Patent Publication Number: US-6985843-B2

Title: Cell modeling in the design of an integrated circuit

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to the manufacture of integrated circuits and more particularly to the design of a model for an integrated circuit input/output cell. More specifically, the invention relates to a method and a software tool for designing an integrated circuit. 
   DESCRIPTION OF THE RELATED ART 
   Digital circuits, no matter how complex, are composed of a small group of identical building blocks. These blocks can be basic logic gates (AND, OR, etc.), memory cells or other structures. But the majority of digital circuits are composed of gates or combinations of gates. Gates are combinations of high-speed electronic switches or transistors. Memory cells are modified versions of basic logic gates. A flip-flop, for example, can be considered as a function block, but it is composed of interconnected gates. A microprocessor is a central processing unit of a computer or other device using thousands (or millions) of gates, flip-flops and memory cells. 
   It is known to manufacture an integrated circuit using conductors separated by a semi-conductor. Circuits are fabricated on a semiconductor, such as silicon, by selectively altering the conductivity of the semiconductor material. Various conductivity levels correspond to elements of a transistor. Transistors, diodes, resistors, and small capacitors are formed on small chips of silicon. Individual components are interconnected by wiring patterns (typically aluminum or gold) that resemble ordinary printed circuit wiring. Integrated circuits are then mounted on etched circuit boards which are used to assemble electronic systems such as personal computers and other data processing equipment. 
   It is known to use commercially available software to model certain features of integrated circuits. For example, Verilog is a hardware description language (HDL) most predominantly used in the United States. Verilog was originally designed by Gateway Design Automation in approximately 1985. Verilog was made available to the public in 1990 and has been adopted as a standard by the Institute of Electrical and Electronic Engineers (IEEE). Verilog is commonly used to develop a list of functions of an integrated circuit. Verilog is also commonly used to identify or list the number of input/out pins required to support the functions identified. When the list of required functions and the list of I/O pins is combined it is commonly referred to as a “netlist.” A netlist can form a basic specification from which a manufacturer can complete the design and manufacture and integrated circuit. 
   Integrated circuits are designed using computer-aided design (CAD) tools. The integrated circuit design process includes constructing the integrated circuit design out of simple circuits (standard cells) that are connected together electrically using wire interconnects. The standard cells and connections between them are stored in databases called “netlists” (i.e., lists of symbolic interconnections). 
   As part of the design process, the design information within a netlist is “placed and routed” by the CAD tool. The CAD tool utilizes placing and routing processes (also called placers and routers) that are typically software programs executed on the CAD tool. The placer determines the optimum location of each standard cell within the integrated circuit layout on the semiconductor surface. The placement location is optimized to reduce the distance between standard cells that are electrically connected to each other by wire interconnects (e.g., input/output lines). This is done to minimize semiconductor area consumed by the integrated circuit and is also done to minimize the lengths of wire interconnects to reduce net capacitance within the design. The router optimizes the routing of input/output lines between connected standard cells so that areas of the integrated circuit layout do not become overly congested by input/output lines. 
   The IC design is next verified at the logical level, using the functions and the timing characteristics supplied in the cell library, to determine whether the design is functionally correct and meets the desired timing requirements. This testing is typically performed using a logic simulation tool, such as Verilog, and other timing analysis tools. Such tools take into account the estimated capacitive loads of physical (mask) interconnections, cell delay times, sequential cell set-up and hold times and other factors important to achieving an accurate simulation of the IC function and performance. Since capacitive loading due to the physical interconnections is not known at this stage, estimates are used. 
   After logic simulation and timing analysis are successfully completed, cell interconnections are physically routed according to the design netlist. Cell placement and routing are typically automated using a placement and route tool as mentioned above. 
   A short netlist for a simple circuit is shown in Table 1: 
   
     
       
         
             
           
             
               TABLE 1 
             
             
                 
             
             
               Exemplary Netlist 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
          
             
                 
               XOR A B C 
             
             
                 
               XOR C CN1 Y 
             
             
                 
               AND A B CA 
             
             
                 
               AND C CN1 CB 
             
             
                 
               NOR CB CA CN 
             
             
                 
                 
             
          
         
       
     
   
   The netlist defines all of the interconnections between the components of the circuit. Each “signal” which interconnects two or more cells, or which represents an input or output for the entire circuit, is actually a node in the circuit which has been assigned a name. Thus the terms “signal” and “node” are often used interchangeably. In the exemplary netlist shown in Table 1, signals A, B and CN1 are input nodes to the entire circuit, Y and CN are output nodes for the entire circuit, and nodes C, CA and CB are internal nodes. 
   Electronic design automation (EDA) tools were originally designed to simulate logic. As electronic design tools became more popular, vendors began to provide enhanced functions. EDA tools are now used to drive synthesis, timing, simulation, test and other tools. Other vendors of EDA tools are: Cadence Corporation, Providence R.I.; Mentor Graphics, Wilsonville, Oreg.; Snyopsys, Mountainview, Calif.; and Snytest Technologies, Inc., Sunnyvale, Calif. These corporations are listed as examples only, other manufactures use proprietary tools for the same purpose. For example the Silicon Ensemble tool provided by Cadence places and routes wires on the integrated circuit driven by timing constraints. 
   Sub-micron designs of integrated circuit chips require accurate timing analysis to prevent operational errors. These timing errors create operational errors which prevent a design, or a manufactured chip, from accomplishing its intended purpose. It is known to use available software tools to design an integrated circuit and to model the functions and timing of the signals on the circuit. For example, the Ensemble tool provided by Cadence develops a design to place and route wires based on timing constraints for the integrated circuit. 
   Referring to  FIG. 1 , integrated circuit chip  110  with perimeter input/output cells (I/O cells) and core area  140  is shown. Specifically, I/O cell  120  is located on the perimeter of the chip. I/O cell  120  includes I/O pad (input/output pad)  130 . As previously discussed, for commercial reasons it is often advantageous for a chip area to be as small as possible. Reducing the size of a chip necessarily reduces the perimeter. When the perimeter of a chip is reduced the area available on the perimeter of the chip is also reduced. The perimeter area on the chip may have insufficient area to support I/O cells of sufficient number and size. The perimeter area can have an I/O cell with an area insufficient to support the function of the cell. For example, an I/O cell may require a larger area than available on the perimeter of the integrated circuit chip when the area of an I/O cell is reduced. In this instance additional area in the core of the chip is utilized. 
   Commercially available software tools can model an I/O cell on the perimeter of an integrated circuit. However, in some cases insufficient area is available on the perimeter of the integrated circuit chip to support the required number of I/O cells. When the area available on the perimeter of a chip is not sufficient to support the function of an I/O cell it is known to utilize a separate area in the core of the chip as a pre-cell. 
   Referring now to  FIG. 2 , main I/O cell  210  is located on the perimeter of integrated circuit chip  210 . However, in this instance the area available on the perimeter integrated circuit chip  210  is deemed to be less than the area necessary to support the function of the I/O cell. In this case, an additional area, referred to as main I/O cell  210 , has been designated in the core of integrated circuit chip  210  to support the function of the cell previously shown as I/O cell  120  (in  FIG. 1 ). The additional area from the core of the chip is indicated on  FIG. 2  as pre-cell  230 . Thus two areas are allocated to support the function of the I/O cell, an area indicated as main I/O cell  210  and an area indicates as pre-cell  230 . Main I/O cell  210  and pre-cell  230  are connected by internal core connection  265 . I/O pad  215  facilitates connections from an external signal to integrated circuit chip  210 . 
   As described above, an I/O cell can be divided into two cells; a main cell located on the perimeter of the integrated circuit and a pre-cell located in the core area. When an I/O cell is divided into two areas to satisfy perimeter area constraints, available EDA tools can introduce errors into the chip design. What is needed is a method to model an I/O cell which has been divided into two areas; one on the perimeter of the chip and a second area in the core of the chip. If would be further advantageous of the method is applicable to modeling an I/O cell which has been divided into three or more areas including one area on the perimeter of the chip and two or more areas in the core area of the integrated circuit. 
   SUMMARY OF THE INVENTION 
   The invention relates to a method for modeling an input/output cell located on the perimeter of an integrated circuit. When the area available on the perimeter of an integrated circuit is not be large enough to support the required number of input/output cells additional area from the core of the cell can be used to support the function of the perimeter cell. A method is taught to more accurately model the function of the integrated circuit. The input/output cell can be modeled in two locations; one location on the perimeter of the cell and a second location in the interior area, or core, of the integrated circuit. The model uses a cover to prevent the area of the core of the integrated circuit from being used for other purposes. When the input/output cell is divided into a main cell and more than one pre-cell, the model uses a cover for each pre-cell. The model adjusts the timing of the signals to compensate for the input/output cell being divided into two areas. In an embodiment a software tool performs the functions of the model. 
   The foregoing is a summary and this contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. 
       FIG. 1  depicts an integrated circuit in the prior art having a core area and an I/O cell on the perimeter. 
       FIG. 2  depicts an integrated circuit in the prior art having a main I/O cell and a pre-cell. (The main I/O cell and pre-cell replace the I/O cell shown in  FIG. 1 .) An I/O pad and core connection has been added. 
       FIG. 3A  depicts an integrated circuit with a main I/O cell and a pre-cell in accordance with the present invention. Additional pins and core connections have been added to support a second block. In accordance with the present invention,  FIG. 3B  further depicts an outline of a cover added to prevent the pre-cell from being used for another function. 
       FIG. 4A  depicts an integrated circuit with a main-cell and two pre-cells in accordance with the present invention.  FIG. 4B  further depicts the outline of a cover placed over the main cell and the two pre-cells. 
       FIG. 5  is a flow diagram in accordance with the present invention.  FIG. 5  depicts the logical steps of a software tool to model an input/output function which utilizes locations on the perimeter and the interior of an integrated circuit. 
       FIG. 6  is a block diagram of a computer system suitable for implementing embodiments of the present invention. 
   

   DETAILED DESCRIPTION 
   The following sets forth a detailed description of a mode for carrying out the invention. The description is intended to be illustrative of the invention and should not be taken to be limiting. 
     FIG. 3A  depicts one embodiment of an integrated circuit with main I/O cell  310  and pre-cell  330  constructed in accordance with the present invention. I/O pad  315  is used to connect an incoming signal from the package to the integrated chip. Pins  340 ,  350  and  360  have been added to support the addition of a separate pre-cell.  FIG. 3B  further depicts virtual cover  370 . As shown in  FIG. 4 , cover  370  is used to prevent the area designated as pre-cell  330  from being used for any other function. 
   The method taught is applicable to an I/O cell which has been divided into a perimeter area and a plurality of areas in the core of the integrated circuit chip.  FIG. 4A  depicts an integrated circuit with a main-cell and two pre-cells. Pre-cell  480  has been added.  FIG. 4B  depicts the outline of virtual cover  490 . Virtual cover  490  prevents the area designated as pre-cell  330  and  480  from being used for any other purpose. Virtual cover  490  like virtual cover  370  (shown previously in  FIG. 3 ) are not actual physical covers or components. 
   The virtual cover (also referred to as a “cover”) is a software function used to designate the areas occupied by certain pre-cells as not available for use. The virtual cover is not physically placed over the cells but is a conceptual function only implemented within the method taught. The virtual cover is theoretically placed over the I/O and the pre-cell. The virtual cover is a theoretical boundary. 
   In an embodiment, the virtual cover is implemented within the circuit design software tool. When used in conjunction with a software tool, the virtual cover is a software function only. The software function of the virtual cover enables two cells (a main I/O cell and a pre-cell) to be modeled as a single cell. The virtual cover prevents the area designated as pre-cells from being identified by the software tool for use as another function and improves the timing accuracy of the model. The software tool determines the boundary of the cover and reserves the area bounded by the cover. The software tool prevents any other use of this area defined by the virtual cover. 
   Still referring to  FIG. 4B , the virtual cover can be thought of as physically covering pre-cell  230  and  480 . Similarly, virtual cover  490  can be thought of as physically covering all pre-cells associated with main I/O cell  210 . However, as previously explained cover  490  is a virtual cover, not an actual cover, and represents a software function as further explained below. Internal connections  491  and  492  are also shown from pre-cells  230  and  480  to main I/O cell  210 . 
   An Embodiment: 
     FIG. 5  illustrates an embodiment of a software program to complete certain implementations of the present invention.  FIG. 5  is a flow diagram of the logical steps of a software tool to model an input/output cell utilizing areas on the perimeter and the interior of an integrated circuit. Referring to  FIG. 5 , a manufacturer or designer determines to use a software tool to facilitate design of the integrated circuit. When a design begins, (identified as start  505 ) netlist  510  is generated. Netlist  510  facilitates the design of an integrated circuit and lists the functions and corresponding input/output cells required. 
   Still referring to  FIG. 5 , Conversion  515  converts netlist  510  from one proprietary format into the manufacturer&#39;s proprietary format. For example, the first proprietary format could be PWC as used by NEC Electronics, Santa Clara, Calif. 
   Thus PWC  525 , the output of Conversion  515 , represents netlist  510  in a manufacturer&#39;s proprietary format. Flatten  530  changes the hierarchical netlist into a flattened netlist. A flattened netlist refers to the function of the same block multiple times. A flattened netlist lists individual reference blocks without referring to the same reference block three times. Flatten PWC  535  is a database storing netlist  510  in a proprietary format with each block listed individually. Listing each block individually removes the hierarchy otherwise found in netlist  510 . 
   Floor-planner  540  identifies the X and Y coordinate of each I/O cell in the integrated circuit. I/O Expanded DEF  545  includes the physical location of each I/O cell divided into a pre-cell and an I/O cell. LEF  550  is a software library describing the physical description of each cell in the integrated circuit. For example I/O cells, core area cell with the boundary and its physical location of the pin of the cells. 
   MkCell  555  combines the representation of the pre-cell and the main cell into a single I/O cell. (MkCell  555  generates the cover and places the cover over the pre-cells as shown previously in  FIGS. 3  and  FIG. 4 .) MkCell  555  provides input for Cell-Rebuilt DEF  567  and Cell-Macro Defined LEF  565 . Cell-Rebuilt DEF  567  combines netlist of pre-cells and main cells and provides input to SE Flow  570 . Cell-Macro Defined LEF  565  is the library describing the merged pre-cell and main cell and also provides input to SE Flow  570 . Complete LEF library  572  are other library elements including all other cells used in the netlist. 
   SE Flow  570  is the entire placement and routing of wires for the integrated circuit. SE Flow  570  provides input to Routed DEF  578  and Netlist with new cell  580 . Routed DEF  578  is the file that includes placement and routing information for each cell and wire on the integrated circuit. Netlist with new cell  580  is the new netlist including changes to the netlist during SE Flow  570 . Netlist with new cell  580  provides input to Cell suffix removal  585 . Cell suffix removal  585  removes the suffix of the original I/O cell type. Cell suffix removal  585  provides input to Netlist with original cell  590 . (The suffix is removed so that the cell type will match the cell type used in netlist from user  510 .) Netlist with original cell  590  is a netlist from  580  without a suffix which matches the cell type used in netlist from user  510 . 
   An Example System for Implementing the Method 
   The present disclosure is applicable to any integrated circuit including data processing systems. Integrated circuits may be found in many components of a typical computer system, for example a central processing unit, memory, cache, audio controller, network interface, I/O controller and I/O device as shown in the example below. Integrated circuits are found in other components within a computer system such as a display monitor, keyboard, floppy and hard disk drive, DVD drive, CD-ROM and printer. However, the example of a computer system is not taken to be limiting. Integrated circuits are ubiquitous and are found in other electrical systems such as stereo systems and mechanical systems including automobiles and aircraft. 
     FIG. 6  is a block diagram of an exemplary computer system  630 .  FIG. 6  is intended to be illustrative of a computer system and should not be taken to be limiting. Computer system  630  includes central processing unit (CPU)  632  connected by host bus  634  to various components including main memory  636 , storage device controller  638 , network interface  640 , audio and video controllers  642 , and input/output devices  644  connected via input/output (I/O) controllers  646 . Those skilled in the art will appreciate that this system encompasses all types of computer systems including, for example, mainframes, minicomputers, workstations, servers, personal computers, Internet terminals, network appliances, notebooks, palm tops, personal digital assistants, and embedded systems. 
   Typically computer system  630  also includes cache memory  650  to facilitate quicker access between processor  632  and main memory  636 . I/O peripheral devices often include speaker systems  652 , graphics devices  654 , and other I/O devices  644  such as display monitors, keyboards, mouse-type input devices, floppy and hard disk drives, DVD drives, CD-ROM drives, and printers. Many computer systems also include network capability, terminal devices, modems, televisions, sound devices, voice recognition devices, electronic pen devices, and mass storage devices such as tape drives. The number of devices available to add to personal computer systems continues to grow, however computer system  630  may include fewer components than shown in  FIG. 6  and described herein. The peripheral devices usually communicate with processor  632  over one or more buses  634 ,  656 ,  658 , with the buses communicating with each other through the use of one or more bridges  660 ,  662 . 
   The method disclosed is not restricted to a specific software, software language or software architecture. Each of the steps of the method disclosed may be performed by a module (e.g., a software module) or a portion of a module executing on a computer system. Thus, the above component organization may be executed on a desk top computer system or other appropriate system. The method may be embodied in a machine-readable and/or computer-readable medium for configuring a computer system to execute the method. Thus, the software modules may be stored within and/or transmitted to a computer system memory to configure the computer system to perform the functions of the module. 
   The operations described above and modules therefor may be executed on a computer system configured to execute the operations of the method and/or may be executed from computer-readable media. The method may be embodied in a machine-readable and/or computer-readable medium for configuring a computer system to execute the method. 
   Those of skill in the art will recognize that, based upon the teachings herein, several modifications may be made to the embodiments shown in  FIGS. 1–6 . For example,  FIG. 6  is used as an example of a computer system containing an integrated circuit. Other electronic devices such as radios, telephones, televisions, calculators and automobiles contain integrated circuits which are subject to including an integrated circuit manufactured by the method disclosed. 
   While particular embodiments of the present invention have been shown and described, it will be recognized to those skilled in the art that, based upon the teachings herein, further changes and modifications may be made without departing from this invention and its broader aspects, and thus, 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.