I've a problem with the definition of an entity. I'm writing a wrapper that accepts several generic. Among these generic there are some boolean values that have the function to enable or disable the transmission of some input signal to the output of the entity. I'd like to have the width of the output signal related to the boolean values. This is what I want to do :

entity of DUT is 
     DATA_1_WIDTH : natural := 16;
     DATA_2_WIDTH : natural := 16;
     SWITCH_1 : boolean := FALSE;
     SWITCH_2 : boolean := FALSE;
   port (
     clk : in std_logic;
     rst : in std_logic;
     DATA_1 : in std_logic_vector(DATA_1_WIDTH - 1 downto 0);
     DATA_2 : in std_logic_vector(DATA_2_WIDTH - 1 downto 0);
     DATA_OUT : out std_logic_vector);

Where the width of "DATA_OUT" change with the values of the switches :

  • When SWITCH_1 = TRUE and SWITCH_2 = TRUE => width of DATA_OUT = DATA_1_WIDTH + DATA_2_WIDTH + constant;
  • When SWITCH_1 = TRUE and SWITCH_2 = FALSE => width of DATA_OUT = DATA_1_WIDTH + constant;

And so on.

Can someone tell me a way to do this ?

  • 1
    \$\begingroup\$ Make your own function that converts boolean to an integer [0,1] and then use that to manipulate your width. Something like DATA_OUT: out std_logic_vector(DATA_1_WIDTH + bool2int(stuff)*DATA_2_WIDTH + constant) \$\endgroup\$
    – uint128_t
    Feb 22, 2016 at 15:47
  • 1
    \$\begingroup\$ Just making sure you know that the size can not change at run time. This is compile time only. \$\endgroup\$
    – mcmiln
    Feb 22, 2016 at 16:40

1 Answer 1


Depending on how the rest of your entity works, you may be able to leave DATA_OUT as an unconstrained array. This means that you don't give it a width in the port map, and that it takes the width of the signal that you connect to it in the instantiating entity. This basically passes the problem up one level in the hierarchy.

You can also achieve this using a function. Create a package for your design, and within this create a function, something like this:

function GetDataOutWidth(Switch1, Switch2 : boolean; Data1Width, Data2Width : integer) return integer is
  constant WidthConstant : integer := 2; -- Or whatever
  if (Switch2) then
    return Data1Width + Data2Width + WidthConstant;
    return Data1Width + WidthConstant;
  end if;
end function;

Your code can then use this function in the port map, something like this:

DATA_OUT : out std_logic_vector(GetDataOutWidth(SWITCH_1, SWITCH_2, DATA_1_WIDTH, DATA_2_WIDTH)-1 downto 0)

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