HDL Functions

If you need to create a function in HDL you're in luck, cause I had to learn it myself last week. And lucky for me it was very simple.

In my particular case I was using an enumerated signal type and I needed to convert that signal back to a std_logic_vector signal such that I could output that signal for debugging purposes. Below is the code I used to convert two enumerated inputs to a single logic vector signal. Notice that the syntax is very similar to "process" definitions. The only key difference is the "return" declaration after the function declaration. This return declaration only requires the data type. It does not need to know what size the vector is (if it is a vector). This is nice since it potentially allows the user to have variable sized inputs...although I have not tried this myself, so it might not be synthesize-able.

function enum2std_logic(
arg1:STATE_TYPE;
arg2:SUBSTATE_TYPE)
return std_logic_vector is
    variable val: std_logic_vector(3 downto 0);
 begin    
 case arg1 is
    when IDLE =>
        val(3 downto 1):="000";
    when READ1 =>
        val(3 downto 1):="001";
    when READ2 =>
        val(3 downto 1):="010";
    when WRITE1 =>
        val(3 downto 1):="011";
    when WRITE2 =>
        val(3 downto 1):="100";
    when others =>
        val(3 downto 1):="000";   
end case;   
case arg2 is
    when Processing =>
        val(0):='0';
    when Waiting =>
        val(0):='1';           
end case;   
return val;
end function enum2std_logic; 

Hope this helps.