Initialization

[spartan]

Hi everybody,
In the following code I need to (and have to) initialize "sum1" and "sum2"
signals. the first idea was to initialize it when rst='1' but I got the
synthesis failure because of this warning: "Multi-source in Unit <test> on
signal <sum1_8> not replaced by logic. Signal is stuck at GND". Of course
I am using ISE 6.02.
Any idea what is wrong with the code? Or how I can initialize "sum1" and
"sum2"?
Thanks a lot.

Library IEEE;
Use ieee.std_logic_1164.all;
Use ieee.std_logic_arith.all;

entity test is
Port ( clk,rst :in std_logic;
I :in std_logic_vector(7 downto 0);
w1,w2 ut std_logic_vector(8 downto 0));
end test;

architecture Behavioral of test is
signal w1i, w2i : signed(8 downto 0);
signal sum1,sum2 : signed(8 downto 0);
begin
process(I,w1i,w2i)
constant alpha : signed(8 downto 0):= "010000000";
variable IC : signed(7 downto 0);
variable d1,d2,ad1,ad2 : signed(8 downto 0);
variable prod1, prod2 : signed(17 downto 0);
variable prodt1, prodt2: signed(8 downto 0);
begin
IC := signed(I);
d1 := conv_signed(IC,9) - conv_signed(w1i,9);
d2 := conv_signed(IC,9) - conv_signed(w2i,9);
ad1:= abs(d1);
ad2:= abs(d2);
if ad1 <= ad2 then
prod1 := alpha * d1;
prodt1:= prod1 (16 downto 8);
sum1 <= w1i+ prodt1;
else
prod2 := alpha * d2;
prodt2:= prod2 (16 downto 8);
sum2 <= w2i+ prodt2;
end if;
end process;


process(clk,rst)
begin
if clk'event and clk='1' then
if rst='1' then
w1i<= "000000000";
w2i<= "000001010";
sum1<= "000000000";
sum2<= "000001010";
else
w1i <= sum1;
w2i <= sum2;
end if;
end if;
end process;
w1<= std_logic_vector(w1i);
w2<= std_logic_vector(w2i);
end Behavioral;

 

[deep]

You are using two proceses...and assigning some values to sum1 and sum2 in
both ...this should be avoided...try assigning values in one process
only...

--deep

 

[Thomas Stanka]

Hello,

You are using two proceses...and assigning some values to sum1 and sum2 in
both ...this should be avoided...try assigning values in one process

It is not only "to be avoided", this is _evil_ until you know exactly
what you are doing.
This construct leads to internal tristate buses which are very nasty
even if you intended to use them.

BTW initialisations in the VHDL-code are only for simulations, they
won't be synthesised.
The OP should rewrite the code in one sequential process with
initialisations done with reset.

bye Thomas

 

[Samuel Fuhrer]

You are using two proceses...and assigning some values to sum1 and sum2
in
both ...this should be avoided...try assigning values in one process
only...

--deep

One way to avoid this kind of mistake is to clearly separate combinatorial
and sequential processes.

Example:

signal sum_present : signed(8 downto 0);
signal sum_next : signed(8 downto 0);

-- sequential process, describing flip flops
process(Clk, Rst)
begin
if Clk'event and Clk='1' then
if rst='1' then
sum_present <= "000000000";
else
sum_present <= sum_next;
end if;
end if;
end process;

-- combinatorial process, describing logic between flip flops
process(sum_present, other_signals)
begin
if ... then
sum_next <= function(sum_present, other_signals);
...
else
sum_next <= ...
...
end if;
end process;

 

[ivero]

Initialize it when rst='1' but in the first process you write.

And one more thing, if signal rst is synchronous you probably do not want to
put it in the sensitivity list of the process (the second you write).