为什么这个移位寄存器不能在 VHDL 中正确加载？

Question

我有一个定制设计的移位寄存器，其输入为 DL（最左侧输入）、DR（最右侧）、清除和加载 DR 的 CLR、向右移位的 S 和加载最左侧的 W。经过测试，最右边的正在加载，而不是左边。我已经多次重读代码，但我无法弄清楚哪里出了问题。代码如下：

    library IEEE;
use IEEE.std_logic_1164.all;

entity shiftregister is
    port (
        CLK, CLR: in STD_LOGIC;
        S: in STD_LOGIC; --Shift right
 W: in STD_LOGIC; --Write
 Cin: in STD_LOGIC; --possible carry in from the addition
        DL: in STD_LOGIC_VECTOR (7 downto 0); --left load for addition result
 DR: in STD_LOGIC_VECTOR (7 downto 0); --right load for initial multiplier
        Q: out STD_LOGIC_VECTOR (15 downto 0)
     );
end shiftregister ;

architecture shiftregister of shiftregister is
signal IQ: std_logic_vector(15 downto 0):= (others => '0');
begin
   process (CLK)
   begin
 if(CLK'event and CLK='1') then 
          if CLR = '1' then 
   IQ(7 downto 0)  <= DR;  --CLR clears and initializes the multiplier
   IQ(15 downto 8) <= (others => '0');
   else 
  if (S='1') then
    IQ <= Cin & IQ(15 downto 1);
  elsif (W='1') then
    IQ(15 downto 8) <= DL;
  end if;
          end if;

 end if;  
    end process;
Q<=IQ;
end shiftregister;

波形

测试台

library IEEE;
use IEEE.std_logic_1164.all;

entity register_tb is
end register_tb;

architecture register_tb of register_tb is
    component shiftregister is port (
        CLK, CLR: in STD_LOGIC;
        S: in STD_LOGIC; --Shift right
        W: in STD_LOGIC; --Write
        Cin: in STD_LOGIC; --possible carry in from the addition
        DL: in STD_LOGIC_VECTOR (7 downto 0); --left load for addition result
        DR: in STD_LOGIC_VECTOR (7 downto 0); --right load for initial multiplier
        Q: out STD_LOGIC_VECTOR (15 downto 0)
    );
    end component;

    signal CLK: std_logic:='0';
    signal CLR: std_logic:='1';
    signal Cin: std_logic:='0';
    signal S: std_logic:='1';
    signal W: std_logic:='0';
    signal DL, DR: std_logic_vector(7 downto 0):="00000000";
    signal Q: std_logic_vector(15 downto 0):="0000000000000000";
begin 
    U0: shiftregister port map (CLK, CLR, S, W, Cin, DL,DR,Q);

    CLR <= not CLR    after 20 ns;
    CLK <= not CLK    after 5 ns;
    W   <= not W      after 10 ns;
    DL  <= "10101010" after 10 ns;
    DR  <= "00110011" after 10 ns;

end register_tb;

Answer 1

您的模拟显示您的S 输入始终很高。您设置条件的方式，这意味着最后一个 elsif 语句将不会执行，因为S 优先于W。如果你希望你的 write 优先于你的 shift 操作，你应该切换你的条件

if (W='1') then
  IQ(15 downto 8) <= DL;
elsif (S='1') then
  IQ <= Cin & IQ(15 downto 1);
end if;

根据您对所需行为的评论，您可以执行以下操作：

if (S='1' and W='1') then
  IQ  <= Cin & DL & IQ(7 downto 1);
elsif (W='1') then -- S=0
  IQ(15 downto 8) <= DL;
elsif (S='1') then -- W=0
  IQ <= Cin & IQ(15 downto 1);
end if; -- W=0 & S=0

Answer 2

一些改进：

(1) 从灵敏度列表中删除除 CLK 之外的所有信号。您的进程没有异步信号，因此灵敏度列表中只需要时钟。

process(CLK)

(2) 仅将零分配给所需的位 -> 品味问题；)

IQ(7 downto 0)  <= DR;              --CLR clears and initializes the multiplier
IQ(15 downto 8) <= (others => '0');

(3) elsif 语句可以明确赋值优先级：

if (S='1') then
  IQ <= Cin & IQ(15 downto 1);
elsif (W='1') then
  IQ(15 downto 8) <= DL;
end if;

(4) 行 Q <= IQ; 生成第二个 16 位寄存器。我认为这不是故意的。将此行移出进程。