编写VHDL优先级编码器的简短方法 [英] short way to write VHDL priority encoder

问题描述

你能告诉我是否有更好的方法在VHDL中编写优先级编码器而不仅仅是使用if-else语句？我的意思是，我有这个代码，它的工作原理，但它可以写得更短吗？ Switch是一个std_logic_vector。

Could you tell me if there is a better way to write a priority encoder in VHDL than just using if-else statements? I mean, I have got this code and it works, but could it be written shorter? Switch is an std_logic_vector.

感谢您的帮助！

shifting : PROCESS(CLK_IN, new_length_ready, switch)

    VARIABLE highest_switch :      INTEGER RANGE 0 TO 15 := 15;

BEGIN

    -- priority encoder, changes the length of the snake track 
    -- ( i.e., number of leds for the snake to move )

    IF switch(15) = '1' THEN
        highest_switch := 15;
    ELSIF switch(14) = '1' THEN
        highest_switch := 14;
    ELSIF switch(13) = '1' THEN
        highest_switch := 13;
    ELSIF switch(12) = '1' THEN
        highest_switch := 12;
    ELSIF switch(11) = '1' THEN
        highest_switch := 11;
    ELSIF switch(10) = '1' THEN
        highest_switch := 10;
    ELSIF switch(9) = '1' THEN
        highest_switch := 9;
    ELSIF switch(8) = '1' THEN
        highest_switch := 8;
    ELSIF switch(7) = '1' THEN
        highest_switch := 7;
    ELSIF switch(6) = '1' THEN
        highest_switch := 6;
    ELSIF switch(5) = '1' THEN
        highest_switch := 5;
    ELSIF switch(4) = '1' THEN
        highest_switch := 4;
    ELSIF switch(3) = '1' THEN
        highest_switch := 3;
    ELSIF switch(2) = '1' THEN
        highest_switch := 2;
    ELSIF switch(1) = '1' THEN
        highest_switch := 1;
    ELSIF switch(0) = '1' THEN
        highest_switch := 0;
    ELSE
        highest_switch := 15;
    END IF

推荐答案

在一个过程中，你可以使用循环。

Within a process, you can use a loop.

type Switches is range 0 to 15;

shifting : PROCESS(switch)
   VARIABLE highest_switch : Switches := 15;
begin
   for i in Switches loop
      if switch(i) = '1' then 
         highest_switch := i;
      end if;
   end loop
...
end process;

请注意，如果设置了多个开关，则会多次分配变量。然而，当循环按递增顺序执行时，最后一个（最高）分配获胜。

Notice that if several switches are set, the variable is assigned multiple times. However as the loop is executed in increasing order, the last (highest) assignment wins.

顺便说一句，这个过程和你的一样是组合的，但我从灵敏度中删除了不必要的信号名单。让它同步会更好;

Incidentally this process, like yours is combinational, but I removed unnecessary signals from the sensitivity list. It would be better to make it synchronous;

shifting : PROCESS(clk)
begin
   if rising_edge(clk) then
      for i in ...
   end if;
end process;

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