11.x XST - The value "-" of std_logic type is interpreted differently by XST for Virtex-6 and Spartan-6 devices
Description
Keywords: interpretation, don't, care
XST for Virtex-6 and Spartan-6 devices interprets the '-' value of std_logic type differently than when I target older devices. Why?
Solution
XST for older devices interprets the '-' value of std_logic type as a "don't care" value. In the following example, XST interprets '-' in the sel = "1-" as "don't care":File: ex_0011.vhd
Compilation Library: work
library ieee;
use ieee.std_logic_1164.all;
entity ex_0011 is
port(sel: in std_logic_vector(1 downto 0);
res: out std_logic
);
end ex_0006;
architecture bhv of ex_0006 is
begin
res <= '1' when sel = "1-" else '0';
end;
As the consequence, XST Standard generates the following netlist:
XST interpretation of above code for older devices
In 11.2, XST introduced a new VHDL/Verilog parser for Virtex-6 and Spartan-6 families. For more information on this change, see(Xilinx Answer 32927).
XST for Virtex-6 and Spartan-6 families does not interpret '-' as a "don't care" value, and therefore, the sel = "1-" expression is evaluated as False and the XST generates the following netlist. This is compliant with VHDL LRM.
XST interpretation of above code for Virtex-6 and Spartan-6
You can use the std_match function from numeric_std package to interpret '-' value as "don't care" in the sel = "1-" expression.
File: ex_0011.vhd
Compilation Library: work
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity ex_0011 is
port(sel: in std_logic_vector(1 downto 0);
res: out std_logic
);
end ex_0006;
architecture bhv of ex_0006 is
begin
res <= '1' when std_match(sel,"1-") else '0';
end;
In this case, the implementation will look like the parser implementation for older devices.