baudgen.vhd

用vhdl语用实现简单的16位cpu功能

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

use STD.TEXTIO.ALL;

--  Uncomment the following lines to use the declarations that are
--  provided for instantiating Xilinx primitive components.
--library UNISIM;
--use UNISIM.VComponents.all;

entity BaudGen is
	Generic(bg_clock_freq : integer; bg_baud_rate  : integer);
    Port( CLK_I  : in  std_logic;
           RST_I : in  std_logic;
           CE_16 : out std_logic
		);
end BaudGen;

architecture Behavioral of BaudGen is

	-- divide bg_clock_freq and bg_baud_rate
	-- by their common divisor...
	--
	function gcd(M, N: integer) return integer is
	begin
		if ((M mod N) = 0) then		return N;
		else						return gcd(N, M mod N);
		end if;
	end;
	constant common_div : integer := gcd(bg_clock_freq, 16 * bg_baud_rate);
	constant clock_freq : integer := bg_clock_freq     / common_div;
	constant baud_freq  : integer := 16 * bg_baud_rate / common_div;
	constant limit      : integer := clock_freq - baud_freq;

	signal COUNTER : integer range 0 to clock_freq - 1;

begin

	process(CLK_I)
	begin
		if (rising_edge(CLK_I)) then
			CE_16 <= '0';		-- make CE_16 stay on for (at most) one cycle

			if (RST_I = '1') then
				COUNTER <= 0;
			elsif (COUNTER >= limit) then
				CE_16 <= '1';
				COUNTER <= COUNTER - limit;
			else
				COUNTER <= COUNTER + baud_freq;
			end if;
		end if;
	end process;

end Behavioral;