[IBIS] Extracting V-T waveform data for the purpose of modeling a CMOS buffer

In section 3.2 of the 09/15/05 "IBIS MODELING COOKBOOK For IBIS Version
4.0" document, it is stated,

"... for each simulation corner, four V-T data sets are generally
recommended. Two waveforms - either featuring a load to ground or a
relatively low voltage - are generated for the buffer output switching
low to high. The other two waveforms - both featuring a load to Vcc or a
relatively high voltage - are generated for the buffer output switching
high to low."

Why do we need two waveforms for each buffer output transition ? 

 

Thanks,

Nam    


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