining the leakage current in CMOS circuit elements. 
         [0003]    2. Description of the Related Art 
         [0004]    Power consumption in electronic circuits has historically been a major concern and a significant design consideration. Modern designs address this concern by using CMOS (complementary-symmetry metal-oxide-semiconductor) devices in digital logic circuits. A recent trend to increase performance of these CMOS devices is to manufacture them with lower threshold voltages (lower switching voltages). These lower voltage threshold devices are termed low-Vt (LVT) devices and the regular voltage threshold devices are termed regular-Vt (RVT) devices. In order to increase CMOS logic circuit performance, low-Vt (LVT) devices have been substituted for regular-Vt (RVT) devices. In recent technologies, performance gains of approximately 30% have been seen. In past designs, entire chips or large groups of circuits used LVT devices. Unfortunately, the increased performance is offset by increased static power dissipation due to increased leakage current or (I off ). In one CMOS process, the I off  for an LVT device has been determined to be approximately 40 times larger than that of a RVT device. Trends in microprocessors show that the power dissipation due to leakage current will actually be higher than the power consumed by switching current in future technology generations. 
         [0005]    In order to maximize the performance of circuits without increasing I off  by a factor of 40, applications of LVT devices should be made judiciously. With a carefully designed library that mixes RVT and LVT devices within the same circuit, nearly all of the performance of an all LVT approach may be gained with only ⅕ to ½ of the leakage delta between the two approaches. This circuit style, termed hybrid-Vt, is used to speed up certain transitions to provide a partial speed up of all performance critical transitions. 
         [0006]    What is needed, therefore, is a method that quickly determines circuit leakage current to assist the designer in the choice and placement of LVT and RVT devices. 
       SUMMARY 
       [0007]    In accordance with the present invention, a method is provided for determining leakage current in a CMOS circuit having several devices. This method includes the steps of reading a netlist data file containing information on each of the several devices and their respective connections within the circuit, reading input state data where each input state data entry specifies the input signals that are provided to the circuit for that input state, determining which devices in the circuit are in an OFF state for each of the several input states, computing leakage current for each device in the OFF state according to the device position in the circuit for each of the several input states, and providing a resulting leakage calculation for each input state. 
         [0008]    In a preferred embodiment of the present invention, the netlist information describing the circuit devices and their interconnections also includes information about the device type and the device characteristics including at least information on the device threshold voltage. In addition, another aspect of the preferred embodiment provides that the input signal data includes leading factors that specify the frequency of occurrence of these input signals. 
         [0009]    Also, in accordance with present invention, a computer program product stored in a computer operable media is provided where the computer operable media includes instructions for execution by a computer which when executed by the computer calls the computer to implement a method for determining leakage current in a CMOS circuit having a plurality of devices and the method including the steps of reading a netlist data file containing information on each of the several devices and their respective connections within the circuit, reading input state data where each input state data entry specifies the input signals that are provided to the circuit for that input state, determining which devices in the circuit are in an OFF state for each of the several input states, computing leakage current for each device in the OFF state according to the device position in the circuit for each of the several input states, and providing a resulting leakage calculation for each input state. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
           [0011]      FIG. 1  is a block diagram illustrating the leakage calculation method with its input information and output report; 
           [0012]      FIG. 2  is a flowchart illustrating the functional flow for the leakage current calculations; 
           [0013]      FIG. 3  is a chart illustrating an example netlist file for a two input NAND gate; 
           [0014]      FIG. 4  is a flow chart depicting the transistor level static logic simulation process; 
           [0015]      FIG. 5  is a flowchart depicting the process of determining which devices are in an OFF state and performing the leakage factor lookup function; 
           [0016]      FIG. 6  is a chart illustrating a Leakage Factor Lookup Table; and 
           [0017]      FIGS. 7A ,  7 B,  7 C and  7 D are schematic diagrams of a NAND CMOS circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following is intended to provide a detailed description of an example of the invention and should not be taken to be limiting of the invention itself. Rather, any number of variations may fall within the scope of the invention, which is defined in the claims following the description. 
         [0019]    The present invention is a method and computer program product that determines leakage current in a CMOS circuit. As discussed previously, the use of low voltage threshold CMOS devices (LVT) has increased in order to improve circuit performance. However, these low-voltage threshold devices (LVT) also exhibit high leakage current characteristics. Therefore, in order to maximize performance of CMOS circuits without increasing the power loss due to leakage current, circuits are designed with both regular voltage threshold devices (RVT) and LVT devices. 
         [0020]    For a simple example, if a designer selected a two input NAND gate and desires to substitute LVT devices for certain RVT devices, the designer must determine the circuit location of the specific devices to substitute. To do so, the designer should consider the possible states of the circuit and analyze the circuit accordingly. In this example (see NAND circuit in  FIG. 7A ), there are four transistors resulting in 2 4  possible variations of LVT and RVT devices. Two factors that must be considered are circuit delays, which are usually determined by computer simulation, and circuit leakage current. 
         [0021]    The present invention may be used to determine the circuit leakage current.  FIG. 1  is a block diagram illustrating the present invention. The two inputs to block  100  are the transistor level netlist  102  and the input signal state file  104 . The netlist is a description of the circuit being analyzed that includes information on the circuit devices and their interconnections. An example of a netlist file is illustrated in  FIG. 3 . The input signal state file  104  is shown in  FIG. 1  as being optional. That is because, in the preferred embodiment of the invention, the invention includes the capability to generate all of the input signal state data or input vectors for the circuit under analysis. However, if an input state data file is provided, it can include weight data to reflect the frequency of occurrence for each of the possible input signal states. For the example of a NAND gate, there are 4 different input combinations for input signals A and B that can be applied to the circuit. This is illustrated in Table 1 showing that each input combination has the same occurrence frequency or weight. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Input State Weight 
                   
                   
                 Circuit 
               
               
                   
                 in percentage 
                 Input A 
                 Input B 
                 Output 
               
               
                   
                   
               
             
             
               
                   
                 25% 
                 0 
                 0 
                 1 
               
               
                   
                 25% 
                 0 
                 1 
                 1 
               
               
                   
                 25% 
                 1 
                 0 
                 1 
               
               
                   
                 25% 
                 1 
                 1 
                 0 
               
               
                   
                   
               
             
          
         
       
     
         [0022]    However, due to other factors, all of these input signals sets may not be provided to a circuit so the circuit should not be analyzed for such input signal sets. Additionally, different weighting factors may need to be considered when applying these input signal sets in order to get a more realistic leakage current computation. Therefore, in the preferred embodiment, the invention includes the capability to receive an input signal state file  104  that may include a more realistic representation of the expected input data and this file is then used as the input signal data in the circuit analysis. 
         [0023]    The analysis process labeled in  FIG. 1  as the leakage calculation block  100  determines the leakage current for each of the devices in the CMOS circuit for each of the input signal states. All of the device leakage current data for all of the input signal states is provided as a leakage report  106 . 
         [0024]      FIG. 2  illustrates, in flowchart form, the leakage calculation process in block  100 . Shown, as in  FIG. 1 , are the transistor level netlist block  102  (i.e., example in  FIG. 3 ) and the input signal state file block  104  as inputs to the process in block  100 . Likewise the output report of leakage data for each input state is the leakage report block  106 . In  FIG. 2 , the program block  100  includes a first step  200  that receives the inputs from the transistor level netlist and the input signal state file. However, as previously discussed, if the input signal state file is not provided, the input signal states are automatically generated in step  200 . Obtaining the input signal states is the initial stage of the circuit analysis. 
         [0025]    Next, in step  202 , a transistor level static logic simulation is performed using the transistor level netlist and the input signal state data provided. The transistor level static logic simulation is shown in more detail in  FIG. 3 . The  FIG. 3  illustrated netlist of a NAND-2 contains a listing of all nets which are inputs A &amp; B, output Z, Nwell, Pwell, GND, VDD, and an intermediate node, symNet35 which is node-C in  FIG. 7  series. Also included in the netlist are the Pin definitions. Lastly, the netlist contains the device definitions and their associated connections. For example, device TNA, is defined as an nfet, having a channel width of 0.42 um, a channel length of 0.08 um, and is a single device as is signified by the m=1 term. The gate(G) is connected to input-A, the drain(D) is connected to the output Z, the source (S) connected to symNet35, and the bulk or body(B) is connected to the PW net. Referring to  FIG. 7A , we see this device  504 . The results of the simulation in step  202  is provided to the netlist reduction process in step  204  which reduces the number of circuit paths by determining the paths with devices in an ON state and the paths with devices in an OFF state. If a path includes devices that are all in an ON state, the path is not analyzed since for device leakage calculations, device leakage only occurs when a device is in an OFF state. Then, in step  206 , the process determines for paths with devices in the OFF state, what the leakage current is for each of the devices. This is done by reference to a Leakage Factor Lookup Table such as one illustrated in  FIG. 6 . Upon completion of the leakage factor lookup for each of the OFF state device paths in step  206 , the process determines in step  208  if each input signal state has been analyzed and, if not, the process loops back via line  210  to the transistor level static logic simulation in step  202  to continue with the next input signal state. Once all of the signal states have been analyzed, the leakage data is provided as a leakage report in block  106 . 
         [0026]      FIG. 4  illustrates the internal process of the transistor level static logic simulation step  202 . In the first step, all nodes are reset to a neutral signal level in step  300  (a numeric value of −1 in the preferred embodiment). Then, in step  302 , all of the input nodes are set to the values defined in the current input signal state and the VDD supply voltage nodes are set to 1 and the ground GND nodes are set to 0. However, in the preferred embodiment, multiple VDD supply voltage values may be specified by the netlist, and these specified values would then be assigned to their respective VDD supply voltage nodes. Likewise, if multiple ground node potential values have been specified by the netlist, then those values would be assigned to their respective ground nodes. Next, the input signals are propagated through the devices to determine which devices are in an ON state and which devices are in an OFF state. In step  306 , for all devices in an ON state, the source node value is passed to the drain node. Then, in step  308 , a check is made that all nodes have assigned values. If not, the process loops back via line  312  and renews step  304 . If all nodes have been assigned values, then the process exits to step  310  to provide the netlist with annotated node values. 
         [0027]      FIG. 5  illustrates the process of step  206  in  FIG. 2 . In step  400 , the annotated netlist has values assigned to each node. In step  402  the analysis starts with an N-type or N channel MOS device that is in an OFF state and which is connected to a node having a 0 value, that is, a propagated ground value or a logic 0 value. Step  402  also starts at OFF P-type or P channel MOS devices which are connected to a node having a logic 1 value and proceeds to step  406  wherein the devices are also traversed through the source/drain connections as long as the drain values are neutral (i.e., neither a 1 nor a 0). In step  408 , the leakage factor multiplier for the stack of devices is then determined. Lastly, in step  410 , the leakage current is computed. The total leakage is calculated by multiplying the device width, the state leakage factor per device as dictated by stack position and applied bias factor, and the associated leakage per micron of device width. Thus, LVT devices would have a significantly higher leakage per micron than RVT devices. PFET and NFET device leakage can be independently addressed as each device type and implant have separate leakage currents. 
         [0028]      FIG. 6  illustrates a leakage factor lookup table. This table is used to provide the leakage factor for devices in the OFF state in a particular circuit path. The location of a device in the off state along with the number of other devices in that path that are in the OFF state determine this leakage factor. In addition, the assigned value at the devices source and drain nodes are used to determine this leakage factor. However, consideration must be given to the fact that NFET and PFET devices do not pass voltage potentials in the same manner. For example, a logic 1 value applied to an NFET whose gate is ON would result in the passing of a logic value of only 0.09. Likewise, PFET connected to ground or logic 0 and whose gate is in the ON state would pass a logic value 0.1. Therefore, these nodes logic values are used in the logic factor lookup table of  FIG. 6  along with the device circuit positions to determine the leakage factor. It should be noted that the 0.9 and 0.1 values to represent the weak logic 1 and weak logic 0, respectively, are not actual voltage values, but are just a means to recognize stack configuration and applied drain-source bias. Values of 0.8 and 0.7 are also used for deeper stack recognition. The table of  FIG. 6  is organized in three distinct columns: Node voltage, Device Circuit Positions and Leakage Factor. The node voltage is that value which is applied to the Drain of the transistor stack. The device circuit positions list the devices in the stack associated with their state as a function of position. Lastly, the leakage factor is the scalar adjustment for the leakage current of the OFF device. 
         [0029]    In order to understand the operation of the leakage current calculator invention, a simple example of a two input NAND gate is provided.  FIGS. 7A ,  7 B,  7 C and  7 D are schematics of this two input NAND gate. In  FIG. 7A , the two inputs A and B=0. Therefore, PFET  500  and PFET  502  are both ON and NFET  504  and NFET  506  are OFF. Since both PFET  500  and  502  are ON, the voltage potential VDD=1 is passed to the Output node. Leakage current occurs in transistors that are in the OFF state. Since both  504  and  506  are OFF, node C has a value of −1. Using the lookup table in  FIG. 6  for two OFF devices with a node voltage of 1.0, it is determined that the leakage factor is 0.125. 
         [0030]    Referring to  FIG. 7B , A=0 and B=1 and PFET transistor  508  is in an ON state, NFET transistor  512  is an OFF state, PFET transistor  510  is an OFF state and NFET transistor  514  is in an ON state. A logic 1 is passed to the Output node through PFET  508 . A logic 0 is passed to node C since NFET transistor  514  is in an ON state. Again, the leakage factor table of  FIG. 6  determines that the leakage factor is 1.0. 
         [0031]    Referring to  FIG. 7C , the input A= 1  and the input B=0. Therefore, the PFET  520  is OFF, the PFET at  522  is ON, NFET  524  is ON, and NFET  526  is OFF. As in the examples of  FIGS. 7A and 7B , the Output node is at a logic 1 value since, in this case, PFET  522  is ON. 
         [0032]    However, the logic 1 value is degraded to a logic 0.9 when passed through the ON NFET  524  to node C. Again, accessing the leakage factor table of  FIG. 6 , it is determined that the leakage factor is 0.764. 
         [0033]    Lastly, and  FIGS. 7D , both A and B=1. Therefore, PFETs  528  and  530  are both OFF and NFETs  532  and  534  are both ON. Since both NFETs are ON, the ground zero potential is passed to the Output node. Using the leakage factor table of  FIG. 6 , it is determined that the leakage factor is 1.0. However, since both PFETs  528  and  530  are OFF, the total leakage has to be added for both resulting in a leakage factor of 2.0. Table 2 below illustrates the resulting leakage factors for the different two input NAND gate input signal configurations. The average leakage factor for all four states is equal to 0.972. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Resulting 
               
               
                 Input State Weight 
                   
                   
                 Circuit 
                 Leakage 
               
               
                 in percentage 
                 Input A 
                 Input B 
                 Output 
                 Factor 
               
               
                   
               
             
             
               
                 25% 
                 0 
                 0 
                 1 
                 0.125 
               
               
                 25% 
                 0 
                 1 
                 1 
                 1.000 
               
               
                 25% 
                 1 
                 0 
                 1 
                 0.764 
               
               
                 25% 
                 1 
                 1 
                 0 
                 2.000 
               
               
                   
               
             
          
         
       
     
         [0034]    It should now be understood that determining the leakage factor and resulting leakage current are important considerations when determining which device is to be a RVT device or a LVT device. The proper design would place the LVT device in the circuit location with a smaller current leakage factor for the desired the performance objective. 
         [0035]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that, based upon the teachings herein, that changes and modifications may be made without departing from this invention and its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of this invention. Furthermore, it is to be understood that the invention is solely defined by the appended claims. It will be understood by those with skill in the art that if a specific number of an introduced claim element is intended, such intent will be explicitly recited in the claim, and in the absence of such recitation no such limitation is present. For non-limiting example, as an aid to understanding, the following appended claims contain usage of the introductory phrases “at least one” and “one or more” to introduce claim elements. However, the use of such phrases should not be construed to imply that the introduction of a claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an”; the same holds true for the use in the claims of definite articles.

Technology Classification (CPC): 6