Difference between Functional and Post-Synthesis Simulation

[Peppe]

Which is the difference between Functional and Post-Synthesis Simulation?

Thanks

Peppe

 

[KJ]

Which is the difference between Functional and Post-Synthesis Simulation?

You write the (VHDL or Verilog) code for the 'functional' sim....you run
that code through some sort of tool to synthesize it and target a specific
device. One of the outputs of synthesis is another (supposedly equivalent)
model (VHDL or Verilog) which also models predicted timing of that code in
the targetted device. If you use that synthesis output model in place of
the code that you wrote you should see no differences in function (i.e.
given the same input stimulus, the outputs using your original code and the
synthesis output code should be the same).

KJ

 

[Peppe]

If the results should be same...why should I do both simulation?

Thanks

Peppe

 

[KJ]

If the results should be same...why should I do both simulation?

Because the results 'should be' the same....and 'should be' does not imply
'must be'. If you're skilled in use of the language you will already have
the knowledge to avoid the constructs that are problematic to implement or
which can cause problems.....but that doesn't mean that you can't
inadvertantly create those things.

If you're doing ASIC designs you are generally required to complete
post-synthesis simulation before signoff because the costs of being wrong
are rather large.

In most FPGA applications by a skilled designer, post-synthesis simulation
would only be done at the tail end of the design cycle once you're fairly
sure that the design is stable. It should not be done as part of the normal
'design/debug' cycle simply because using the post-synthesis model runs much
slower (since it much more detailed since it maps relatively closely to the
actual physical implementation).

If you're somewhat less skilled (or supervising someone you're not certain
of their skill level) you might want to have post-synthesis simulations
completed as just another check that the design is correct since even in the
FPGA world, debug and finding problems on hardware are more expensive than
during simulation.

KJ