Patent Document

BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to integrated circuits. In particular, the present invention relates to a method for adding decoupling capacitance in an integrated circuit during the floor planning stage of integrated circuit design. 
     2. Related Art 
     One current trend in semiconductor design, especially for application specific integrated circuits (ASICs) and other advanced/complex semiconductor integrated circuit devices, such as microprocessors, is to lower the operating power. This trend drives power supply and device threshold (i.e., turn-on) voltages to lower levels. Another trend emphasizing the need for decoupling is that voltage scaling has lagged area/capacitance scaling. As the power grid supply voltage (VDD) and device threshold voltage (Vt) drop, the ratio of noise voltages to Vt and VDD increases, since noise levels do not scale down at the same rate as Vt and VDD. Consequently, sensitivity to noise in these types of semiconductor integrated circuit devices increases. 
     The increased sensitivity to noise is further exacerbated by local drops in VDD caused by local high current use. This may be caused, for example, by high current circuits or high duty cycle circuits. The effect is that some circuits or groups of circuits do not see the full VDD voltage for a short period of time, further increasing the noise to VDD ratio. Often, such a noise sensitivity problem does not become apparent until very late in the design process, or during actual device fabrication or testing, leading to expensive and time consuming remodeling, simulation, and/or design activity. Currently, the noise sensitivity problem is often overcome by over-designing the integrated circuits to make them more tolerant of noise or power drops. Unfortunately, this solution often results in decreased performance, increased power consumption, increased chip area, and more expensive chips. 
     One method of compensating for local power grid voltage drops is through the use of decoupling capacitors. The amount of decoupling capacitance required is a local requirement dependent upon such factors as the number of nearby circuits that are switching at one time and the sequence of the switching. The current state of the art accomplishes the task of adding decoupling capacitance to an integrated circuit design using one of two methods. In one method, the amount of decoupling capacitance per input/output (I/O) cell or clock buffer is estimated and then placed near the corresponding circuit. For the rest of the design, rule of thumb methods are used. One is to place the decoupling capacitors close to each latch. Another is to place decoupling capacitors on empty spaces in the chip area after placement. This often results in too much decoupling capacitance being added to the design. In another method, the amount of decoupling capacitance is calculated based on power drops in the grid voltage for the completed circuit as designed. Decoupling capacitors are then placed in those regions of the macros of the chip where there is space for them. One drawback of this method is that available space may not be close enough to where the decoupling capacitance is needed, requiring more decoupling capacitance to eliminate the voltage drop. 
     Thus, traditional methods of adding or placing decoupling capacitance in an integrated circuit often create other problems that can only be addressed after the design of an integrated circuit is complete. These include, inter alia,: wasted chip space and increased power requirements due to excess decoupling capacitance (and the support circuitry (e.g., switching buffer trees) required to implement the decoupling capacitance); and inadequate compensation for power drops in the grid voltage caused by the post-design placement of decoupling capacitance too far away from the circuit that requires it, increasing wiring requirements. 
     A need therefore exists for a method for adding decoupling capacitance during the design of an integrated circuit which solves these and other problems associated with currently available capacitance placement techniques. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for adding decoupling capacitance in an integrated circuit during the floor planning stage of integrated circuit design. 
     Generally, the present invention provides a method for adding decoupling capacitance in an integrated circuit design, comprising: 
     creating a floor plan for an integrated circuit, the floor plan comprising the relative locations of a plurality of functional units; 
     overlaying a power grid on the floor plan; 
     dividing the floor plan and the power grid into a plurality of regions; and, for each region: 
     determining a support decoupling capacitance value required to support a voltage of the power grid; 
     determining a native capacitance value; 
     determining a required decoupling capacitance value based on the support decoupling capacitance value and the native capacitance value; 
     determining a decoupling capacitor area for the required decoupling capacitance value; and 
     modifying a circuit area in the region based on the decoupling capacitor area. 
     The present invention additionally provides a method, comprising: 
     creating a floor plan for an integrated circuit; 
     dividing the floor plan into a plurality of regions; and, for each region: 
     determining a support decoupling capacitance value required to support a voltage of a power grid of the floor plan; 
     determining a native capacitance value; 
     determining a required decoupling capacitance value based on the support decoupling capacitance value and the native capacitance value; 
     determining a decoupling capacitor area for the required decoupling capacitance value; and 
     modifying a circuit area in the region based on the decoupling capacitor area. 
     The present invention further provides a computer program product, comprising: 
     a computer usable medium having a computer readable program code embodied therein for performing a method for adding decoupling capacitance in an integrated circuit design, the computer readable program code including: 
     code for causing a computer system to create a floor plan for an integrated circuit; 
     code for causing a computer system to divide the floor plan into a plurality of regions; and, for each region: 
     code for causing a computer system to determine a support decoupling capacitance value required to support a voltage of a power grid of the floor plan; 
     code for causing the computer system to determine a native capacitance value; 
     code for causing the computer system to determine a required decoupling capacitance value based on the support decoupling capacitance value and the native capacitance value; 
     code for causing the computer system to determine a decoupling capacitor area for the required decoupling capacitance value; and 
     code for causing the computer system to modify a circuit area in the region based on the decoupling capacitor area. 
     The foregoing and other features of the invention will be apparent from the following more particular description of the embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
     FIG. 1 illustrates a floor plan for an integrated circuit chip; 
     FIG. 2 illustrates the floor plan of FIG. 1 with a power grid overlaid thereon; 
     FIG. 3 illustrates a flowchart illustrating the method of the present invention; 
     FIG. 4 illustrates the floor plan of FIG. 2 divided into a plurality of regions, in accordance with the present invention; and 
     FIG. 5 illustrates a representative computer system for practicing the method of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Although certain embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc. Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale. 
     An exemplary floor plan  12  for an integrated circuit chip  10  is illustrated in FIG.  1 . The floor plan  12  comprises the relative locations of a plurality of functional units  14 , often referred to as “macros,” arranged in a logical relationship to one another to provide a specific function. Many products are available for generating the floor plan  12 , including Cplace™ and MCplace™ available from International Business Machines Corporation. In FIG. 1, the heavy lines  16  are used to illustratively define the boundaries of the macros  14  within the floor plan  12 . 
     Each macro  14  provides a predesigned and preverified function, and has well established functional, operational, and electrical characteristics, including, e.g., a built-in decoupling capacitance space requirement (i.e., the amount of area in the macro available for the placement of decoupling capacitors). As illustrated in the flowchart  20  of FIG. 3, a plurality or collection of different macros  14  are typically provided by a macro vendor in a macro library  22 . The floor plan  12  for the integrated circuit chip  10  is synthesized  24  in a known manner by mapping the desired circuit functions and requirements  26  for the integrated circuit chip  10 , typically defined in a hardware description language (HDL) such as VHDL or Verilog®, to a set of the macros  14  provided in the macro library  22 . These circuits may range from the simplest functions, such as inverters, NAND and NOR gates, and flip-flops and latches, to more complex circuits such as adders, counters, mixed signal circuits, phase-lock-loops, memory arrays (e.g., SRAM), digital signal processors, microcontrollers, and even microprocessors. 
     A power grid  28  for the floor plan  12  is generated in a known manner and is overlaid or otherwise incorporated into the floor plan  12 . The power grid  28  has grid-type structure usually with the same periodicity on all metal layers. In some cases, it is possible to have different power grid pitches within the same floor plan if pre-designed macros such as cores or arrays are used. The resultant structure is shown schematically in FIG. 2, wherein the power grid  28  is represented by lines  30 . It should be noted that the power grid  28  illustrated in FIG. 2 is only representative of the fact that a power grid is present, and is not intended to represent the actual structure or layout of a power grid. 
     In step  30  of FIG. 3, the native capacitance per macro  14 , and/or native capacitance distribution per macro  14 , is determined. The native capacitance refers to the naturally occurring supply rail (e.g., VDD) to ground capacitance which exists in the circuits comprising the macro  14 . The native capacitance for each macro  14  in the macro library  22  may be predefined for each macro  14 , using well known methods, and/or may be determined as necessary, also using well known methods. 
     The native capacitance of a macro  14  is typically dependent upon many factors, including the design and layout of the devices and circuits forming the macro  14 , and the semiconductor technology used to implement the macro  14 . For example, in silicon devices formed on a bulk silicon substrate, the native capacitance may comprise n or p diffusion junction capacitance to the substrate or nwell, respectively, the capacitance of grounded sources, the static capacitance load arising from unused gates, etc. Many other sources of native capacitance may also be present. 
     In step  32  of FIG. 3, and as illustrated in FIG. 4, the floor plan  12  of the integrated circuit chip  10  is divided into a grid having a plurality of regions  18 . In the embodiment illustrated in FIG. 4, for example, the entire floor plan  12  is divided into a grid having a plurality of equal size regions  18 . In an alternate embodiment of the present invention, the size of the regions  18  may vary for different macros  14 , depending, for example, on topological power dissipation differences. In another alternate embodiment of the present invention, one or more macros  14  may not be divided into regions  18 . This may occur, for example, if the macro  14  has a large relative size (e.g., the macro  14  comprises a SRAM) and the correct decoupling capacitance has already been built into the macro  14 . 
     The size of the grid regions  18  is determined based on decoupling capacitor and supply impedance  34 . In one embodiment of the present invention, it is assumed that the power VDD is being fed to the design represented by the floor plan  12  through pins on the boundary of the chip. If the power grid is uniform, the longest distance between a cell in the design and a power pin is at the center of the chip. The supply impedance comprises the resistance seen by the cell looking toward the VDD pin. The time to charge a capacitance (cell) at the center of the chip is a function of the size of the capacitance (cell intrinsic capacitance and load) and the resistance between VDD and the cell location. Likewise, if the decoupling capacitance is viewed as a temporary power supply, the previous relationship applies. Because the decoupling capacitor is closer to the cell, however, the resistance between the decoupling capacitor and the load capacitance is smaller. How much smaller is a function of the distance between them. The size of the grid regions  18 , therefore, is a function of how far a decoupling capacitance can be away from the load capacitance and still be effective in preventing the power grid from dropping under a predetermined voltage. At a minimum, the size of the grid regions  18  is the size of the regions of the power grid. A less dense grid (i.e., a grid having larger grid regions  18 ), however, can be used in order to make the determination of required decoupling capacitance faster yet still accurate. One way to find the size of the grid is to peak one decoupling capacitance from the cell library (the largest for example for the most coarse grid) and determine how much load it can drive faster (preventing to much voltage drop) than the same load being driven by the power grid. If the load is translated into an average number of cells per region, the distance of the furthest cell from the decoupling capacitor can be determined. If the decoupling capacitor is placed at the center of a circle, for example, the diameter of the circle would be the diagonal of a grid region, and the grid pitch would be the largest square internal to the circle. For simplicity, the calculated grid region can be adjusted to be a multiple of the power grid. Repeating the same process for the smallest decoupling we get an upper bound on the grid size (the previous being the lower bound or less dense grid). In case the power VDD is being fed to the design not from the periphery but with a uniform distribution within the chip, the previous description still applies. However, the typical impedance from the power supply is smaller than before. 
     In step  36 , after the size of the grid regions  18  has been established, the current requirements of the circuits located within each of the regions  18  are determined for various predetermined operational conditions (including, for example, activity factor based on average instruction stream, DC circuit topologies, as well as standby currents). Many circuit simulation tools exist for determining the current requirements within areas of an integrated circuit chip, including the program PowerMill® available from Synopsys. Generally, such tools calculate the AC and DC current draw or power requirements of discrete regions of an integrated circuit chip under simulated operational conditions. 
     In step  38 , which may be performed in parallel with step  36 , the native capacitance value within each region  18  is determined based on the native capacitance of the macro  14  in which the region  18  is located, as determined in step  30 . 
     A support decoupling capacitance value required to support a voltage of the power grid  28  in each region  18  under predetermined current conditions (as determined in step  36 ) is provided in step  40 . For example, the support decoupling capacitance value may be chosen to support the power grid voltage within a predetermined, application specific, range within each region  18  for all circuit elements operating at predetermined current levels (e.g., average or maximum operational current levels) within the region  18 . The required decoupling capacitance value in each region  18  is then calculated by subtracting the native capacitance value determined for that region in step  38  from the support decoupling capacitance value for that region determined in step  40 . 
     In step  42 , the area required to implement the required decoupling capacitance value in each region  18  is determined, based on the applicable design rules  44  for the decoupling capacitors. Decoupling capacitors are then added to the design to provide the required decoupling capacitance value. The design rules  44  for the decoupling capacitors are typically dependent upon several factors, including gate oxide integrity, diffusion isolation defect control, minimum spacing between polysilicon anode plates, polysilicon and diffusion sheet resistance, poly/diffusion contact technology, etc. 
     Once the required decoupling capacitor area for each region  18  is determined in step  42 , the process flow in FIG. 3 branches at step  46  depending upon the type of integrated circuit chip  10  that is being designed. The branching at step  46  is provided because of the differences (examples of which are described infra) in decoupling capacitor accounting built into the design methodology commonly employed for different chip types. For example, for ASICs (branch  48 ), a decoupling capacitance space requirement is typically built into each macro  18 , while for microprocessors (branch  50 ), decoupling capacitance is typically added in empty regions of the macros  18 , or even the whole integrated circuit chip  10 . An additional branch  52  is provided for integrated circuit chips, such as digital signal processors (DSPs), analog communication chips, microcontrollers, etc., that may not fall under the above-described typical decoupling capacitance accounting practices of either ASICs or microprocessors. Branch  52  may comprise, for example, a modified version of the steps performed in branches  48  or  50 , a combination of a set of the steps performed in branches  48  and  50 , etc. It should also be noted that ASICs and microprocessors are not strictly limited to following the flow defined in branches  48  and  50 , respectively. Indeed, depending on the specific design methodologies and decoupling capacitance accounting practices that are followed for particular ASIC and microprocessor designs, and well as other possible factors, flow for either type of integrated circuit chip may follow one or more of the branches  48  and  50  in the flowchart  20 . Other chip types which may not necessarily fall under the categories of ASIC or microprocessor chips, may also follow one or both of the branches  48  and  50  in the flowchart  20 . 
     In the ASIC branch  48 , the area available for circuits in each region  18  is reduced in step  54  by the required decoupling capacitor area for that region  18 . For example, if the design rules of a macro  14  (and/or of the integrated circuit chip  10 ) indicate that  40  circuits could be placed within a region  18 , but the required decoupling capacitor area for that region  18  is equivalent to the area occupied by  10  circuits, a design placement rule exception for that region  18  is created (step  56 ) that limits the circuit placement to  30  circuits. Conversely, if an excessive amount of native capacitance is determined for a region  18  in step  38 , resulting in a negative required decoupling capacitor area for that region  18  in step  40 , then additional circuits could be placed in that region  18 . Again, a design placement rule exception for that region  18  is created in step  56  to increase the circuit placement value for that region  18 . This process is performed for each region  18  of the integrated circuit chip  10 . Thereafter, in step  58 , a new floor plan  12 ′ is created based on the design placement rule exceptions generated in branch  48  of the flowchart  20 . 
     In the microprocessor branch  50 , the total required decoupling capacitor area for each macro  14  is determined in step  60  by summing up the required decoupling capacitor area calculated in step  42  for each region  18  comprising the macro  14 . The total required decoupling capacitor area for each macro  14  is then compared in step  62  with the decoupling capacitor area available for that macro  14 . If there is excess available decoupling capacitor area in a macro  14  (i.e., the available decoupling capacitor area minus the total required decoupling capacitor area yields a positive result), flow proceeds along branch  64  to step  66 . In step  66 , the space allocated to that macro  14  is reduced, for example, by altering the size, layout, and/or design of the macro  14 . If it is determined in step  62  that there is not enough available decoupling capacitor area in a macro  14 , flow proceeds along branch  68  to step  70 , where additional decoupling capacitor space is added to that macro  14 . This may be accomplished, for example, by providing additional space along the edges of the macro  14  for the placement of decoupling capacitors. These steps are performed for each macro  14  in the floor plan  12  of the integrated circuit chip  10 , and the floor plan is modified in step  72  as necessary. 
     A representative computer system  100  for practicing the decoupling capacitor placement method of the present invention, is illustrated in FIG.  5 . The computer system  100  comprises at least one processor  102  (e.g., a central processing unit (CPU)). The processor  102  is interconnected via bus  104  to a random access memory (RAM)  106 , read only memory (ROM)  108 , and input/output (I/O) adapter  110  for connecting various peripheral devices  112  to the bus  104 . The peripheral devices  112  may include, inter alia, hard drives, compact disc (CD-ROM, CD-R, CD-RW) drives, tape drives, floppy disc drives, scanners, printers, video cameras, etc. A user interface adapter  114  is provided for connecting at least one user input device, such as a keyboard  116 , mouse  118 , microphone  120 , speaker  122 , etc., to the bus  104 . A display adapter  124  is provided to interconnect at least one display device  126  to the bus  104 . At least one data network  128  is connected to the bus  104  via one or more communication adapters  130 . While FIG. 5 shows the computer system  100  as a particular configuration of hardware and software, any suitable configuration of hardware and software, as would be known to a person of ordinary skill in the art, may be utilized to perform the decoupling capacitor placement method of the present invention. 
     Another embodiment of the decoupling capacitor placement method of the present invention, such as the embodiment detailed in FIG. 3, can be implemented as one or more sets of instructions (e.g., computer readable program code). The instructions may be resident in the random access memory  106 , or other suitable location, of one or more computer systems  100  configured generally as described with reference to FIG.  5 . Until required by the computer system  100 , the set of instructions may be stored in a removable or fixed computer usable medium, including, for example, a hard drive, floppy disk, magnetic tape, compact disc, optical disc, etc. The computer usable medium may be connected directly to the bus  104 , or may be accessed through a suitable device, such as CD-ROM drive, floppy disk drive, etc. Further, the sets of instructions may be stored in the memory of another computer, or in a computer usable medium attached to another computer, and transmitted over a network, such as a local area network (LAN), wide area network (e.g., the Internet), etc. One skilled in the art would appreciate that the physical storage of the sets of instructions physically changes the medium upon which it is stored electrically, mechanically, or chemically, such that the medium carries computer usable information. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.

Technology Category: 3