Constant value of ROM array

Question

I am trying to crate a 64 bit ROM type ROM is array (7 downto 0, 7 downto 0) of std_logic;

then I want create constant R :ROM :=.

Where R(0, 7 downto 0 ) would be value of "00010011",

R(0, ) R(1, ) is selected by an 3bit pointer value and then it output by formula :

for i in 0 to 7  loop
      Data_Out(i) <= R(conv_integer(AArray),i);
end loop;.

But which ever way I am trying define the constant value of ROM I get an error, could someone point me to right way of defining constant for this array.

Morten Zilmer · Accepted Answer · 2016-06-06 09:50:24Z

4

For more complex initialization values, it is often helpful to make a function that returns the constant value to assign, like:

type ROM is array (7 downto 0, 7 downto 0) of std_logic;

function ROM_init return ROM is
  variable res_v : ROM;
begin
  -- Write code that assigns initial value to res_v
  return res_v;
end function;

constant R : ROM := ROM_init;

If an array of vectors is actually needed, you may consider instead doing:

type ROM is array (7 downto 0) of std_logic_vector(7 downto 0);
function ROM_init return ROM is
  variable res_v : ROM;
begin
  res_v(0) := "00010011";  -- Value from question
  -- Write more code that assigns initial value to res_v;
  return res_v;
end function;
constant R : ROM := ROM_init;

Note that the synthesis tool will probably not implement this as a ROM memory block, but simply as a simple logic network.

edited Jun 6, 2016 at 9:50

answered Jun 6, 2016 at 9:36

Morten Zilmer

15.9k4 gold badges35 silver badges52 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Seb.Sz Over a year ago

line function ROM_init return ROM is gives error "parse error, unexpected IS, expecting SEMICOLON" . Any idea why I am using ISE 14.7

Seb.Sz Over a year ago

Would it be because type ROM is in the package?

Morten Zilmer Over a year ago

If the ROM_init function is placed in a package, then the public part of the package (package) should only declare function ROM_init return ROM;, and the implementation of the package (package body) should have the function implementation itself with function ROM_init return ROM is ... end function;.

Seb.Sz Over a year ago

Yes you are 100% right. I have just find this out and so far so good I will run small test bench and see if it works correctly now, thank you.

Collectives™ on Stack Overflow

Constant value of ROM array

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related