help about operation :s = c1 * 0.2+ c2 * 0.6 + c3 * 0.1

[VHDL_HELP]

Hi every body ,
i hope that you can help me , i want to do this operation:
s = c1 * 0.2+ c2 * 0.6 + c3 * 0.1
when i check the syntax , i have these errors
ERROR:HDLParsers:808 - "C:/Xilinx/projet/operation.vhd" Line 44. * can
not have such operands in this context.

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library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity operation is
Port ( clk : in STD_LOGIC;
c1 : in STD_LOGIC_VECTOR (7 downto 0);
c2 : in STD_LOGIC_VECTOR (7 downto 0);
c 3: in STD_LOGIC_VECTOR (7 downto 0);
s: out STD_LOGIC_VECTOR (7 downto 0);

end operation;

architecture Behavioral of operation is

begin

s <=c1 * 0.2 + c2 * 0.6 + c3 * 0.1 ;

end Behavioral;

 

[KJ]

1. Don't use std_logic_arith package, use numeric_std instead.
2. In any case, with either package you can not multiply a
std_logic_vector by a floating point number (i.e. 'c1 * 0.2', etc.)
3. The result of a floating point expression is not a std_logic_vector
(i.e. everything on the right hand side of your equation for 's
<= ...' would have to be floating point).
4. If you intend to synthesize this into real hardware you might want
to consider multiplying by slightly different coefficients that have
an exact binary representatio (if possible), otherwise look into fixed
point representation methods. If you're simply trying to simulate but
have no intention of putting this into real hardware then this point
does not apply.

Kevin Jennings.

 

[weber]

1. Don't use std_logic_arith package, use numeric_std instead.
2. In any case, with either package you can not multiply a
std_logic_vector by a floating point number (i.e. 'c1 * 0.2', etc.)
3. The result of a floating point expression is not a std_logic_vector
(i.e. everything on the right hand side of your equation for 's
<= ...' would have to be floating point).
4. If you intend to synthesize this into real hardware you might want
to consider multiplying by slightly different coefficients that have
an exact binary representatio (if possible), otherwise look into fixed
point representation methods. If you're simply trying to simulate but
have no intention of putting this into real hardware then this point
does not apply.

Kevin Jennings.

I never implemented multiplication using a '*'... what will be
synthesized?
A good idea is to use hardwired multipliers, if possible.
Cheers,
weber

 

[KJ]

I never implemented multiplication using a '*'... what will be
synthesized?

Logic to implement a multiply operation.

A good idea is to use hardwired multipliers, if possible.

If...
- The targeted part has a multiplier
- The synthesis tools that you're using support the multiply operator
(i.e. '*')
- The manner that you use the '*' operator doesn't conflict with the
way that the hardware can implement it

Then it will use a hardwired multiplier. The easiest way is to try it
out in some code and see what the result is. Usually the only gotcha
with being able to use the hardware multipliers has to do with getting
data in and out because the hardware may have some input or output
flops or something.

If that's the case, then you'll find that "a <= b * c" will get fitted
into the hardware multiplier only if it's in a clocked process. Like
I said though, try it out with something simple or check the
documentation for any restrictions/suggestions on inferring the
hardware multiplier from your source code.

Usually it either works straight off or there is some minor usage
issue to resolve so that it doesn't generate the multiplier using
logic blocks.

Kevin Jennings