Accurate delay estimation model for lumped CMOS logic gates

Recently a novel slope delay model for timing verification in digital CMOS circuits has been presented by H.G. Yang and D.M. Holburn (see ibid vol.137, p.405-12, 1990). Three different classifications were made, based on the risetime (slope) of the input waveforms, as follows: quick-no effect of input risetime on the output voltage; intermediate-small effect of input risetime on the output delays; and slow-significant effect of input risetime on the output delays. In the analysis of the slow waveform, the input voltage to the inverter was taken as 0.5 (V_dd-V_tn+kt₀ ), because otherwise there is no analytic solution. This assumption is valid as long as the input voltage is rising. If this approximation is continued after the input voltage reaches V _dd, then the current through the NMOS transistor is consistently underestimated, and the fall-time is overestimated. Hence, a more accurate representation of input waveform is necessary to obtain more reliable delay estimates