Need help in Reading contents of memory array and get a count in vhdl - process

Question

input array: Ram = {D D D D D 3 3 3 3 7 7 7 9 9 8 8};
I am expecting the output as below:
Count = {0 0 0 4 0 0 0 3 2 2 0 0 0 5 0 0};
That is to count the number of occurances, like there are five D's , four 3's , three 7's, two 9's and two 8's so that Count array has 5 in Dth index, 4 in 3rd index, 3 in 7th index and so on.
my code is as below:
architecture behav of Bitcount is
signal cnt: count_type := (others=> (others=>'0'));
use ieee.numeric_std.all;
begin
Countproc:process(clk)
begin
if (clk'event and clk='1' ) then
if Enable='1' then
if (Read_bit='1') then
for k in 0 to 15 loop
for i in 0 to 15 loop
if ( Ram(k) = std_logic_vector(to_unsigned(i, 4)) )then
cnt(i) <= cnt(i) + "01";
end if;
end loop;
end loop;
end if;
end if;
Count <= cnt;
end if;
end process Countproc;
end behav;
but its not working as my expectation . pls help

Answer 1

Your problem is that cnt is a signal. It needs to be a variable.
The lines of code within a process are executed sequentially. Whenever a line of code containing a signal assignment (<=) is executed, an event is put on the event queue to drive the target signal (cnt in this case) on the next delta cycle (assuming there is some change to the target signal as a result). The event queue is the simulator's "to do" list; a delta cycle is one iteration of the simulator; the next iteration will occur once all the processes that are executing in the current iteration suspend.
If, during any execution of a process, another signal assignment to the same target signal is encountered, then (usually*) any other events on the event queue relating to that signal are deleted. So, in your example, every time this line is encountered (which it will be 16*16=256 times):
cnt(i) <= cnt(i) + "01";
Instead of incrementing cnt(i) (which is what I assume you are expecting to happen), all previous events relating to cnt(i) are deleted and replaced with a new event to drive cnt(i) with the value it had before the process started executing plus one. The signal cnt(i) does not get its new value until the process completes executed (and all other processes too).
Variables do not exhibit this behaviour; variables behave just like variables do in any software language. When a line of code is encountered with a variable assignment (:=), the variable is updated immediately and so its value is immediately available for further calculations.
So, how about changing cnt to a variable:
architecture behav of Bitcount is
use ieee.numeric_std.all;
begin
Countproc:process(clk)
variable cnt : count_type;
begin
if (clk'event and clk='1' ) then
if Enable='1' then
if (Read_bit='1') then
cnt := (others=> (others=>'0'));
for k in 0 to 15 loop
for i in 0 to 15 loop
if ( Ram(k) = std_logic_vector(to_unsigned(i, 4)) )then
cnt (i) := cnt (i) + "01";
end if;
end loop;
end loop;
Count <= cnt;
end if;
end if;
end if;
end process Countproc;
end behav;
I can't be sure this will work, because you haven't provided an MCVE, so I haven't tested it. But you original solution will not work for the reason I have given.
--
* this is nearly always true, because of the way that VHDL models delays as inertial delays and it is certainly true in this example. But strictly it's not always true.