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AR# 4592

FPGA Express: Using don't cares in VHDL


Keywords: Express, don't care, VHDL, synth

Urgency: Standard

General Description:
When trying to synthesize VHDL code with don't cares, Express ignores anything
that contains don't cares and synthesizes that section of code to a constant.

For example, a code segement like this

if (a= "1--1" and b = "--00") then
c <= '1';
c <= '0';
end if;

would result in warnings like this:

Comparisons to a 'don't care' are treated as always being false.
This may cause simulation to vary from synthesis.



One solution is to compare the individual bits rather than the entire bus with
the don't cares in them. Instead of the code above, you would write:

if ((a(3)= '1' and a(0)='1') and (b(1)= '0' and b(0)='0')) then
c <= '1';
c <= '0';
end if;


Another solution is to build your own std_match function to
compare the vector to a mask string. This procedure becomes
easier to implement (compared to the first solution) as the
comparisons become more complex.

--first, declare the match type that allows don't cares
type match_t is ('0', '1', '-');
type match_a_t is array (natural range <>) of match_t;

--next, create the function that will perform the compare
function std_match (l : std_logic_vector; r : match_a_t) return boolean is
variable flag : boolean;
flag := true;
for i in l'range loop
flag := flag and ((r(i)='-') or (r(i)='1' and l(i)='1') or (r(i)='0' and l(i)='0'));
end loop;
return flag;
end std_match;

--declare the mask that will be used in the comparison
constant maskA : match_a_t (A'range) := "-11-";
constant maskB : match_a_t (B'range) := "--00";

signal comp1, comp2 : boolean;


-- compare the signal against the mask
comp1 <= std_match(A, maskA);
comp2 <= std_match(B, maskB);

--this process creates the same logic as resolution 1
process(comp1, comp2) begin
if (comp1 and comp2) then
C <= '1';
C <= '0';
end if;
end process;
AR# 4592
Date Created 09/10/1998
Last Updated 08/11/2003
Status Archive
Type General Article