How to calculate the number of bits needed for an std_logic_vector when its size is parametrable?

Question

I have an entity that instanciates a FIFO which depth is a generic:

DEPTH_FIFO : natural range 2 to 64 := 3; -- number of vectors in the FIFO

I have to declare a counter that could store the index of the FIFO but I need to know which size has to be the counter.

signal cnt_FIFO : unsigned(length_cnt_FIFO-1 downto 0);

My problem is to find a way to calculate the constant length_cnt_FIFO.

I have tried this :

constant length_cnt_FIFO : natural := CEIL(LOG(2, DEPTH_FIFO));

with the library use ieee.MATH_REAL.all;

but I get problems of type conversion.

Anyone has an idea to make this work, or any other solution ?

Thanks in advance,

SLP

Answer 1

1. Your DEPTH_FIFO is natural, but you have to convert it to a Real to pass it to the logarithmic function.
2. The CEIL function returns a real: you have to convert it back to a natural to store it in a constant of natural type.
3. The parameters of LOG function are real, so passing a literal 2 causes problems because it is handled as an integer. Use e.g. 2.0 instead or the LOG2 function.

This did the trick for me:

constant length_cnt_FIFO : natural := natural(CEIL(LOG2(real(DEPTH_FIFO))));