ccs (composite current source)

**CCS (Composite Current Source)** is Synopsys's advanced **waveform-based timing and noise model** that represents cell output behavior as **time-varying current sources** rather than simple delay/slew tables — providing significantly more accurate timing, noise, and power analysis than NLDM, especially at advanced process nodes. **Why CCS Is More Accurate Than NLDM** - **NLDM**: Models the output as a single delay value and a linear ramp (one slew number). The actual waveform shape is lost. - **CCS**: Models the output as a **current waveform** that interacts with the actual load network — capturing the real voltage waveform shape, including non-linear transitions and load-dependent behavior. - This matters because at advanced nodes: - Waveforms are not linear ramps — they have distinct shapes that affect downstream cell switching. - The interaction between driving cell and load (Miller effect, crosstalk) depends on the actual waveform. - Setup/hold timing is sensitive to waveform shape, not just arrival time. **CCS Model Components** - **CCS Timing**: Current source model for output driving behavior. - Stores **output current vs. time** waveforms for each (input_slew, output_load) combination. - The STA tool convolves this current with the actual RC load network to compute the precise output voltage waveform. - Result: More accurate delay and transition time that accounts for the specific downstream network. - **CCS Noise**: Noise immunity and propagation model. - Models how noise glitches on inputs propagate to outputs. - Captures the cell's noise rejection characteristics. - Used for signal integrity analysis to determine if crosstalk-induced glitches cause functional failures. - **CCS Power**: Current-based power model. - Provides more accurate dynamic power estimation than NLDM's energy tables. - Captures the actual current draw profile during switching. **CCS vs. NLDM Accuracy** - **Delay**: CCS is typically **2–5%** more accurate than NLDM for single cells, with larger improvements for cells driving complex RC networks. - **Setup/Hold**: CCS can be **10–20%** more accurate for setup/hold time computation — critical for timing closure at advanced nodes. - **Noise**: NLDM has no noise model. CCS provides full noise analysis capability. - **Waveform**: CCS produces realistic non-linear waveforms; NLDM produces only linear ramps. **CCS in the Design Flow** - CCS data is stored in Liberty (.lib) files with additional CCS-specific sections. - **Characterization**: More data must be extracted during library characterization — current waveforms in addition to delay tables. - **File Size**: CCS Liberty files are **3–10×** larger than NLDM files — more data per timing arc. - **Runtime**: CCS-based STA is **10–30%** slower than NLDM due to more complex calculations. - **Sign-Off**: CCS is the recommended (or required) model for sign-off timing at 28 nm and below in Synopsys flows. CCS is the **state-of-the-art timing model** for Synopsys-based design flows — it provides the waveform accuracy needed for reliable timing closure at advanced semiconductor nodes.