VHDL设计8位可逆计数器，数码管显示

以下是一个设计8位可逆计数器并使用数码管显示的VHDL代码：

entity reversible_counter_display is
    port (
        clk : in std_logic;
        rst : in std_logic;
        seg : out std_logic_vector(7 downto 0)
    );
end entity reversible_counter_display;

architecture rtl of reversible_counter_display is
    signal counter : std_logic_vector(7 downto 0) := (others => '0');
    signal count   : integer range 0 to 99 := 0;
begin
    process(clk, rst)
    begin
        if rst = '1' then
            counter <= (others => '0');
            count   <= 0;
        elsif rising_edge(clk) then
            if count = 99 then
                count   <= 0;
                counter <= (others => '0');
            else
                count   <= count + 1;
                counter <= std_logic_vector(to_unsigned(count, 8));
            end if;
        end if;
    end process;

    seg <= to_7seg(counter);
end architecture rtl;

-- 数码管显示模块
library ieee;
use ieee.std_logic_1164.all;

entity to_7seg is
    port (
        num : in std_logic_vector(7 downto 0);
        seg : out std_logic_vector(7 downto 0)
    );
end entity to_7seg;

architecture rtl of to_7seg is
begin
    with num select
        seg <=  "0000001" when "00000000",  -- 0
                "1001111" when "00000001",  -- 1
                "0010010" when "00000010",  -- 2
                "0000110" when "00000011",  -- 3
                "1001100" when "00000100",  -- 4
                "0100100" when "00000101",  -- 5
                "0100000" when "00000110",  -- 6
                "0001111" when "00000111",  -- 7
                "0000000" when "00001000",  -- 8
                "0001100" when "00001001",  -- 9
                "0010000" when "00001010",  -- A
                "0001000" when "00001011",  -- b
                "0000010" when "00001100",  -- C
                "1000110" when "00001101",  -- d
                "0100001" when "00001110",  -- E
                "0100011" when "00001111",  -- F
                "1111111" when others;       -- Off
end architecture rtl;

在这个代码中，我们使用了一个8位的计数器，使用可逆计数算法实现。当计数器的值达到99时，它将重新开始计数，而不是继续增加到100。这个算法允许计数器在任何时候都能回到0，因此它是可逆的。

我们还使用了一个 `to_7seg` 模块来将计数器的值转换为七段显示格式。这个模块使用一个 `with select` 语句，将数字转换为相应的七段显示码。它还包括一个 "Off" 选项，用于在计数器值超出范围时关闭数码管显示。

为了测试这个设计，你可以使用一个测试台来提供时钟和复位信号，并监视数码管显示的输出。以下是一个简单的测试台：

entity tb_reversible_counter_display is
end entity tb_reversible_counter_display;

architecture testbench of tb_reversible_counter_display is
    signal clk : std_logic := '0';
    signal rst : std_logic := '0';
    signal seg : std_logic_vector(7 downto 0);
begin
    dut : entity work.reversible_counter_display
        port map (
            clk => clk,
            rst => rst,
            seg => seg
        );

    process
    begin
        clk <= '0';
        wait for 10 ns;
        clk <= '1';
        wait for 10 ns;
    end process;

    process
    begin
        rst <= '1';
        wait for 5 ns;
        rst <= '0';
        wait for 5 ns;
        rst <= '1';
        wait;
    end process;

    process
    begin
        wait for 100 ns;
        assert seg = "00000001" report "Error: Expected '0' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00000010" report "Error: Expected '1' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00000011" report "Error: Expected '2' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00000100" report "Error: Expected '3' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00000101" report "Error: Expected '4' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00000110" report "Error: Expected '5' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00000111" report "Error: Expected '6' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00001000" report "Error: Expected '7' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00001001" report "Error: Expected '8' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00001010" report "Error: Expected '9' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "00001100" report "Error: Expected 'C' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "1000110" report "Error: Expected 'd' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "0100001" report "Error: Expected 'E' but got '" & std_logic'image(seg) & "'" severity error;
        wait for 100 ns;
        assert seg = "0100011" report "Error: Expected 'F' but got '" & std_logic'image(seg) & "'" severity error;
        wait;
    end process;
end architecture testbench;

这个测试台使用了一个 `assert` 语句，来检查数码管显示的输出是否正确。你可以将这个代码与可逆计数器和数码管显示的代码一起编译，并运行一个仿真，以测试设计的行为。