Description

The following error occurs in XST when I target Virtex-6 or Spartan-6 devices, but I do not have any issues when I target older devices. Why?

ERROR:HDLCompiler:545 - "<file>.vhd" Line xx: Initial value for constant declaration is not constant

Example code:

library ieee;

use ieee.std_logic_1164.all;

entity ex_0009 is

port(clk,a : in std_logic;

res : out std_logic

);

end ex_0009;

architecture bhv of ex_0009 is

constant my_const: std_logic:='0';

signal tmp: std_logic;

begin

tmp <= my_const;

process (clk)

constant local_const: std_logic := tmp; -- Note: Error points here

begin

if clk'event and clk='1' then

res <= a or local_const;

end if;

end process;

end;

Solution

The example above is not a VHDL LRM compliant code. To solve this issue, initialize local_const constant directly with my_const constant.

library ieee;

use ieee.std_logic_1164.all;

entity ex_0009 is

port(clk,a : in std_logic;

res : out std_logic

);

end ex_0009;

architecture bhv of ex_0009 is

constant my_const: std_logic:='0';

signal tmp: std_logic;

begin

tmp <= my_const;

process (clk)

constant local_const: std_logic := my_const; -- Note: Error points here

begin

if clk'event and clk='1' then

res <= a or local_const;

end if;

end process;

end;

In 11.2 XST introduced a new VHDL/Verilog parser for Virtex-6 and Spartan-6 families. For more information on this change please refer (Xilinx Answer 32927)