Can't assign value to integer signal in VHDL - vhdl

Question

I am using a programmable-logic to decode a sequence of long or short impulses into latin letters according to morse code. I am using VHDL to describe our design, to be precise I'm using Quartus Prime for the design and ModelSim for the simulations. My CPLD is an ALTERA MAX-V 5M160ZE64C5.
Here is my code :
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all ;
use ieee.std_logic_arith.all;
entity SauvezLesMorses is
port
(
-- Input ports
clk : in std_logic;
message : in std_logic;
display : in std_logic;
start : in std_logic;
-- Output ports
seg14 : out std_logic_vector (13 downto 0);
lengthLED : out std_logic := '0'
);
end entity SauvezLesMorses;
architecture SauvezLesMorses_arch of SauvezLesMorses is
type state_t is (A, B, C);
signal state : state_t;
signal count : integer range 0 to 4 := 0;
signal clk_cnt : integer range 0 to 21 := 0;
signal morse : std_logic_vector (3 downto 0);
begin
process (clk, start)
variable vectorDummy : std_logic_vector (3 downto 0);
begin
if (start = '1') then
state <= A;
count <= 0;
seg14 <= "00000010001000";
morse <= "0000";
lengthLED <= '0';
elsif (rising_edge(clk)) then
case state is
-- Idle, listening
when A =>
if (display = '0') then
if (message = '1' and count < 4) then
state <= B;
seg14 <= "00000010001000";
count <= count;
morse <= morse;
lengthLED <= '0';
clk_cnt <= 0;
else
state <= A;
seg14 <= "00000010001000";
count <= count;
morse <= morse;
lengthLED <= '0';
end if;
else
state <= C;
count <= count;
morse <= morse;
lengthLED <= '0';
seg14 <= "00000010001000";
end if;
-- Measuring impulse length
when B =>
if (display = '0') then
if (message = '1') then
state <= B;
count <= count;
morse <= morse;
seg14 <= "00000010001000";
if (clk_cnt < 20) then
clk_cnt <= (1 + clk_cnt);
lengthLED <= '0';
else
clk_cnt <= 21;
lengthLED <= '1';
end if;
else
state <= A;
if (clk_cnt < 21) then
morse <= morse;
else
case count is
when 0 => vectorDummy := "1000";
when 1 => vectorDummy := "0100";
when 2 => vectorDummy := "0010";
when 3 => vectorDummy := "0001";
when others => vectorDummy := "0000";
end case;
morse <= morse or vectorDummy;
end if;
count <= count + 1;
lengthLED <= '0';
seg14 <= "00000010001000";
end if;
else
state <= C;
count <= count;
morse <= morse;
lengthLED <= '0';
seg14 <= "00000010001000";
end if;
-- Displaying converted character to user
when C =>
if (display = '0') then
state <= A;
count <= 0;
seg14 <= "00000010001000";
lengthLED <= '0';
morse <= "0000";
else
state <= C;
count <= count;
morse <= morse;
lengthLED <= '0';
if(count = 1) then
case morse is
when "0000" => seg14 <= "10011110001000"; --E
when "1000" => seg14 <= "10000000100010"; --T
when others => seg14 <= "11111111111111"; --unknown character
end case;
elsif(count = 2) then
case morse is
when "0100" => seg14 <= "11101110001000"; --A
when "1000" => seg14 <= "01101101000100"; --N
when "1100" => seg14 <= "01101101010000"; --M
when "0000" => seg14 <= "00000000100010"; --I
when others => seg14 <= "11111111111111"; --unknown character
end case;
elsif(count = 3) then
case morse is
when "0000" => seg14 <= "10110110001000"; --S
when "0010" => seg14 <= "01111100000000"; --U
when "0100" => seg14 <= "11001110001100"; --R
when "0110" => seg14 <= "01101100000101"; --W
when "1000" => seg14 <= "11110000100010"; --D
when "1010" => seg14 <= "00001110010100"; --K
when "1100" => seg14 <= "10111100001000"; --G
when "1110" => seg14 <= "11111100000000"; --O
when others => seg14 <= "11111111111111"; --unknown character
end case;
elsif(count = 4) then
case morse is
when "0000" => seg14 <= "01101110001000"; --H
when "0001" => seg14 <= "00001100010001"; --V
when "0010" => seg14 <= "10001110001000"; --F
when "0100" => seg14 <= "00011100000000"; --L
when "0110" => seg14 <= "11001110001000"; --P
when "0111" => seg14 <= "01111000000000"; --J
when "1000" => seg14 <= "11110000101010"; --B
when "1001" => seg14 <= "00000001010101"; --X
when "1010" => seg14 <= "10011100000000"; --C
when "1011" => seg14 <= "00000001010010"; --Y
when "1100" => seg14 <= "10010000010001"; --Z
when "1101" => seg14 <= "11111100000100"; --Q
when others => seg14 <= "11111111111111"; --unknown character
end case;
else
seg14 <= "11111111111111";
end if ;
end if;
end case;
end if;
end process;
end architecture SauvezLesMorses_arch ;
A modelsim simulation with parameters
force -freeze sim:/sauvezlesmorses/clk 1 0, 0 {25000000000 ps} -r {50 ms}
force -freeze sim:/sauvezlesmorses/display 0 0, 1 {9000000000000 ps} -r {18 sec}
force -freeze sim:/sauvezlesmorses/message 0 0, 1 {3200000000000 ps} -r {6.4 sec}
force -freeze sim:/sauvezlesmorses/start 1 0 -cancel {0.5 sec}
run 40 sec
which yields :
Modelsim Simulation
clearly shows that :
clk_cnt never increases but rather remains zero for 40 seconds
count is neither set to 0 by the activation of start nor from the desactivation of display (i.e. the transition of state from C to A).
Would you have any idea why?
P.S. I know that I am positively not running a proper testbench. So even if I should, please do not remind it to me unless you know it is part of the answer to my question.

Answer 1

A force updating a signal value doesn't generate an event.
See IEEE Std 1076-2008 14.7.3.4 Signal update, para 3
... If updating a signal causes the current value of that signal to change, then an event is said to have occurred on the signal, unless the update occurs by application of the vhpi_put_value function with an update mode of vhpiDeposit or vhpiForce to an object that represents the signal. ...
Likely the same mechanism used by Modelsim's force or FLI.
With a testbench:
library ieee;
use ieee.std_logic_1164.all;
entity slm_tb is
end entity;
architecture foo of slm_tb is
-- Input ports
signal clk: std_logic := '1';
signal message: std_logic := '0';
signal display: std_logic := '0';
signal start: std_logic := '1';
-- Output ports
signal seg14: std_logic_vector (13 downto 0);
signal lengthLED: std_logic;
begin
DUT:
entity work.sauvezlesmorses
port map (
clk => clk,
message => message,
display => display,
start => start,
seg14 => seg14,
lengthLED => lengthLED
);
-- force -freeze sim:/sauvezlesmorses/clk 1 0, 0 {25000000000 ps} -r {50 ms}
-- force -freeze sim:/sauvezlesmorses/display 0 0, 1 {9000000000000 ps} -r {18 sec}
-- force -freeze sim:/sauvezlesmorses/message 0 0, 1 {3200000000000 ps} -r {6.4 sec}
-- force -freeze sim:/sauvezlesmorses/start 1 0 -cancel {0.5 sec}
-- run 40 sec
-- stimulus generators:
CLOCK:
process
begin
wait for 25 ms;
clk <= not clk;
if now > 40 sec then
wait;
end if;
end process;
DISP:
process
begin
wait for 9 sec;
display <= not display;
if now > 35 sec then -- stop simulation at 40 sec
wait;
end if;
end process;
MSG:
process
begin
wait for 3.2 sec;
message <= not message;
if now > 35 sec then
wait;
end if;
end process;
ST:
process
begin
wait for 0.5 sec;
start <= 'U';
wait;
end process;
end architecture;
You do get events: