CODEWITHSUNDEEP
vhdlhdlvlsi
user2624915
user2624915

Reputation: 65

What is difference between index(9) and index(9 downto 9) in vhdl?

logic index : unsigned(9 downto 0) ;  
type fft_data  is array (3 downto 0) of unsigned(16 downto 0);  
signal tmp,signal fmax_data :fft_data;  
tmp = fmax_data(to_integer(index(9)));

Above part of the code gives following compilation error; "subprogram call or operator argument type mismatch 87"

if I do following modification it works. 

logic index : unsigned(9 downto 0) ;  
type fft_data  is array (3 downto 0) of unsigned(16 downto 0);  
signal tmp,signal fmax_data :fft_data;;  
tmp = fmax_data(to_integer(index(**9 downto 9**)));

Can anyone please explain what is the difference between above two implementation? I am using vhdl-93 std and ncvhdl. Thank you

Upvotes: 4

Views: 425

Answers (1)

Parth Parikh
Parth Parikh

Reputation: 166

In numeric_std, the to_integer function is only defined for unsigned and signed type.

Unsigned is derived from std_logic as type UNSIGNED is array ( NATURAL range <> ) of STD_LOGIC;

When you use to_integer(index(9)) you are passing index(9) which is of type std_logic .

When you use to_integer(index(9 downto 9)) you are passing a range of index which is of size 1, however, it is an unsigned type.

You can also create your custom function if you want. To convert std_logic to integer. 

function to_integer (ARG : std_logic)
  return integer is
begin
  if ARG = '1' OR ARG = 'H' then
    return 1;
  else
    return 0;
  end if;
end function to_integer;`

OR wrap around to_integer

function to_integer (ARG : std_logic)
    return integer is
    variable t : unsigned(0 downto 0) := "0";
begin
    t(0) := ARG;
    return to_integer(t);
end function to_integer;

Upvotes: 4

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