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Hi! I've been trying to do a 16 bit multiplier block using vhdl:   library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all; entity Multiplier is port( x,B: in std_logic_vector(15 downto 0); R: out std_logic_vector(15 downto 0) ); end Multiplier; architecture Behavioral of Multiplier is signal s32_R: std_logic_vector(31 downto 0); begin Multiplication: process(x,B) begin s32_R <= std_logic_vector(signed(x)*signed(B)); R <= s32_R(15 downto 0); end process Multiplication; end Behavioral;   When the simulation is done the result is at the first 16 bits of the 32 bits value at s32_R signal but it does not move to the output port labeled R, instead of the desire result, port R keep the unitialized value 'UUUUUUUUUUUUUUUU'. Is it wrong to use "R <= s32_R(15 downto 0);"? Is it a better or correctly way to describe this operation? It's just moving the 16 less significant bits of the s32_R signal into the entity output R. Thanks for reading! :)
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hi, a classical one: Esteban wrote: > Multiplication: process(x,B) sensitive list incomplete
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This seems, I guess, to be an issue regarding the verification process. How exactly did you do it? How long was the simulation? How long was it in relation to the time, the multiplier takes? How long keeps R the uninitialized state? The main point is, that you possibly forgot, that you describe structures rather than writing programs. While the multiplication is done, the second line, assigning (or better say transportation) of s32_R to R is at and during the very same time in effect,  or say: parallel to , the multiplication. So, if you stop simulation when the multiplication is done, the transfer to R had just begun, not finished, as you may assume. Whether that assumption matters, depends in turn on whether you simulated a real design or not. It is, additionally, true, that such a simple "putthrough" or say "and identity" in terms of math, is optimized away in a synthesized design. So I assume, that you showed us a simplified process, whose real version, make use of the intermediate result s32_R, so that it wasn't optimized away, and shows that "delayed" behavior, which you don't recognize because you stopped the simulation too early.
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Esteban wrote:
Multiplication: process(x,B)  begin s32_R <= std_logic_vector(signed(x)*signed(B)); R <= s32_R(15 downto 0);  end process Multiplication; 
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Sensitivity list seems incomplete to me. Maybe it works after adding s32_R to it.
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Instead of adding s32_R to the sensitivity list I suggest to use a variable.  library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all; entity Multiplier is port( x,B: in std_logic_vector(15 downto 0); R: out std_logic_vector(15 downto 0) ); end Multiplier; architecture Behavioral of Multiplier is begin Multiplication: process(x,B) variable s32_R: std_logic_vector(31 downto 0); begin s32_R := std_logic_vector(signed(x)*signed(B)); R <= s32_R(15 downto 0); end process Multiplication; end Behavioral; 