Using carry-in adders with Synopsys

I'm trying to get Synopsys Design Compiler to synthesize
an adder where I can control the carry-in input. The manual
says:

>>>>
Merging Cascaded Adders With a Carry
If your design has two cascaded adders and one has a bit input, VHDL
Compiler replaces the two adders with a simple adder that has a carry
input.
Example: z <= a + b + cin;
<<<<

Now, "cin" can not be of type "bit", because bits can not be added,
right? So I have tried using other types
(natural range 0 to 1, unsigned(0 downto 0))
but I can't get it to work: Synopsys synthesizes always two adders
which is insane.

Does anyone know how to make it to synthesize only one adder
e.g. from the following code:

library IEEE;
use IEEE.STD_LOGIC_1164.all;
use IEEE.NUMERIC_STD.all;
entity CADD is port(
A: in unsigned(23 downto 0);
B: in unsigned(23 downto 0);
C: in std_ulogic;
R: out unsigned(23 downto 0));
end;
architecture RTL of CADD is
function caddf(A: unsigned(23 downto 0); B: unsigned(23 downto 0); C : std_ulogic) return unsigned is
variable n : natural range 0 to 1;
variable r : unsigned(23 downto 0);
begin
if C='0' then
n := 0;
else
n := 1;
end if;
r := A + B + n;
return r;
end;
begin
R <= caddf(A,B,C);
end;

Tuukka Toivonen

The following may help:
res:=(l&'1')+(r&cin);

Notice that the operands, l and r, are extended on the right with '1' and
'cin' rsp.
(types of l, r and res are unsigned)
The LSB bit of res is not be used.

Egbert Molenkamp

"Tuukka Toivonen" <> schreef in
bericht news:...
> I'm trying to get Synopsys Design Compiler to synthesize
> an adder where I can control the carry-in input. The manual
> says:
>
> >>>>
> Merging Cascaded Adders With a Carry
> If your design has two cascaded adders and one has a bit input, VHDL
> Compiler replaces the two adders with a simple adder that has a carry
> input.
> Example: z <= a + b + cin;
> <<<<
>
> Now, "cin" can not be of type "bit", because bits can not be added,
> right? So I have tried using other types
> (natural range 0 to 1, unsigned(0 downto 0))
> but I can't get it to work: Synopsys synthesizes always two adders
> which is insane.
>
> Does anyone know how to make it to synthesize only one adder
> e.g. from the following code:
>
> library IEEE;
> use IEEE.STD_LOGIC_1164.all;
> use IEEE.NUMERIC_STD.all;
> entity CADD is port(
> A: in unsigned(23 downto 0);
> B: in unsigned(23 downto 0);
> C: in std_ulogic;
> R: out unsigned(23 downto 0));
> end;
> architecture RTL of CADD is
> function caddf(A: unsigned(23 downto 0); B: unsigned(23 downto 0); C :
std_ulogic) return unsigned is
> variable n : natural range 0 to 1;
> variable r : unsigned(23 downto 0);
> begin
> if C='0' then
> n := 0;
> else
> n := 1;
> end if;
> r := A + B + n;
> return r;
> end;
> begin
> R <= caddf(A,B,C);
> end;

Egbert Molenkamp

In article <bqg8ui$88b$>, Egbert Molenkamp wrote:
> The following may help:
> res:=(l&'1')+(r&cin);

Thanks! This made my design 22% smaller. I thought about this myself
too but it feels a bit clumsy compared to if the synthesizer could
infer it more automatically... but this will do it for now.

Tuukka Toivonen

In Synopsys Design Analyzer, set the variable "hdlin_use_cin" to "TRUE".
When I looked at the full list of variables using the Design Analyzer GUI, I
could only find a variable called "hdlin_use_carry_in". This second variable
appears to have no effect!

There is, of course, an issue with synthesis tool portability if you go
setting variables in the Synopsys tool - the solution Egbert gave is
portable, but slightly harder VHDL...

Ian
--
Ian Poole, Consultant

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are not the views of Doulos Ltd., unless specifically stated.


"Tuukka Toivonen" <> wrote in
message news:...
> I'm trying to get Synopsys Design Compiler to synthesize
> an adder where I can control the carry-in input. The manual
> says:
>
> >>>>
> Merging Cascaded Adders With a Carry
> If your design has two cascaded adders and one has a bit input, VHDL
> Compiler replaces the two adders with a simple adder that has a carry
> input.
> Example: z <= a + b + cin;
> <<<<
>
> Now, "cin" can not be of type "bit", because bits can not be added,
> right? So I have tried using other types
> (natural range 0 to 1, unsigned(0 downto 0))
> but I can't get it to work: Synopsys synthesizes always two adders
> which is insane.
>
> Does anyone know how to make it to synthesize only one adder
> e.g. from the following code:
>
> library IEEE;
> use IEEE.STD_LOGIC_1164.all;
> use IEEE.NUMERIC_STD.all;
> entity CADD is port(
> A: in unsigned(23 downto 0);
> B: in unsigned(23 downto 0);
> C: in std_ulogic;
> R: out unsigned(23 downto 0));
> end;
> architecture RTL of CADD is
> function caddf(A: unsigned(23 downto 0); B: unsigned(23 downto 0); C :
std_ulogic) return unsigned is
> variable n : natural range 0 to 1;
> variable r : unsigned(23 downto 0);
> begin
> if C='0' then
> n := 0;
> else
> n := 1;
> end if;
> r := A + B + n;
> return r;
> end;
> begin
> R <= caddf(A,B,C);
> end;

Ian Poole