General Description: When using the IEEE.numeric_std library to perform a comparison between an unsigned number and a natural number, the following error may occur:
Error: Target 'L' is incompatible with assigned value in routine "=" line 490 in file 'C:/fndtn/synth/lib/packages/IEEE/src/numeric_std.vhd' called from test line 17 in file 'C:/myproj/test.vhd' (HDL-40)
If you examine the numeric_std.vhd file, you will see that the function called on line 490 (C.27) is a comparison between an unsigned and a natural where the natural is expected first. A similar function is called on line 502 (C.29) where the unsigned is expected first, but that function is not called. The following code will produce this error:
<PRE> library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all;
entity test is port (a : in unsigned(1 downto 0); x : out boolean; y : out boolean); end test;
architecture arch of test is constant zero : unsigned(1 downto 0) := "00"; constant natural_zero : natural := 0; begin
x <= a = zero; -- works y <= a = natural_zero; -- fails with HDL-40 error
end arch; </PRE>
ソリューション
1
Reverse the order of the comparison, so the natural is first, like so:
y <= natural_zero = a;
2
Use the IEEE.std_logic_arith library instead of IEEE.numeric_std.