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VHDL - GHDL Initialise std_logic_vector with smaller bit length

I have a signal dataIn : std_logic_vector ( 15 downto 0);

I want to give an input less than 16-bits for example dataIn <= x"000a" and those bits occupy the most significant bits and the rest to be zero. In verilog you can do that very easy but in VHDL you get the error:

"string length does not match that of the anonymous integer subtype defined t... ".

I know that if you use 16x"bit_string" solves the problem but this is only for VHDL-2008 and ghdl doesn't support yet VHDL-2008.

Are there any method for IEEE Std 1076-2002?

Solution

  • For VHDL-87/93/2002 you could use the resize function from the numeric_std package.

    library ieee;
use ieee.numeric_std.all;
...
constant FOO : std_logic_vector(2 downto 0) := "010";
signal dataIn : std_logic_vector(15 downto 0) := std_logic_vector(resize(unsigned(FOO), 16));

    Note that the resize function is only defined for types signed and unsigned.
    If you want the short bit string to be placed into the MSBs you may need to use the 'reverse_order attribute.

    Often you will find it easier to define a dedicated function which encapsulates more complicated initializations. 

    constant FOO : std_logic_vector(2 downto 0) := "010";

function init_dataIn (bar : std_logic_vector; len : integer) return std_logic_vector is
begin
  return bar & (len - bar'length - 1 downto 0 => '0');
end function init_dataIn;

signal dataIn : std_logic_vector(15 downto 0) := init_dataIn(FOO, 16);