不能推断出......的寄存器在......，因为它在时钟沿之外不保持其值

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问题描述

这一定是VHDL新手中最常见的问题，但是我看不到我在这里做错了什么！这似乎符合我在适当的状态机设计上看到的所有习语.我在Altera Quartus 9.2中编译，以达到价值.实际错误是:

"不能在[file] [line]上推断" spiclk_out"寄存器，因为它不会在时钟边缘之外保持其值

ENTITY spi_state_machine IS
    PORT(
            spiclk_internal : IN STD_LOGIC;
            reset : IN STD_LOGIC;
            spiclk_out : BUFFER STD_LOGIC
    );
END spi_state_machine;

PROCESS(spiclk_internal, reset)
BEGIN
    IF reset = '1' THEN
        spiclk_out <= '0';
    END IF;

    IF spiclk_internal = '1' AND spiclk_internal'EVENT THEN --error here
        spiclk_out <= NOT spiclk_out;
    END IF;
END PROCESS;

感谢您的时间.

推荐答案

正如书面的，即使reset处于活动状态，该过程也会导致spiclk_out在spiclk_internal边缘上切换，这不是与异步重置的触发器的行为.

您可能想要的是

SPICLK: process(spiclk_internal, reset)
    if reset = '1' then
        spiclk_out <= '0';
    elsif spiclk_internal'event and spiclk_internal='1' then
        spiclk_out <= not spiclk_out;
    end if;
end process SPICLK;

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问题描述

This must be the most common problem among people new to VHDL, but I don't see what I'm doing wrong here! This seems to conform to all of the idioms that I've seen on proper state machine design. I'm compiling in Altera Quartus 9.2, for what it's worth. The actual error is:

"Can't infer register for "spiclk_out" at [file] [line] because it does not hold its value outside the clock edge"

ENTITY spi_state_machine IS
    PORT(
            spiclk_internal : IN STD_LOGIC;
            reset : IN STD_LOGIC;
            spiclk_out : BUFFER STD_LOGIC
    );
END spi_state_machine;

PROCESS(spiclk_internal, reset)
BEGIN
    IF reset = '1' THEN
        spiclk_out <= '0';
    END IF;

    IF spiclk_internal = '1' AND spiclk_internal'EVENT THEN --error here
        spiclk_out <= NOT spiclk_out;
    END IF;
END PROCESS;

Thanks for your time.

推荐答案

As written, the process would cause spiclk_out to toggle on spiclk_internal edges even when reset is active, which is not how flip-flops with asynchronous resets should behave.

What you probably want is

SPICLK: process(spiclk_internal, reset)
    if reset = '1' then
        spiclk_out <= '0';
    elsif spiclk_internal'event and spiclk_internal='1' then
        spiclk_out <= not spiclk_out;
    end if;
end process SPICLK;

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