VHDL initalize signed of variable length to maximum value

I've got the following function that returns the value variable when it can be represented with the length given as a parameter. If value is out of limits, it should return the maximum possible value or the maximum negative value depending on the sign of value.

function truncate (
value : in signed;
length : in integer)
return signed is
constant max_positive_value : signed(length-1 downto 0) := ((length-1) => '0', others => '1');
constant max_negative_value : signed(length-1 downto 0) := ((length-1) => '1', others => '0');
variable return_value : signed(length-1 downto 0) := (others => '0');
begin
if (value >= max_positive_value) then
return_value := max_positive_value;
elsif (value <= max_negative_value) then
return_value := max_negative_value;
else
return_value := resize(value, length);
end if;
return return_value;
end;


The problem is the initialization of the max_positive_value and max_negative_value. GHDL complains about not static choice exclude others choice.
How can I initalize the limit values if the length is variable?
I'm using VHDL 93.

max_positive_value

max_negative_value

not static choice exclude others choice

3 Answers
3

Fortunately, since your odd-man-out is on the left, you can do this:

constant max_positive_value : signed(length-1 downto 0) := ('0', others => '1');
constant max_negative_value : signed(length-1 downto 0) := ('1', others => '0');


https://www.edaplayground.com/x/64S7

The LRM says

Apart from a final element association with the single choice others,
the rest (if any) of the element associations of an array aggregate
shall be either all positional or all named. A named association of an
array aggregate is allowed to have a choice that is not locally
static, or likewise a choice that is a null range, only if the
aggregate includes a single element association and this element
association has a single choice. An others choice is locally static if
the applicable index constraint is locally static.

which is as clear as mud.

Non-locally static OTHERS choice is allowed only if it is the only choice of the only association.

V5 := truncate(V, 5);

I found another way to do it that Modelsim accepts without a warning:

constant max_positive_value : signed(length-1 downto 0) := to_signed(2**(length-1)-1, length);
constant max_negative_value : signed(length-1 downto 0) := to_signed(-2**(length-1), length);


The other way to do it is to declare a constant locally to work around the good old "not locally static" VHDL issues:

constant C_LEN : integer := length
constant max_negative_value : signed(length-1 downto 0) := ((C_LEN-1) => '1', others => '0');


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