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Functional vs, Timing

 
 
ALuPin
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      04-15-2005
Hi out there,

maybe someone can give her/his opinion:

I have the following assignment in my top level file:

entiy top is
port ( ...
Sdram_csn : out std_logic_vector(1 downto 0);
...
);
end top;

architecture rtl of top is

....
begin

Sdram_csn <= ('1' & l_sdram_csn);
....
end rtl;

When performing a functional simulation with Modelsim
Sdram_csn gets "10" at a certain point. (Reset: Sdram_csn="11")

The Timing Simulation with Modelsim instead shows that
Sdram_csn gets "11".

Is it possible that l_sdram_csn is '0' and yet
Sdram_csn gets "11" at the pin ? I tried to make the signal "l_sdram_csn"
visible in Modelsim, but the problem is that it is spread over
various slices after place and routing...Any tipps to find that signal ?

Rgds
André
 
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dutchgoldtony
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      04-15-2005
Look at creating a .do macro with Modelsim that adds the signal with
the add wave command. Look at the syntax that the input and output
signals are declared with in the signals window when you've simulated
and follow that format.

 
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Tim Hubberstey
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      04-15-2005
ALuPin wrote:
>
> When performing a functional simulation with Modelsim
> Sdram_csn gets "10" at a certain point. (Reset: Sdram_csn="11")
>
> The Timing Simulation with Modelsim instead shows that
> Sdram_csn gets "11".


The very first thing you should do when you get a function <-> gate
mismatch is to go back to your static timing analysis report and verify
that there are no timing violations reported.

The second thing is to verify that your testbench meets the setup & hold
requirements for the "real" part. This applies both to outputs from the
tb and inputs to it.

The third thing is to make sure that any combinational processes in your
RTL have *all* of the input signals in the sensitivity list.

This will take care of 99.9% of mismatches. Any remaining issues will be
the proverbial 5% that takes 95% of the time.
--
Tim Hubberstey, P.Eng. . . . . . Hardware/Software Consulting Engineer
Marmot Engineering . . . . . . . VHDL, ASICs, FPGAs, embedded systems
Vancouver, BC, Canada . . . . . . . . . . . http://www.marmot-eng.com
 
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