vhdl "for loop" with step size not equal to 1

Question

I have a simple question . Is it possible to write a VHDL for loop with step size not equal to 1 e.g 16

The loop should go like

0--> 16 --> 32--> 48.... to some value

any immediate help is appreciated

Answer 1

A possible solution is to use a range that is 1/16th of the desired range and unroll the loop inside it to generate the desired range:

for i in 0 to 3 -- Actually 0 to 48
loop
    x(16*i) <= ...
    x((16*i)+1) <= ...
    (...)
    x((16*i)+15) <= ...
end loop;

Another solution would be to use a while instead. Assuming your count variable is an integer:

while (i < 48)
loop
   --Do something
   i := count + 16;
end loop;

Edit: I haven't tested the code above, you might be unable to change the variable count inside the loop, I'm not sure. Maybe the first solution is the best one.

It is not possible to have a for loop with a step different then 1. You are not even allowed to change it inside the for, like this:

--THIS WILL NOT WORK
for i in 0 to 48 loop
    --Do Something
    i := i + 15; -- This will NOT increment the loop index by 16
end loop; 

And finally, for steps of 2 or 3 you might use nested for's.

But anyway, What are you trying to accomplish? VHDL is a low-level hardware description language, you should be able to achieve whatever you are trying to without fancy for loops.

Answer 2

VHDL has no step parameter in for loop, so the step is always 1 for to range direction and -1 for downto range direction.

So if you need loop with start and step value, you can do:

...
constant FOR_START : natural := 1;
constant FOR_STEP  : natural := 2;
variable idx_v     : natural;       -- Support index variable
...
for idx_pre in 0 to 3 loop
  idx_v := FOR_START + FOR_STEP * idx_pre;
  z_o(idx_v) <= a_i(idx_v);
end loop;

A while loop can also be used as alternative:

constant FOR_START : natural := 1;
constant FOR_STEP  : natural := 2;
constant FOR_MAX   : natural := 7;
variable idx_v     : natural;
...
idx_v := FOR_START;
while idx_v <= FOR_MAX loop
  z_o(idx_v) <= a_i(idx_v);
  idx_v := idx_v + FOR_STEP;
end loop;