Understanding Standard Cell Characterization

Cell characterization is a process of analyzing a circuit using static and dynamic methods to generate models suitable for chip implementation flows.

Why is cell characterization needed?

No digital chip is possible without cell models. These cell models are produced by cell characterization using commercial softwares like guna. Every digital chip implementation (rtl-to-gdsII) flow requires cell models for analysis (logic simulation, verification, timing, power, noise etc), implementation (synthesis, test insertion, placement, clock tree synthesis, routing) and fixing (engineering change order, rule fixing etc).

How is cell characterization performed?

Cell characterization typically takes cell design extracted as spice circuit and spice technology models. Characterization software like guna from Paripath, analyzes this information to 

1. acquire or recognize cell’s function,
2. generates stimulus appropriate to determine characteristic (like delay, transition time etc),
3. simulates it using circuit simulator,
4. gather simulations output to measure characteristic and
5. finally writes this data into a standard like liberty^TM, veriog or IBIS.

This entire flow is depicted into the picture shown below

Cell characterization output

Cell characterization produces following outputs, in no particular order

1. verilog
2. IBIS
3. liberty^TM Format
   1. Delay
   2. Transition Time
   3. Tristate
   4. Input Capacitance
   5. Hidden Power
   6. Glitch Power
   7. Dynamic Power
   8. Leakage Power
   9. Setup Time
   10. Hold Time
   11. Recovery Time
   12. Removal Time
   13. Minimum Pulse Width
   14. Output Current Waveform
   15. Input Receiver Capacitance
   16. Power Supply Waveforms
   17. Ground Waveforms
   18. Leakage Current
   19. Gate Leakage Current
   20. CCB Output VIVO
   21. CCB Output Voltage Waveform
   22. CCB Input Miller Capacitance
   23. CCB Noise Propagation Model
   24. … and other

Summary

This article answers main questions on standard cell characterization and model generation. Scope of this topic is VLSI cell/circuits. Please google for solar cell characterization and bio-cell characterization.

This is a guest post by Paripath. Read more on VLSI characterization on Paripath Blog.