list_ch04_07_shift.vhd

VHDL FFT SOURCECODE great one

-- Listing 4.7
library ieee;
use ieee.std_logic_1164.all;
entity free_run_shift_reg is
   generic(N: integer := 8);
   port(
      clk, reset: in std_logic;
      s_in: in std_logic;
      s_out: out std_logic
   );
end free_run_shift_reg;

architecture arch of free_run_shift_reg is
   signal r_reg: std_logic_vector(N-1 downto 0);
   signal r_next: std_logic_vector(N-1 downto 0);
begin
   -- register
   process(clk,reset)
   begin
      if (reset='1') then
         r_reg <= (others=>'0');
      elsif (clk'event and clk='1') then
         r_reg <= r_next;
      end if;
   end process;
   -- next-state logic (shift right 1 bit)
   r_next <= s_in & r_reg(N-1 downto 1);
   -- output
   s_out <= r_reg(0);
end arch;