vhdl case语句给两个值赋值
时间: 2023-08-04 11:13:49 浏览: 62
在VHDL中,使用case语句可以根据不同的条件执行不同的操作。如果要给两个值赋值,可以在case语句中使用两个分支,每个分支都赋值一个值。
以下是一个简单的例子:
```
process (input)
begin
case input is
when '0' =>
a <= '1';
b <= '0';
when '1' =>
a <= '0';
b <= '1';
when others =>
a <= 'Z';
b <= 'Z';
end case;
end process;
```
在上面的例子中,当输入信号为'0'时,a被赋值为'1',b被赋值为'0';当输入信号为'1'时,a被赋值为'0',b被赋值为'1';否则,a和b都被赋值为'Z'。根据实际情况修改分支即可。
相关问题
给出真值表,写一段用到CASE语句的VHDL语言
以下是一个四输入逻辑门的真值表:
| A | B | C | D | OUT |
|---|---|---|---|-----|
| 0 | 0 | 0 | 0 | 1 |
| 0 | 0 | 0 | 1 | 0 |
| 0 | 0 | 1 | 0 | 0 |
| 0 | 0 | 1 | 1 | 1 |
| 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 | 1 |
| 0 | 1 | 1 | 0 | 1 |
| 0 | 1 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 | 0 |
| 1 | 0 | 0 | 1 | 1 |
| 1 | 0 | 1 | 0 | 1 |
| 1 | 0 | 1 | 1 | 0 |
| 1 | 1 | 0 | 0 | 1 |
| 1 | 1 | 0 | 1 | 0 |
| 1 | 1 | 1 | 0 | 0 |
| 1 | 1 | 1 | 1 | 1 |
下面是相应的VHDL代码:
```vhdl
library ieee;
use ieee.std_logic_1164.all;
entity four_input_logic_gate is
port (
A, B, C, D : in std_logic;
OUT : out std_logic
);
end four_input_logic_gate;
architecture rtl of four_input_logic_gate is
begin
process (A, B, C, D)
begin
case (A & B & C & D) is
when "0000" =>
OUT <= '1';
when "0001" =>
OUT <= '0';
when "0010" =>
OUT <= '0';
when "0011" =>
OUT <= '1';
when "0100" =>
OUT <= '0';
when "0101" =>
OUT <= '1';
when "0110" =>
OUT <= '1';
when "0111" =>
OUT <= '0';
when "1000" =>
OUT <= '0';
when "1001" =>
OUT <= '1';
when "1010" =>
OUT <= '1';
when "1011" =>
OUT <= '0';
when "1100" =>
OUT <= '1';
when "1101" =>
OUT <= '0';
when "1110" =>
OUT <= '0';
when "1111" =>
OUT <= '1';
end case;
end process;
end rtl;
```
这段代码中,我们使用了一个process块,来响应输入的变化。在process块中,我们使用了`case`语句,对输入进行判断,并根据真值表的结果来赋值输出。`&`符号表示逻辑与运算。
vhdl用case语句实现四选一选择器
可以使用以下代码实现四选一选择器:
```vhdl
entity selector is
port (
A : in std_logic_vector(1 downto 0);
B : in std_logic_vector(1 downto 0);
C : in std_logic_vector(1 downto 0);
D : in std_logic_vector(1 downto 0);
S : in std_logic_vector(1 downto 0);
Y : out std_logic_vector(1 downto 0)
);
end entity;
architecture Behavioral of selector is
begin
process(S, A, B, C, D)
begin
case S is
when "00" =>
Y <= A;
when "01" =>
Y <= B;
when "10" =>
Y <= C;
when "11" =>
Y <= D;
when others =>
Y <= (others => 'X');
end case;
end process;
end architecture;
```
在这个例子中,我们使用了一个 `process` 进程来对输入信号 `S` 进行判断,并将对应的输出信号 `Y` 赋值为对应的输入信号 `A`、`B`、`C` 或 `D`。如果输入信号 `S` 不符合任何一个条件,输出信号 `Y` 将被赋值为 `'X'`。