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CCS Timing Model vs ECSM Timing Model

posted Aug 12, 2014, 11:25 AM by Rohit Sharma   [ updated Nov 24, 2015, 8:29 PM ]
CCS stands for Composite Current Source and ECSM stands for Effective Current Source Model. Both of these are current source models and have ability to abstract circuit models for static timing, power, noise and voltage-drop analysis.

Timing model consists of a driver model and a receiver model as shown in next picture.
Delay Calculation Model with Driver and Receiver

Delay calculator of static timing analysis engine looks up, interpolate or extrapolates these two models in liberty. CCS driver model captures output current flowing through load capacitor. Thus CCS model forces characterization engine to have a non-zero capacitance connected to cell's output. ECSM driver model captures voltage waveform at cell's output. ECSM driver model can capture load sensitivity starting from zero load capacitance, whereas CCS must start from non-zero load capacitor. Next picture shows output voltage V(CY) captured in ECSM driver model and output current I(CY) captured in CCS driver model.

CMOS Inverter Voltage and Currents

Next picture shows miller capacitance of inverter contributed by both transistors - nmos and pmos. Receiver model of both models captures miller capacitance of receiver with sensitivity to slope and load. 

Miller capacitance (C_gs.nmos + C_gd.pmos)

In the next blog, we'll relate these two driver model using basic circuit theory. Current source receiver models are essentially same with different granularity. Both these CCS and ECSM models capture miller effect as input capacitance with different sensitivity to input transition, input transition times, output load etc.