Generic bitslip module

Question

I want to implement a generic bitslip module. Below is an example of what I want to do for 4 and 8. I cannot figure out how to write code so I can pass some generic N and the code will be generated automatically using for loops or something.

---- 4-bitslip
bits_slipped <= 
           bits_in(3 downto 0)                       when tap_sel = "00" else
           bits_in(2 downto 0) & bits_in(3)          when tap_sel = "01" else
           bits_in(1 downto 0) & bits_in(3 downto 2) when tap_sel = "10" else
           bits_in(0)          & bits_in(3 downto 1) when tap_sel = "11";

-- 8-bitslip
bits_slipped <= 
           bits_in(7 downto 0)                       when tap_sel = "000" else
           bits_in(6 downto 0) & bits_in(7)          when tap_sel = "001" else
           bits_in(5 downto 0) & bits_in(7 downto 6) when tap_sel = "010" else
           bits_in(4 downto 0) & bits_in(7 downto 5) when tap_sel = "011" else            
           bits_in(3 downto 0) & bits_in(7 downto 4) when tap_sel = "100" else
           bits_in(2 downto 0) & bits_in(7 downto 3) when tap_sel = "101" else
           bits_in(1 downto 0) & bits_in(7 downto 2) when tap_sel = "110" else               
           bits_in(0)          & bits_in(7 downto 1) when tap_sel = "111"; 

-- N-bitslip ????

Answer 1

You can use the rotate_right() function from numeric_std. You can make this work for any size without a generic by just using unconstrained signals on a port. If you want, a generic can be added to force bits_in to match the size of bits_slipped.

library ieee;
ise ieee.std_logic_1164.all;
use ieee.numeric_std.all;
...

port (
  bits_in : in unsigned;
  tap_sel : in unsigned;
  bits_slipped : out unsigned -- Must be same length as bits_in
);
...

bits_slipped <= rotate_right(bits_in, to_integer(tap_sel));