  Faithfully representing small gate delay variations caused by input
switchings on different inputs in close temporal proximity is a very
challenging task for digital delay models. In this paper, we use the example of
a 2-input NOR gate to show that a simple hybrid model leads to a surprisingly
accurate digital delay model. Our model utilizes simple first-order ordinary
differential equations (ODEs) in all modes, resulting from considering
transistors as ideal switches in a simple RC model of the gate. By analytically
solving the resulting ODEs, we derive expressions for the gate delays, as well
as formulas that facilitate model parametrization. It turns out that our model
almost faithfully captures the Charlie effect, except in just one specific
situation. In addition, we experimentally compare our model's predictions both
to SPICE simulations, using some 15 nm technology, and to some existing delay
models. Our results show a significant improvement of the achievable modeling
accuracy.
