problems with generate statement

I am generating a 39 bit adder which takes an input from a multiplier
in ise 7.1. The problem is that when the inputs (the summands) are
defined as signals, XST will completely omit the adder from the design
and gives warnings. If the inputs are taken directly from pins in the
entity declaration, the adder synthesizes correctly.

Relevant code:

entity testAdd is
GENERIC ( n : INTEGER := 39 ) ;
port (

IN1,IN2 : in std_logic_vector(38 downto 0);
S : OUT STD_LOGIC_VECTOR(n-1 DOWNTO 0)
);
end entity testAdd;

architecture behv of testAdd is
SIGNAL s1,s2 : STD_LOGIC_VECTOR (38 downto 0);
SIGNAL C : STD_LOGIC_VECTOR(0 TO n) ;
begin

....
....
....
C(0) <= '0' ;
Generate_label:
FOR i IN 0 TO n-1 GENERATE
--stage: fulladd PORT MAP ( C(i), s1(i), s2(i), S(i), C(i+1) );
--doesn't work
stage: fulladd PORT MAP ( C(i),
IN1(i), IN2(i), S(i), C(i+1) ); --works
END GENERATE ;
....
....
....
end architecture behv;


Low Level Synthesis *
=========================================================================
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage11/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage10/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage9/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage8/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage7/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage6/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage38/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage37/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage0/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage1/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage2/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage3/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage4/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage5/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance is
removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage36/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage35/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage34/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage33/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage32/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage31/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage30/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage29/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage28/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage27/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage26/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage25/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage24/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage23/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage22/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage21/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage20/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage19/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage18/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage17/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage16/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage15/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage14/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed
WARNING:Xst:1989 - Unit <testadd>: instances <stage12/Mxor_s>,
<stage13/Mxor_s> of unit <LPM_XOR3_1> are equivalent, second instance
is removed


Does anybody understand why this happens?

Thanks!


sarah

 

[dipesh.trivedi]

hi firend,
Assign the inputs to the signal then try to generate.
s1<= IN1;
s2<= IN2;

i dont know will it work or not.
but try this. i dont have VHDL simulator otherwise i would have tried
it.

 

[Rtafas]

Agreed. It probably would work.

dipesh.trivedi escreveu:

hi firend,
Assign the inputs to the signal then try to generate.
s1<= IN1;
s2<= IN2;

i dont know will it work or not.
but try this. i dont have VHDL simulator otherwise i would have tried
it.

 

Thanks for your reply. Actually, the inputs pass through a multiplier
stage prior to entering the adder. Connecting the inputs to
intermediary signals will work, as you suggest, but it doesn't fit the
design.

sarah

dipesh.trivedi wrote:

hi firend,
Assign the inputs to the signal then try to generate.
s1<= IN1;
s2<= IN2;

i dont know will it work or not.
but try this. i dont have VHDL simulator otherwise i would have tried
it.

 

[Ralf Hildebrandt]

eengineer101@gmail.com wrote:

I am generating a 39 bit adder which takes an input from a multiplier
in ise 7.1.

library IEEE;
use IEEE.Numeric_std.ALL;

signal A,B,result : std_ulogic_vector(38 downto 0);

result<=std_ulogic_vector( unsigned(A) + unsigned(B) );

That's all. It synthesizes to a CRA if you don't set speed constraints
and to a faster adder (e.g. CLA) if your speed constraints are harder.
Why do you need such a bunch of code?

The problem is that when the inputs (the summands) are
defined as signals, XST will completely omit the adder from the design
and gives warnings. If the inputs are taken directly from pins in the
entity declaration, the adder synthesizes correctly.

So you have written something that makes the synthesis tool think, that
this adder is not needed.

entity testAdd is
GENERIC ( n : INTEGER := 39 ) ;
port (

IN1,IN2 : in std_logic_vector(38 downto 0);
S : OUT STD_LOGIC_VECTOR(n-1 DOWNTO 0)
);
end entity testAdd;

I am just curious: Why do you need such an adder inside a component
which furthermore is manually build by instances of fulladders?

IN1 and IN2 should also depend on the generic n to be flexible.

architecture behv of testAdd is
SIGNAL s1,s2 : STD_LOGIC_VECTOR (38 downto 0);

Why do you need extra signals s1, s2? What is the purpose of them? Do
they hold the input summands? Why don't you just read IN1 and IN2 if I
guess right?

SIGNAL C : STD_LOGIC_VECTOR(0 TO n) ;

Why not downto? It often makes designs harder to read if you use
different directions.

C(0) <= '0' ;
Generate_label:
FOR i IN 0 TO n-1 GENERATE
--stage: fulladd PORT MAP ( C(i), s1(i), s2(i), S(i), C(i+1) );
--doesn't work
stage: fulladd PORT MAP ( C(i),
IN1(i), IN2(i), S(i), C(i+1) ); --works
END GENERATE ;

If you read s1 and s2 but never write to them, it is obvious, that the
synthesis tool is smart enough to optimize these useless signals away.


Ralf