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.