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Forum: FPGA, VHDL & Verilog VHDL multiplication for std_logic_vector


Author: Miguel (Guest)
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When simulating I get a run time error, so I'm trying to run a RTL 
analysis in Vivado to see if the schematic of the component can be 
created at least. The code is the following
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity multiplicator_test is
  generic(
      WORD_SIZE: natural := 8;
      EXP_SIZE: natural := 3
    );
    port(
      input_1: in std_logic_vector(WORD_SIZE-1 downto 0);
      input_2: in std_logic_vector(WORD_SIZE-1 downto 0);
      result: out std_logic_vector(WORD_SIZE-1 downto 0)
    );
end entity multiplicator_test;

architecture multiplicator_test_arch of multiplicator_test is
  constant SIGNIFICAND_SIZE: natural := WORD_SIZE - EXP_SIZE - 1;

  signal significand: std_logic_vector(SIGNIFICAND_SIZE-1 downto 0) := (others => '0');
  signal exponent: std_logic_vector(EXP_SIZE-1 downto 0) := (others => '0');
  signal sign: std_logic := '0';
  signal aux: std_logic_vector((2*SIGNIFICAND_SIZE)-1 downto 0) := (others => '0');
begin
    aux <= std_logic_vector(signed(input_1(SIGNIFICAND_SIZE-1 downto 0))*signed(input_2(SIGNIFICAND_SIZE - 1 downto 0)));
    significand <= aux(SIGNIFICAND_SIZE - 1 downto 0);
    exponent <= std_logic_vector(unsigned(input_1(WORD_SIZE-2 downto WORD_SIZE-EXP_SIZE-2))+unsigned(input_2(WORD_SIZE-2 downto WORD_SIZE-EXP_SIZE-2)));
    sign <= input_1(WORD_SIZE-1) or input_2(WORD_SIZE-1);
    result <= sign & exponent & significand;
end architecture multiplicator_test_arch;

When running the analysis, I get
ERROR: [Synth 8-690] width mismatch in assignment; target has 3 bits, source has 4 bits [(...)/multiplicador.vhd:27]

The line with the error is 27,
aux <= std_logic_vector(signed(input_1(SIGNIFICAND_SIZE-1 downto 0))*signed(input_2(SIGNIFICAND_SIZE - 1 downto 0)));

Apparently the target (aux) is 3 bits, but really it should be 8.

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