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00.050.10.150.20.250.10.20.30.40.50.60.70.80.90.0020.0740ivcBecause the interaction between slew and delay characterize a composite waveform, predicting delayalone cannot ensure accurate waveform prediction. For example, traditional models could supply a singleCeff value to ensure a single correct crossover point (50% VDD). But, this value is completely inadequatefor ensuring three correct crossover points (10%, 50%, and 90% VDD).SignalStorm NDC offers a new model to solve these problems for nanometer designs. This model, calledtime-quantized Ceff, can apply the dynamic relationships between voltage, current, and load capacitanceto ensure correct slew and delay calculation. This unique technology closely tracks the actual effectivecapacitance as it changes with signal voltage by calculating the load capacitance of receiver gates andthen computing the effective capacitance over several time steps during the signal ramp. The process begins with a specific, set time interval. The initial current is derived from the set voltage,capacitance, and input slew rate. (A signal slew rate is the time required for a signal to transition froma low to a high logic level.) These values drive the RC network, and based on the response, the modelcalculates a new effective capacitance derived from the current, voltage, and time of the instance.This process is repeated for each time point, using the new current value to drive the RC network,thus obtaining a new voltage and effective capacitance. Time-quantized Ceff provides delay calculationaccuracy to within ± 5% of SPICE models.3.3.2 Variable current source modeling with effective current source models (ECSMs)The ECSM model in SignalStorm NDC uses input slew and output load data to fit curves of current source,which are nonlinear. Calculations of delay for multiple driver cells, clock meshes, long interconnects,and modeling IR drop effects—all important in nanometer designs—become significantly more accurate.ECSM is an optional model that can be used when maximum accuracy is paramount for the mostdemanding Nanometer designs. ECSM is a variable current source model, the most sophisticated to datebecause it can more accurately represent complex topologies (see Figure 3). It provides measurementsof current over multiple time intervals, with different combinations of input slew and output loadingcapacitance. This nonlinear model includes output driver characteristics and variable driver currentsources. Using the drive current, ECSM can determine voltages by simulating the drive of RC networks.Drive voltages and receiving voltages are used to generate timing parameters of the RC networks.ECSM closely models nets with multiple drivers, such as clock meshes, with the additional benefit ofreducing on-the-side clock simulations and manual overrides. The key to ECSM is its nonlinear behavior,which more closely predicts actual silicon performance—so accurately that it performs to within ± 2%of SPICE models.Figure 3: Variable current source modeling with ECSM4