Functional Verification Interview Question


One day at the lunch table in the cafeteria, your manager says that she recently learned that X-SIM, the VHDL simulator that you have been using, has split their simulator into two products: one for functional simulation and one for timing simulation. She is considering buying either just the functional simulator or just the timing simulator, and using the money that she saves to buy more compute to speed up simulation.


Your current functional verification methodology uses a mixture of functional simulation, timing simulation, and running on an FPGA board. Your manager describes three options: “FTB”, “FCB”, and “TCB”; where “F” means functional simulation, “T” means “timing simulation”, “C” means new computers, and “B” means FPGA board. All three options cost the same.

FTB Buy both the functional simulator and timing simulator, and continue with the current methodology.

FCB Don’t buy the timing simulator; use functional simulation and FPGA boards for functional verification. Use the money saved by not buying the timing simulator to buy more computers, which will allow you to run functional simulation 10-times faster than you do now.

TCB Don’t buy the functional simulator; use timing simulation and FPGA boards for functional verification. Use the money saved by not buying the functional simulator to buy more computers, which will allow you to run timing simulation at the same speed as you currently run functional simulation.

For each option, answer whether you think it should be chosen as the best option, considered as a possibility, or rejected. If you recommend that an option be chosen, then you must reject the other two options. For each option, briefly justify your recommendation in terms of its advantages and/or disadvantages.

Hint: There is no single right or wrong answer. Please try to argue through analysis.

Try to think through these ...

  • FTB allows current methodology to be continued: methodology is trusted andreliable, no need for additional training.
  • Functional simulation is insufficient; need timing simulation or running on board to detect timing errors
  • Running functional simulation faster provides a minimal increase in actual coverage, because functional coverage is so low
  • Debugging is much easier with functional simulation than with timing simulation, and debugging with timing simulation is much easier than debugging on the board.
  • Could use the board as an alternative to timing simulation.
  • Without functional simulation, the design would need to be synthesizable before it could be simulated.

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