Abstract:
A method and apparatus for allocating decoupling capacitance in a power grid is provided. A power grid model of an RLC network is evaluated to identify the power nodes that have peak voltages exceeding a minimum voltage threshold and a maximum voltage threshold. For each noisy node that violates the thresholds, a decoupling capacitor is inserted. The power grid is once again re-evaluated to determine of any of the power nodes still violate the voltage thresholds. For those violating power nodes, the size of the decoupling capacitor is incremented. This process of incrementing the size of the decoupling capacitance and re-evaluating the power grid is repeated until all the power nodes are within a noise budget or the noise level for each power node is within the thresholds. The decoupling capacitance is then re-evaluated as it is iteratively decremented while maintaining the power noise within the thresholds. This method of inserting and sizing decoupling capacitance allows the power noise thresholds, to be satisfied while optimizing the size of the decoupling capacitance.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to field of integrated circuit (IC) design. More specifically, the present invention is directed to a method and apparatus for sizing and insertion of decoupling capacitance. 
     BACKGROUND 
     High-speed digital integrated circuits (ICs) require a stable reference voltage, regardless of the input current required. However, spike currents generated during switching can cause Vcc and ground (GND) voltage level to fluctuate, causing ringing in the output waveform or a delay in response speed. The switching noise is caused by changes in current through various parasitic inductances. The simultaneous switching of I/O drivers and internal circuits can increase the voltage drop on the power supply. This power supply noise not only will introduce additional signal delay, but also may cause false switching of logic gates. Decoupling capacitors are used to reduce the impedance between power and ground, minimizing the effects of current spikes and board noise on the IC, keeping power supply within specification, and providing signal integrity. 
     For low-frequency power drop problems, it may be adequate to use only the off-chip decoupling capacitors. However, for high-frequency power drop problems, the on-chip decoupling capacitor is more effective due to its proximity to the switching activities. A problem with having on-chip decoupling capacitance to control the power noise is the size of the decoupling capacitors. The size of the decoupling capacitors can consume a significant part of die size, ranging generally from 3% to 15% of the die size. 
     One existing method of sizing decoupling capacitance uses a maximum-frequency analysis. Another method of sizing decoupling capacitance uses a worst-case pre-allocation technique where allocation is based on a percentage of layout area in a specific layout window. The percentage of the layout area and the layout window are given as design guidelines. The design guidelines are determined based on the study of the power noise budget and power supply network topologies on chip and on package. For example, the design guideline specifies that within 200 μm layout window, 10-20% area should be used for the decoupling capacitance. Generally, these methods will result in a conservative estimate of the area reserved for decoupling capacitance. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a method for allocating decoupling capacitance in a power grid is provided. The power grid is evaluated to identify noisy nodes. For each noisy node that violates voltage thresholds, a decoupling capacitor is inserted. The power grid is re-evaluated to determine the power nodes that still violate the voltage thresholds. The size of the decoupling capacitor at each noisy node is incremented until the corresponding power node is brought to within the voltage thresholds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example in the following drawings in which like references indicate similar elements. The following drawings disclose various embodiments of the present invention for purposes of illustration only and are not intended to limit the scope of the invention. 
     FIG. 1 shows a graphical representation of the relationship between the voltage level and the chip size in high-speed designs and in low power designs. 
     FIG. 2 illustrates an exemplary diagram of a prior art technique of device size preallocation for decoupling capacitance. 
     FIG. 3 illustrates an exemplary flow diagram of a process to identify noisy nodes and to size decoupling capacitance to be inserted at the noisy nodes in accordance with one embodiment of the present invention. 
     FIG. 4 illustrates an exemplary diagram of an embodiment where the decoupling capacitance is added only for noise violation nodes. 
     FIG. 5 illustrates exemplary waveforms of power nodes that violate noise margin. 
     FIG. 6 illustrates exemplary waveforms of power nodes after insertion of decoupling capacitance with all nodes having no violation of noise specification. 
     FIG. 7 illustrates an exemplary flow diagram of another embodiment of a method to insert and to size decoupling capacitance where the decoupling capacitance can be reduced at the last stage. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description sets forth numerous specific details to provide a thorough understanding of the invention. However, those of ordinary skill in the art will appreciate that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, protocols, components, algorithms, and circuits have not been described in detail so as not to obscure the invention. 
     Some portions of the detailed descriptions which follow are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of processes leading to a desired result. The processes are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. 
     The present invention also relates to apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type; of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. 
     FIG. 1 illustrates in graphical representation the correlation between supply voltage and chip size in high-speed chip design and in low-power chip design. The y-axis represents the supply voltage or Vdd. The x-axis represents the minimum feature size. Graph  101  represents the correlation between the supply voltage and minimum feature size of the chip processing technology in high-speed designs such as high performance microprocessor. As the amount of transistors per chip increases or as the size of the chips decreases, the supply voltage level is reduced correspondingly to accommodate the smaller transistors. Graph  102  represents the similar correlation in low power designs. 
     FIG. 2 illustrates a power grid using the prior art method of pre-allocating decoupling capacitance. The decoupling capacitors are evenly distributed in the power grid. In this example, five percent (5%) of the layout area is reserved for the decoupling capacitance in each layout window. This technique of setting a pre-allocated percentage is inefficient as it assumes the power noise is roughly the same at all nodes in the power grids. Therefore, using this technique will typically result in some over estimation of the power noise, and as a result may result in over-estimation of decoupling capacitance and sacrifice of layout density. 
     Generally, the determination of the number of decoupling capacitors and the size of the decoupling capacitors is based upon the maximum switching rate of the transistors and the power supply noise that the internal logic can tolerate. Although there may be multiple noisy power nodes having the power drop ΔV that goes beyond a voltage threshold, for example, a minimum voltage threshold, not all of these noisy power nodes have the same peak level voltages. Furthermore, there may be noisy power nodes that are within the maximum voltage threshold and the minimum voltage threshold. Therefore, it may be that not every power node needs to have a similarly sized decoupling capacitors, if at all, as is currently done in the prior art shown in FIG.  2 . 
     In one embodiment of the present invention, the power grid is examined to identify those power nodes that violate the thresholds. Depending on the application, the thresholds may be set such that the power supply noise at a power node can fluctuate within a certain range and still be considered to be tolerable. For example, in one embodiment, the voltage drop ΔV may be considered acceptable if it is between 3 to 10 percent of the power supply voltage. After the violating power nodes are identified, decoupling capacitors are placed at the violating power nodes and a process of sizing the decoupling capacitors is performed until the voltage drop at the violating nodes are within the thresholds. 
     FIG. 3 is an exemplary flow diagram showing a method of one embodiment. At block  301 , a power grid RLC model is evaluated to identify the power nodes that violate the voltage thresholds or noise margin. In general, peak voltages in a power node are acceptable if they are within a predetermined percentage deviation of the nominal voltage level or Vdd. For example, for a nominal Vdd being 1.5V, and with a 10% deviation, the voltage peaks within a range of 1.3V to 1.8V are considered acceptable. In this example, the noise margin or voltage thresholds are from 1.3V to 1.8V. Thus any power node that has peak voltages larger than 1.8V or less than 1.3V is considered to be a noisy node. The identification of noisy nodes is performed by generating scenarios that have worst-case circuit switching. 
     After the violating power nodes are identified, decoupling capacitor is added to each of the violating power nodes. At block  302 , the decoupling capacitors are inserted into the noisy nodes of the power grid. In one embodiment, a predetermined decoupling capacitor of a certain size is used. No decoupling capacitor is necessary in the other power nodes that do not violate the noise margin. Since each power node may have different voltage peaks, the decoupling capacitors are sized accordingly to bring the violating power nodes or noisy nodes to within the required noise margin, as shown in block  303 . By incrementally adding the decoupling capacitors to the noisy nodes, eventually the peak voltages of these noisy nodes will be brought to within the noise margin. As a result, the power grid will have a smaller amount of required decoupling capacitance, reducing over estimation of decoupling capacitance while keeping the number of noisy nodes low. That is, all the nodes in the power grid have the peak voltages within the noise margin. 
     FIG. 4 illustrates an exemplary power grid having decoupling capacitors  401  inserted using the technique according to one embodiment. The decoupling capacitors  401  are added only for noisy nodes or violating nodes, and are not distributed evenly as in the prior art. The placement of the decoupling capacitors  401  in the current invention increases the device area while maintaining power noise budget. As illustrated in FIG. 4, there is not a decoupling capacitor  401  at every power node. Only the nodes highlighted are considered noisy nodes and need to have decoupling capacitors  401  inserted. 
     In the present embodiment, a power grid model is a RLC network. The RLC network is defined by a layout extraction tool or by power grid designers. If the RLC network is too big, each power line can be modeled as a series of RLC elements. These RLC elements can then be linked with one another based on the connections of the power grid. A transistor circuit model is used with the power grid RLC model. Each transistor connected to the power grid can be modeled as a current source. Current simulation is done for the transistor circuit to get the current source model. In one embodiment, a linearized ramped current source model is used for each transistor. Other current source models may be used without departing from the scope of the current invention. 
     In one embodiment, using the power grid RLC model, the transistor circuit model, and the current source model as input, a Power Noise Verifier (PNV) tool is used to analyze the power grid in functional blocks or units. Alternatively, other tools designed to provide the same functionality as the PNV tool may also be used. The PNV tool detects those nodes in the power grids that violate the power noise margin. In one embodiment, an input configuration file is used to provide the tolerable noise margin, including a maximum voltage threshold and a minimum voltage threshold. For example, the tolerable voltage level for each power node is between 1.15V and 1.5V. Therefore, a power node having voltage peaks larger than 1.5V or lower than 1.15V is considered a violating node. The PNV tool gets the specified voltage thresholds from the configuration file and tests each power node against the threshold voltages to identify the violating nodes in power grids. 
     An example of the violating power nodes identified by the PNV tool is shown in voltage waveforms in FIG.  5 . In this example, the minimum voltage threshold is set at 1.15V. The first violating node corresponds to waveform  501  and is found to have a minimum peak voltage level at approximately 1.14V. The second violating node corresponds to waveform  502  and is found to have a minimum peak voltage level at approximately 0.47V. 
     In one embodiment, a Decap Sizing Analyzer (DSA) tool inserts and calculates the sizes of the decoupling capacitors at the noisy nodes. Alternatively, other tools providing the same functionality may also be used. The DSA gets the voltage simulation results for the nodes of the power grids from the PNV tool outputs. The DSA reads the configuration file to retrieve the allowable voltage thresholds and the decoupling capacitor increment size. Based on the identification of violating nodes from the PNV tool, the DSA uses the decoupling capacitor increment information from the configuration file and increments the size of the decoupling capacitor at each violating node in the power grid. The PNV tool is then applied to the power grid to identify power nodes that still violate the specified thresholds. The size of the decoupling capacitor for each violating node is incremented until the peak voltages at the power nodes are within the specified thresholds. The threshold information is specified in the configuration file. 
     This process of applying the PNV tool and the DSA tool iterates until there are no violating nodes in the power grid or until all the power nodes are within the voltage thresholds. The size of the decoupling capacitor for each violating node and the location of the violating nodes are reported in an output file. FIG. 6 illustrates exemplary voltage waveforms for three power nodes after applying the method of one embodiment of the present invention. Waveform  601  has a minimum peak voltage at approximately 1.28V. Waveform  602  has a minimum peak voltage at approximately 1.21V. Waveform  603  has a minimum peak voltage at approximately 1.18V. The power nodes represented by the waveforms  602  and  603  are the same power nodes represented by waveforms  501  and  502  in FIG.  5 . By applying the DSA tool, the necessary decoupling capacitor is added to each node and sized to bring the minimum peak voltages to within the specified minimum threshold voltage of 1.15V. In this example, the power node represented by waveform  603  has the lowest peak voltage at approximately 1.18V. 
     FIG. 7 illustrates, in another embodiment, an exemplary flow diagram showing how the technique of one embodiment is used to improve the insertion and sizing of decoupling capacitance in the power grid. In this embodiment, the insertion and sizing of the decoupling capacitor is improved by first incrementing the size of the decoupling capacitors and then decrementing the size of the decoupling capacitors to prevent any over allocation. In block  701 , the PNV tool is used to identify violating nodes in the power grid. Initially, the decoupling capacitor is inserted at each violating node, as shown in block  702 . After this initial insertion, some of the violating power nodes may now be within the voltage thresholds while some remains to be violating power nodes. 
     In a first pass, at block  703 , the DSA tool is applied to increment the size of the decoupling capacitors for the power nodes that are not within the voltage thresholds. This is repeated until the peak voltage levels at these power nodes are within the voltage thresholds. Therefore, after the first pass, there are no noisy nodes in the power grid since the all the nodes are already within the noise margin or voltage thresholds. However, there may be some over-allocation of decoupling capacitance since not all of the violating power nodes have the same peak level voltages. This includes the violating power nodes that were brought to within the noise margin after the initial insertion of the decoupling capacitance. 
     In a second pass, at block  704 , the DSA tool is applied to decrement the size of the decoupling capacitor of each power node while the power nodes are within the voltage thresholds. In this embodiment, since the noise margin has been satisfied in the first pass of the optimization process, the noise margin will remain satisfied in the second pass. Therefore, the noise margin of the power grid is satisfied with the optimized number of decoupling capacitors. 
     In one embodiment, a computer-readable medium containing various sets of instructions, code sequences, configuration information, and other data used by a computer or other processing device to carry out the functions described in the present invention. The embodiment is suitable for use with the functions of identifying the violating power nodes, the insertion of decoupling capacitance in the violating power nodes, and the sizing of the decoupling capacitance until all the power nodes are within noise budgets or voltage thresholds. The various information stored on the computer-readable medium is used to perform various data processing operations. The computer-readable medium is also referred to as a processor-readable medium. The computer-readable medium can be any type of magnetic, optical, or electrical storage medium including a diskette, magnetic tape, CD-ROM, memory device, or other storage medium. The computer-readable medium may include interface code that controls the flow of information between various devices or components in the computer system. The interface code may control the transfer of information within a device (e.g., between the processor and a memory device), or between an input/output port and a storage device. Additionally, the interface code may control the transfer of information from one device to another. 
     From the above description and drawings, it will be understood by those of ordinary skill in the art that the particular embodiments shown and described are for purposes of illustration only and are not intended to limit the scope of the invention. Those of ordinary skill in the art will recognize that the invention may be embodied in other specific forms without departing from its spirit or essential characteristics. References to details of particular embodiments are not intended to limit the scope of the claims.