Description

I get the following error in XST when targeting Virtex-6/Spartan-6 devices, but do not have any issues when I target older devices. Why?

ERROR:HDLCompiler:236 - "<file>.vhd" Line xx: Attribute last_value on multiple bits is not synthesizable

Example code:

library ieee;

use ieee.std_logic_1164.all;

entity ex_46_1_0 is

port(in_port : in std_logic_vector(3 downto 0);

out_port: out std_logic_vector(3 downto 0));

end ex_46_1_0;

architecture beh of ex_46_1_0 is

begin

out_port <= in_port'last_value;

end;

Solution

In the above example we use 'last_value predefined attribute is used on a signal assignment. This is not synthesizable construct. The code has to be rewritten without 'last_value.

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)