problem with using "Real" type signals in VHDL-AMS on ncsim

[Manivannan]

hai all,

i am having problems with the usage of real type signals.Based on the
code the real value in the SimVision waveform viewer is required to
change discretely but it is changing continuously.The code looks
simple but i am clueless about the problem.

Description of the problem:
i am trying to desing a simple bit to real converter ( a simple dac).A
bit_real function convertes the bit vector to real value.

Code:
_____________________________________________________________________

library ieee;
use ieee.electrical_systems.all;

entity tester is
generic ( constant BITS: integer:=16;
constant REFER: real:=5.0);
port ( signal Input : in bit_vector(BITS-1 downto 0) ;
signal actual_valut real);
end entity tester;

architecture realtest of tester is

signal over: bit;
signal a1: real:=0.0;

function bit_real(inp : bit_vector; ref: real) return real is
variable value:integer:=0;
variable i: integer:=0;
variable result: real:=0.0;
begin
for i in BITS-1 downto 0 loop
if inp(i)='1'then
value:=value*2+1;
else
value:=value*2;
end if;
end loop;
result:=ref*real(value)/real(2**BITS-1);
return result;
end bit_real;


begin
calc: process(Input)is
begin
a1<=bit_real(Input,REFER);
end process;
actual_val<=a1;

end architecture;


Testbench:
___________________________________________________________________
library ieee;
use ieee.electrical_systems.all;

entity toptest is
end entity;

architecture tb of toptest is
constant BITS:integer:=16;
signal inptb :bit_vector(BITS-1 downto 0);
signal outtb :real:=0.0;
begin
d2a: entity work.tester(realtest)
port map( Input=>inptb,actual_val=>outtb);

inp: process
begin
inptb<="1000000000000000";
wait for 3 ns;
inptb<="1100000000000000";
wait for 3 ns;
inptb<="1111000000000000";
wait for 3 ns;
wait;
end process;
end architecture;



This code was compiled & elaborated using ncvhdl & ncelab with -ams &
-v93 option. and simulated using the ncsim, with the following analog
control file

simulator lang=spectre
saveNodes options save=all rawfmt=sst2 rawfile="tran1.tran"
tran1 tran stop=10ns errpreset=moderate maxiters=10 cmin=10f


Output:
Here it is required that the value of a1 & actual_val to change only
when the input is changing, i.e must remain at constant value (2.5)
till the first 3ns. But it dosent happen so,.. the curve looks like
there is some kind of slew associated with it .. the value of a1 and
actual_val raises almost like a Ramp to the next value

I just dont know if i am missing out anything.The bit_real function is
not overloaded anywhere in my design or the work library.What could be
the reason for this problem?,Should i specify the max time step, or
any other options?

It will be great if anybody can answer this question.


Thanks
Manivannan

DPL
Univ of Cincinnati
OH:45220,USA