Current-source model (CCS)
Beginner
A more detailed way of describing a building block’s speed, which records the shape of its output signal instead of just two numbers.
Novice
A Liberty timing model that stores the cell’s output current over time instead of only a delay and a transition. Tools can rebuild the real output waveform from it, which is more accurate when wires are long and resistive.
Expert
CCS (and the similar ECSM) store current or voltage waveforms per slew/load point, so the receiving net’s resistance and the Miller effect can be handled. It costs larger libraries and slower analysis; NLDM stays the default for synthesis and early placement.
Explained in From devices to a cell library (Transistors).
See also: NLDM (non-linear delay model).