新手请教一点语法问题

[复制链接]
2756|3
 楼主| kun5979 发表于 2011-8-7 14:12 | 显示全部楼层 |阅读模式
自学FPGA没多久,对<=,=>,<=这几个符号老是搞混,比如下面这段程序中的语句
entity key_led is
port (
key_in : in std_logic_vector (3 downto 0);   --KEY INPUT
led_out : out std_logic_vector (5 downto 0)  --LED OUTPUT
);
end entity;
architecture key_led_arch of key_led is
begin
process(key_in)
begin
led_out <= (others => '1');   就是这段语句,到底是实现什么功能
case key_in is
when "1110" => led_out <= "111110";
when "1101" => led_out <= "111100";
when "1011" => led_out <= "111000";
when "0111" => led_out <= "110000";
when others => NULL;
end case;
end process;
end architecture;
xiamingmin163 发表于 2011-8-8 17:33 | 显示全部楼层
给led_out的6位全部置为'1'
混杰奋斗 发表于 2011-8-11 15:24 | 显示全部楼层
led_out <= (others => '1');   --led_out<=111111;因为是6位的。
when "1110" => --执行操作,当key_in=1110时,执行后面的操作
led_out <= "110000";--赋值操作,给6位led_out赋值。
mmax 发表于 2011-8-11 21:05 | 显示全部楼层
我问大家一个问题:
led_out <= (others => '1');   
case key_in is
when "1110" => led_out <= "111110";
when "1101" => led_out <= "111100";
when "1011" => led_out <= "111000";
when "0111" => led_out <= "110000";
when others => NULL;


跟下面这样:
case key_in is
when "1110" => led_out <= "111110";
when "1101" => led_out <= "111100";
when "1011" => led_out <= "111000";
when "0111" => led_out <= "110000";
when others => led_out <= (others => '1');  

执行结果有没有区别? 占用的LE大小有没有区别?
您需要登录后才可以回帖 登录 | 注册

本版积分规则

0

主题

16

帖子

0

粉丝
快速回复 在线客服 返回列表 返回顶部