DE2 board USB communication

I assume that you are using tmp to detect a change in buffer_full ?

In that case, in the clocked part put something like:

if (tmp = '0') and (buffer_full = '1') then
  tmp <= '1';
  -- do your start stuff here
end if;

Don't forget to give an initial value to tmp.

And choose another name for tmp, to make your code more readable and maintainable. Something like 'active' or 'enabled' as an example.