nco in modelsim and quartusII

[rameshbalan]

i create one code for nco in modelsim. this works correctly. but while simulating in Quartus it is not works i dont know why.
code is,
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
use ieee.math_real.all;

entity ncocode is
GENERIC (amp_res : INTEGER := 8; -- output amplitude resolution
phas_res : INTEGER := 12; -- phase resolution
acc_size:INTEGER:=32 --accumulater size
);
port(fcw : in std_logic_vector(phas_res-1 downto 0);
clk: in std_logic;
opval : out std_logic_vector(amp_res-1 downto 0));
end ncocode;
architecture behav of ncocode is
constant romsize:integer:=2**phas_res-1;
constant romwidth:integer:=2**(amp_res-1)-1;
constant wavesize:integer:=2**(phas_res-1);
constant beta : integer := integer(((2**amp_res)-1)/2);
signal temp : std_logic_vector(phas_res-1 downto 0):="000000000000";
signal romvar : std_logic_vector(amp_res-1 downto 0);

begin
process(clk)
VARIABLE x: REAL;
begin
if rising_edge(clk) then
temp<= temp + fcw;
x:= (SIN(real(conv_integer(temp))*MATH_PI/real(wavesize)));
romvar<=STD_LOGIC_VECTOR(CONV_SIGNED(INTEGER(x*real(beta))-(1/2),amp_res));
end if;
end process;
opval <= romvar;
end behav;

--------------------------------------------------------------------------
please tell immediately.

 

[FvM]

The question will be precisely answered by a Quartus error message:

Error (10414): VHDL Unsupported Feature error at ncocode.vhd(26): cannot synthesize non-constant real objects or values

You'll need to implement the sine table as a ROM. It can be calculated in VHDL at compile time.

 

[rameshbalan]

thank you sir. i need why it cant calculate like that. i tried using ROM table. it give correct answer. what difference does it make. please explain .

 

[FvM]

QSIM is a gate level simulator, it can only simulate synthesizable logic. Real data type and sine() function aren't synthesizable.

 

[rameshbalan]

thank you sir.