下面这串代码如何改才不会报错“process variable r_cnt: std_logic_vector(9 downto 0):="00000000000000000000" ; begin wait until rising_edge(i_ADC_clk); if i_rst = '1' then r_cnt <= "00000000000000000000"; else r_cnt <= r_cnt + 1; end if; wait until rising_edge(i_ADC_clk); case r_cnt is when 20000 => i_ADC <= "0000001010"; when 400000 => i_ADC <= "0101010111"; when 1500000 => i_ADC <= "0110110100"; when 1950000 => i_ADC <= "1000010111"; when 2000000 => i_ADC <= "1010000000"; when 4000000 => i_ADC <= "0111000111"; when 6000000 => i_ADC <= "0010000010"; when 8000000 => i_ADC <= "1100011000"; when 10000000 => i_ADC <= "0011111000"; when others => null; end case; end process;”

这段代码中使用了一个变量 r_cnt，需要将其改为信号类型，因为在 VHDL 中只有信号才能在不同的过程之间传递数据。同时，需要注意在时序逻辑中，对信号的赋值必须在时钟上升沿时进行。修改后的代码如下：

process
    variable r_cnt : natural := 0;
begin
    wait until rising_edge(i_ADC_clk);
    if i_rst = '1' then
        r_cnt := 0;
    else
        r_cnt := r_cnt + 1;
    end if;
    wait until rising_edge(i_ADC_clk);
    case r_cnt is
        when 20000 =>
            i_ADC <= "0000001010";
        when 400000 =>
            i_ADC <= "0101010111";
        when 1500000 =>
            i_ADC <= "0110110100";
        when 1950000 =>
            i_ADC <= "1000010111";
        when 2000000 =>
            i_ADC <= "1010000000";
        when 4000000 =>
            i_ADC <= "0111000111";
        when 6000000 =>
            i_ADC <= "0010000010";
        when 8000000 =>
            i_ADC <= "1100011000";
        when 10000000 =>
            i_ADC <= "0011111000";
        when others =>
            i_ADC <= (others => '0');
    end case;
end process;

修改后，r_cnt 变成了一个自然数类型的变量，并且在 process 中使用了一个等待语句等待 ADC 时钟上升沿。在 case 语句中，当 r_cnt 的值匹配到某个特定的值时，给 i_ADC 赋以相应的值。注意在 case 的 others 分支中，需要对 i_ADC 进行赋值，否则会产生 X 值。