Why am getting inferred latches?

问题

I am aware that inferred latches occur when not every possible path is defined, but I have made considerations to avoid this in my process:

The signal is:

signal BothButtons : std_logic_vector (1 downto 0) ;

The process is:

Signaling : process(button0, button1)
begin

    if (button0= '0') AND (button1 = '0') then
        BothButtons <= "00";
    elsif (button0= '0') AND (button1 = '1') then
        BothButtons <= "01";
    elsif (button0= '1') AND (button1 = '0') then
        BothButtons <= "10";
    elsif (button0= '1') AND (button1 = '1') then
        BothButtons <= "11";    
    end if;
end process;

This is driving me crazy, any help is appreciated, maybe I lack an understanding of something really simple!

The error is:

Warning (10631): VHDL Process Statement warning at swDisplay.vhd(28): inferring latch(es) for signal or variable "BothButtons", which holds its previous value in one or more paths through the process

As far as I am aware I am not assigning to the signal two values at the same time, rather it is in receiving values in different situations?

this time I am using that previous signal to drive the output of another process but another latch is appearing within it, this time I did take into account any other value and place an "else" statement to take care of that but no luck:

Counting : process(BothButtons) 

variable count0 : integer range 0 to 9; -- to hold the counter value

begin 


if BothButtons = "00" then
           count0 := 0;
elsif BothButtons = "01" then
           count0 := count0 + 1;
elsif BothButtons = "10" then
            count0 := count0;
elsif BothButtons = "11" then
            count0 := count0;
else
               count0 := 0;
end if;

For those of you who are wondering, yes this is part of an academic exercise!

回答1:

What happens if button0 is neither '0' nor '1'? There's your latch. (ditto button1) Even 'H' and 'L' will confuse it, even though these have a clear meaning to you or me...

Now what do you need that BothButtons <= button0 & button1; doesn't do? (I may have misunderstood the problem you are having)

回答2:

You have latches because you have a process with memory without a clock.

In the first example you give you just need an else in the end of the if-case. Otherwise it is forced to use a previous value, which requires that it has some memory of this previous value. Memory requires latches or flip-flops - and without a clock it is forced to use latches.

In the second example the lines count0 := count0 + 1; and count0 := count0; uses the value from a previous iteration of the process. This requires memory. And memory without a clock gives you latches.

来源：https://stackoverflow.com/questions/33279873/why-am-getting-inferred-latches

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