信号temp2无法合成，同步描述不好

问题描述

entity timer is
    Port ( click : in  STD_LOGIC;
           clear : out  STD_LOGIC;
           t_unlock : out  STD_LOGIC);
end timer;

architecture Behavioral of timer is
    signal temp2 : integer range 0 to 20 := 0;
begin
    process
    begin
        if rising_edge(click) then
            temp2<=0;
            clear<='0';
            t_unlock<='0';
        else
            temp2<=temp2+1 after 15 ns;
        end if;
        if temp2=6 then
            clear<='1';
        elsif temp2=20 then
            t_unlock<='1';
        end if;
    end process;
end Behavioral;

我写了这段代码.编译器说:

I have writted this code.And the complier say:

Signal temp2 cannot be synthesized, bad synchronous description. The description style you are using to describe a synchronous element (register, memory, etc.) is not supported in the current software release.

我在 stackoverflow 上搜索过.他们说 错误'bad synchronousdescription' 通常表示您描述了硬件中不存在的寄存器(时钟元素).但我不知道如何解决我的问题.

I have searched on stackoverflow.They say The error 'bad synchronous description' usually means that you have described a register (clocked element) that does not exist in the hardware.But I don't know how to solve my problem.

推荐答案

VHDL 必须遵循一些综合工具特定的编码指南，以便该工具能够将 VHDL 代码转换为 FPGA 实现.对于异步复位触发器的实现，样式可以是:

The VHDL has to follow some synthesis tool specific coding guidelines, for the tool to be able to translate the VHDL code into the FPGA implementation. For implementation to a flip-flop with asynchronous reset, the style can be:

process (clk, rst) is
begin
  -- Clock
  if rising_edge(clk) then
    ... -- Update at clock
  end if;
  -- Asynchronous reset
  if rst = '1' then
    ... -- Update at reset
  end if;
end process;

就您的代码而言，您似乎没有使用异步重置，因此模板可能会简化为:

In the case of your code it looks like you are not using the asynchronous reset, thus the template may be reduced to:

process (clk) is
begin
  if rising_edge(clk) then
    ... -- Update at clock
  end if;
end process;

现在的练习是让您将代码放入该模板中，不幸的是，很难根据提供的代码确定确切的意图.

The exercise is now for you to fit your code into that template, and unfortunately it is pretty hard to determine the exact intention based on the provided code.

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