400ns分频器VHDL代码

在VHDL（VHSIC Hardware Description Language）中，设计一个400ns分频器，通常会涉及到计数器和同步逻辑。以下是一个简单的示例代码，使用的是同步分频器的设计思路：

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity four_hundred_ns_divider is
    Port ( clk : in  STD_LOGIC;
           input : in  STD_LOGIC;
           output : out STD_LOGIC);
end four_hundred_ns_divider;

architecture Behavioral of four_hundred_ns_divider is
    type counter_type is array (0 to 3) of integer;
    signal counter : counter_type := (others => 0); -- 4-bit counter
    signal next_counter : counter_type;
    constant PERIOD : time := 400 ns; -- 400 ns period for the clock

begin
    process(clk)
    begin
        if rising_edge(clk) then
            if input = '1' then
                next_counter := counter + 1;
            else
                next_counter := counter;
            end if;
            counter <= next_counter when counter < 4 else (others => 0); -- reset when overflow
        end if;
    end process;

    -- Output logic: divide by 4 (shift left)
    output <= not counter(0);
    -- Optional: add more outputs for higher order bits if needed
end Behavioral;

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