Using carry-in adders with Synopsys

[Tuukka Toivonen]

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]

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]

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.

 

[Ian Poole]

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

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * Perl * Tcl/Tk * Verification * Project Services

Doulos Ltd. Church Hatch, 22 Market Place, Ringwood, Hampshire, BH24 1AW, UK
Tel: +44 (0)1425 471223 mail: (e-mail address removed)
Fax: +44 (0)1425 471573 Web: http://www.doulos.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.