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Driver Waveform Computation for Timing Analysis with Multiple Voltage Threshold Driver Models Peter Feldmann*, Soroush Abbaspour, Debjit Sinha, Gregory Schaeffer, Revanta Banerji, Hemlata Gupta IBM T.J. Watson Research, Yorktown Heights, NY* IBM Systems & Technology Group, Hopewell Junction, NY June 11, 2008 DAC 2008, Anaheim, CA

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2 Introduction Traditional gate delay characterization Capacitive loads only Output signal assumed to be a ramp Delays and slews functions of input slew and capacitive load Sources of inaccuracy Non-ramp-like waveforms Highly resistive modern day interconnects Inductive effects

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3 Concept of effective capacitance (C eff ) Ideally match output waveform using C eff instead of RC load Match charge in interval [t v=0, t v=0.5Vdd ] Single number – not accurate enough Time (ps) Current / Voltage at A RC Line C eff A A

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4 Recent industry trends Non-linear current source models (CSM) Effective Current Source Model (ECSM) Cadence, Magma Driving current I = f I (V, C dyn ) For given input slew, characterization data stored as: Time = T (V, C) CLoad InSlew dotLib + ECSM Op Voltage Waveform Table t1t1 t2t2 t3t3 t4t4 t5t5 t6t6 v1v1 v2v2 v3v3 v4v4 v5v5 v6v6 Op Voltage Time PWL* *PWL = Piece Wise Linear

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5 Recent industry trends (contd.) Non-linear current source models Composite Current Source (CCS) Synopsys Driving current I = f I (V, C dyn ) For given input slew, characterization data stored as: I = I (T, C) CLoad InSlew dotLib + CCS Op Current Waveform Table PWL t1t1 t2t2 t3t3 t4t4 t5t5 t6t6 I1I1 I2I2 I3I3 I4I4 I5I5 I6I6 Io t0t0 Output Current Time

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6 Simulating current source models For a given input ramp (slew) Transformations required (T ~ time) T = T (V, C) I = f I (V, C) I = I (T, C) I = f I (V, C) Approximation, accuracy loss Accurate transformation requires High degree of continuity Smoothness

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7 Contributions Accurate and efficient analytical framework for driver waveform computation Novel algorithm for simulation of CSM Simulation along V axis and not T axis Avoids time domain integration (requires smooth data, time step control etc.) Requires model in MVTM * form: T= T (V, C) Same as industry standard characterized data No transformation to I = f I (V, C) Eliminates approximations Assumes monotonic piecewise linear output voltage waveform * Multiple Voltage Threshold Model

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8 Dynamic capacitance concept (C dyn ) A driver’s time varying instantaneous equivalent load capacitance Generalization for multiple voltage threshold model RC Line i(t) v(t) Time

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9 Driver waveform computation Assume current state T p, V p Goal: Given V p+1, calculate T p+1 Charge supplied by driver Assuming change in V linear for T p Output Voltage Time TpTp unknown

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10 Driver waveform computation (contd.) Charge flowing into load in T p : Q p Can be expressed analytically as f(T p+1 ) Equate charge Unknowns: T p+1, C d,p Use information from driver model (characterization table) RC Line EQ1 EQ2

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11 Small testcase setup Test common gates (INV, BUFF, AND, XOR) driving RC interconnect loads Compare near-end waveforms SPICE Traditional C eff approach Proposed approach denoted as MVTM (Multi voltage threshold model) Ramp of 20ps slew at gate input Krylov method used to compute interconnect delay

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12 Results Cn (fF) R (Ohms) Cf (fF) Gate DelayNear-end SlewWire Delay CeffMVTMSpiceCeffMVTMSpiceCeffMVTMSpice Delay and slew values are in psecs RR CnCn CfCf 100 m RC Line

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13 Small testcase results (contd.) In this experiment, the CMOS gate under test is a NAND2. The C n =20fF, C f =480fF, R =500. This experiment shows that MVTM follow SPICE while the Ceff technique incurs about 20% error in gate delay and about 40% in slew calculation.

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14 Large testcase setup and results Tested on large microprocessor units 65 nm designs Design, runtime and memory stats +5% +20%

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15 Large testcase results (contd.) Timing comparison with traditional C eff based gate delay calculation Largest difference paths analyzed in SPICE Observed MTVM models more accurate Within 3% of SPICE Design Num. timing points compared Comparison type Max diff (ps) Avg. diff (ps) 12.45M Arrival time Slew M Arrival time Slew

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16 Summary Accurate and efficient timing analysis Based on Multiple Voltage Threshold Models Realistic load models can be handled Novel algorithm for simulation of CSMs Eliminates need of intermediate transformation of models to I = f I (V, C) Compatible with industry standards Acceptable runtime Limitations, assumptions Driver input voltage waveform ramp Monotonic output voltage waveform

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