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ASIC Physical Design:: transition values in libraries

rupeshbamane

 In the timing libraries used in ASIC sta the transition tables are normally extrapolated to 10%~90%. WHY?

The delay calculator has to again multiply the values in transition table by slew_derate_from_library to get the transition_value for the actual characterised portion of the waveform and use it for delay calculation.
Why in first place the actual transition values for the characterised say 30%~70% not written in the libraries?
Could some one help me understand?

  •  The output waveforms at the ends are asymptotic and it is hard to determine the exact start and
    end points of the transition. Consequently, the transition time is defined
    with respect to specific threshold levels(10%-90%).

    The slew thresholds are chosen to correspond to the linear portion of the
    waveform. As technology becomes finer, the portion where the actual
    waveform is most linear is typically between 30% and 70% points. Thus,
    most of the newer generation timing libraries specify slew measurement
    points as 30% and 70% of Vdd. However, because the transition times were
    previously measured between 10% and 90%, the transition times measured
    between 30% and 70% are usually doubled for populating the library. This
    is specified by the slew derate factor which is typically specified as 0.5. The
    slew thresholds of 30% and 70% with slew derate as 0.5 results in equivalent
    measurement points of 10% and 90%.

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