NLDM VS CCS

NLDM: nonlinear delay models.

CCS:  current composite models.

The basic difference between CCS and NLDM is :

NLDM uses a voltage source for driver modeling (thevenin's theorem), where as a CCS uses a current source for driver modeling (norton's theorem).

130nm library (NLDM - say 5% variation with SPICE simulation) characterisation.

130nm library (CCS - say 3% variation with SPCIE simulation) characterisation.

NLDM Driver Model:

NLDM driver model characterizes input-to-output delay and output transition times with sensitivity to input transition time, output load and side input states. These characteristics are obtained using a circuit simulator with appropriate stimulus to cause output transition.

The problem with NLDM modeling is that, when the drive resistance RD becomes much less than Znet(network load impedance), then ideal condition arises i.e Vout=Vin. This is impossible in practical conditions.

NLDM only captures 3 output points, which is not sufficient to reflect non-linearities of circuits at lower geometries (65nm and below) in synthesized driver model during static timing analysis.

So with NLDM modeling parameters like the cell delay calculation, skew calculation will be inaccurate.

CCS Driver Model:

CCS driver model is characterized by capturing current waveform flowing into the load capacitor of the cell. CCS driver model also has sensitivity to input transition time, output load and side input states. CCS driver model is essentially a current source with infinite driver resistance, hence it provides better accuracy in cases where net impedance is very very high. Note, CCS timing model does not require synthesis of driver model, captured current waveform is driver model itself.

CCS Receiver Model:

CCS receiver model is characterized much like NLDM receiver model with additional granularity to reflect sensitivities like miller capacitance, state of side inputs, input transition times and output load. To accurately reflect effect of miller capacitance on input capacitance and net-delay, it is divided into two parts - C1 and C2. For STA delay calculation, C1 is used in net delay calculation before receiver waveform hits delay threshold point and C2 is used in net delay calculation after receiver waveform hits delay threshold point.

That is the reason why we prefer CCS to NLDM