[VHDL Beginner] About ressources used

[Pascal Peyremorte]

Hi,

I am beginning in VHDL and FPGA world, and I want to understand a bit
more about better code for a 4 bits adder :

entity ADD4 is
port(VAL1 : in std_logic_vector(3 downto 0);
VAL2 : in std_logic_vector(3 downto 0);
SOMME : out std_logic_vector(3 downto 0);
RETENUE : out std_logic);
end ADD4;
Note : SOMME = French(SUM); RETENUE = French(CARRY) (.. Sorry !)

I found this architecture in a tutorial :

architecture ADD4_INT of ADD4 is
signal INT_SOMME : std_logic_vector(4 downto 0);
signal INT_VAL1 : std_logic_vector(4 downto 0);
signal INT_VAL2 : std_logic_vector(4 downto 0);
begin
INT_VAL1 <= '0' & VAL1;
INT_VAL2 <= '0' & VAL2;
INT_SOMME <= INT_VAL1 + INT_VAL2;
SOMME <= INT_SOMME(3 downto 0);
RETENUE <= INT_SOMME(4);
end ADD4_INT;

and write that one :

architecture ADD4_BOOL of ADD4 is
signal C0, C1, C2 : std_logic;
begin
SOMME(0) <= VAL1(0) xor VAL2(0);
C0 <= VAL1(0) and VAL2(0);

SOMME(1) <= VAL1(1) xor VAL2(1) xor C0;
C1 <= (VAL1(1) and VAL2(1)) or (C0 and (Val1(1) or Val2(1)));

SOMME(2) <= VAL1(2) xor VAL2(2) xor C1;
C2 <= (VAL1(2) and VAL2(2)) or (C1 and (Val1(2) or Val2(2)));

SOMME(3) <= VAL1(3) xor VAL2(3) xor C2;
RETENUE <= (VAL1(3) and VAL2(3)) or (C2 and (Val1(3) or Val2(3)));
end ADD4_BOOL;

It seems to me that the first one look like shorter but may allocate
more resources because of the many 5 bits "SIGNAL"

Am I right ?
Have you any comment about the two architectures ?

Thank you very much
Pascal

 

[Jonathan Bromley]

I am beginning in VHDL and FPGA world, and I want to understand a bit
more about better code for a 4 bits adder : [...]
I found this architecture in a tutorial :

architecture ADD4_INT of ADD4 is
signal INT_SOMME : std_logic_vector(4 downto 0);
signal INT_VAL1 : std_logic_vector(4 downto 0);
signal INT_VAL2 : std_logic_vector(4 downto 0);
begin
INT_VAL1 <= '0' & VAL1;
INT_VAL2 <= '0' & VAL2;
INT_SOMME <= INT_VAL1 + INT_VAL2;
SOMME <= INT_SOMME(3 downto 0);
RETENUE <= INT_SOMME(4);
end ADD4_INT;

That will work if you include

use IEEE.std_logic_unsigned.all;

at the beginning of your code.
However, I suggest you try to find out about the "numeric_std"
package, which offers a better way to do arithmetic on
vector signals.

architecture ADD4_BOOL of ADD4 is
signal C0, C1, C2 : std_logic;
begin
SOMME(0) <= VAL1(0) xor VAL2(0);
C0 <= VAL1(0) and VAL2(0);

SOMME(1) <= VAL1(1) xor VAL2(1) xor C0;
C1 <= (VAL1(1) and VAL2(1)) or (C0 and (Val1(1) or Val2(1)));

SOMME(2) <= VAL1(2) xor VAL2(2) xor C1;
C2 <= (VAL1(2) and VAL2(2)) or (C1 and (Val1(2) or Val2(2)));

SOMME(3) <= VAL1(3) xor VAL2(3) xor C2;
RETENUE <= (VAL1(3) and VAL2(3)) or (C2 and (Val1(3) or Val2(3)));
end ADD4_BOOL;

This is a structural description of a ripple-carry adder.
Although your VHDL looks correct, it is crazy to use VHDL like
this. The whole point of an HDL is to allow you to AVOID this
kind of dreary work.

It seems to me that the first one look like shorter but may allocate
more resources because of the many 5 bits "SIGNAL"

It's OK. Any good synthesis tool will note that the most significant
bit of the adder has two constant inputs '0' and only one variable
input (carry from bit 3) - so it becomes a simple wire.

Am I right ?

In principle, yes. In practice, synthesis will remove the unused
logic.

Good luck with your learning.
--
Jonathan Bromley, Consultant

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[Pascal Peyremorte]

Jonathan Bromley a écrit :

[...]
Am I right ?

In principle, yes. In practice, synthesis will remove the unused
logic.

Good luck with your learning.

Thank you very much for all that.

Pascal