UART communication between computer and BASYS 3 FPGA - vhdl

Question

I have a project in which I need to send data from a Windows 10 computer to a BASYS 3 board(ARTIX7 FPGA). I use UART to do so. The data to send is entered in a PuTTY serial console.
For testing purposes, I decided to display the received data using 8 LEDs on the board.
I am using Vivado 2016.4.
The problem I have is that the data I get on the LEDs is totally different from what it should be. I suppose it's a problem of synchronization between PuTTY's baud rate and my VHDL module.
Please find hereafter the .vhd file and the .xdc file of this project :
The .vhd is based on a finite state machine (FSM), and there are two signals allowing the synchronization :
tick_UART : it ticks every 10417 clock period. Since the clock period is 10 ns, tick_UART rises 9600 times per second (I intend a use at 9600 bauds).
double_tick_UART : twice the frequency of tick_UART, used to sample the bits in the middle.
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;
entity UART_RX is
Port ( RxD : in STD_LOGIC;
clk : in STD_LOGIC;
RAZ : in STD_LOGIC;
data_out : out STD_LOGIC_VECTOR (7 downto 0));
end UART_RX;
architecture Behavioral of UART_RX is
signal tick_UART : STD_LOGIC; -- Signal "top" passage d'un état à l'autre selon vitesse connexion série
signal double_tick_UART : STD_LOGIC; -- Signal précédent, fréquence * 2
signal compteur_tick_UART : integer range 0 to 10420; -- Compteur pour tick_UART
signal double_compteur_tick_UART : integer range 0 to 5210; -- Compteur pour demi-périodes
type state_type is (idle, start, demiStart, b0, b1, b2, b3, b4, b5, b6, b7, stop); -- Etats de la FSM
signal state :state_type := idle; -- Etat par défaut
signal RAZ_tick_UART : STD_LOGIC; -- RAZ du signal tick_UART;
begin
process(clk, RAZ, state, RAZ_tick_UART) -- Compteur classique (tick_UART)
begin
if (raz='1') or (state = idle) or (RAZ_tick_UART = '1') then
compteur_tick_UART <= 0;
tick_UART <= '0';
elsif clk = '1' and clk'event then
if compteur_tick_UART = 10417 then
tick_UART <= '1';
compteur_tick_UART <= 0;
else
compteur_tick_UART <= compteur_tick_UART + 1;
tick_UART <= '0';
end if;
end if;
end process;
process(clk, RAZ, state) -- Compteur demi-périodes (double_tick_UART car fréquence double)
begin
if (raz='1') or (state = idle) then
double_compteur_tick_UART <= 0;
double_tick_UART <= '0';
elsif clk = '1' and clk'event then
if double_compteur_tick_UART = 5209 then
double_tick_UART <= '1';
double_compteur_tick_UART <= 0;
else
double_compteur_tick_UART <= double_compteur_tick_UART + 1;
double_tick_UART <= '0';
end if;
end if;
end process;
fsm:process(clk, RAZ) -- Machine à état
begin
if (RAZ = '1') then
state <= idle;
data_out <= "00000000";
RAZ_tick_UART <= '1';
elsif clk = '1' and clk'event then
case state is
when idle => if RxD = '0' then -- Si front descendant de RxD et en idle
state <= start;
RAZ_tick_UART <= '1';
end if;
when start => if double_tick_UART = '1' then
state <= demiStart;
RAZ_tick_UART <= '0';
end if;
data_out <= "00000000";
when demiStart => if tick_UART = '1' then
state <= b0;
RAZ_tick_UART <= '0';
end if;
data_out(0) <= RxD; -- Acquisition bit 0
when b0 => if tick_UART = '1' then
state <= b1;
end if;
data_out(1) <= RxD; -- Acquisition bit 1
when b1 => if tick_UART = '1' then
state <= b2;
end if;
data_out(2) <= RxD; -- Acquisition bit 2
when b2 => if tick_UART = '1' then
state <= b3;
end if;
data_out(3) <= RxD; -- Acquisition bit 3
when b3 => if tick_UART = '1' then
state <= b4;
end if;
data_out(4) <= RxD; -- Acquisition bit 4
when b4 => if tick_UART = '1' then
state <= b5;
end if;
data_out(5) <= RxD; -- Acquisition bit 5
when b5 => if tick_UART = '1' then
state <= b6;
end if;
data_out(6) <= RxD; -- Acquisition bit 6
when b6 => if tick_UART = '1' then
state <= b7;
end if;
data_out(7) <= RxD; -- Acquisition bit 7
when b7 => if tick_UART = '1' then
state <= stop;
end if;
when stop => if tick_UART = '1' then
state <= idle; -- Renvoi en idle
end if;
end case;
end if;
end process;
end Behavioral;
XDC file :
## Clock signal
set_property PACKAGE_PIN W5 [get_ports clk]
set_property IOSTANDARD LVCMOS33 [get_ports clk]
create_clock -add -name sys_clk_pin -period 10.00 -waveform {0 5} [get_ports clk]
## LEDs
set_property PACKAGE_PIN U16 [get_ports data_out[0]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[0]]
set_property PACKAGE_PIN E19 [get_ports data_out[1]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[1]]
set_property PACKAGE_PIN U19 [get_ports data_out[2]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[2]]
set_property PACKAGE_PIN V19 [get_ports data_out[3]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[3]]
set_property PACKAGE_PIN W18 [get_ports data_out[4]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[4]]
set_property PACKAGE_PIN U15 [get_ports data_out[5]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[5]]
set_property PACKAGE_PIN U14 [get_ports data_out[6]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[6]]
set_property PACKAGE_PIN V14 [get_ports data_out[7]]
set_property IOSTANDARD LVCMOS33 [get_ports data_out[7]]
##Buttons
set_property PACKAGE_PIN T18 [get_ports RAZ]
set_property IOSTANDARD LVCMOS33 [get_ports RAZ]
##USB-RS232 Interface
set_property PACKAGE_PIN B18 [get_ports RxD]
set_property IOSTANDARD LVCMOS33 [get_ports RxD]
Do you spot any mistake ?
I also tried to use another .vhd (not written by myself and supposed to work).
This didn't work either: https://www.nandland.com/vhdl/modules/module-uart-serial-port-rs232.html
(I well modified the generic g_CLKS_PER_BIT in accordance with my clock & baud rate)
The issue could come from PuTTY but I've set a baud rate of 9600 baud, 8 data bits, 1 stop bit, no parity, so I don't see what could be wrong !
If you have further ideas / comments, because I can't find what's wrong !
Thank you very much !
EDIT March 16, 2017 :
Following #J.H.Bonarius & #user1155120 recommandations, I added a 2 stage flip flop synchronizer to synchronize the RxD incoming signal with my 100 MHz clock domain.
I also modified modified some asynchronous resets.
Nevertheless, I still have the same issue (LEDs not corresponding to what's sent via PuTTY).
Find hearafter the new .vhd code:
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;
entity UART_RX is
Port ( RxD_in : in STD_LOGIC;
clk : in STD_LOGIC;
RAZ : in STD_LOGIC;
data_out : out STD_LOGIC_VECTOR (7 downto 0));
end UART_RX;
architecture Behavioral of UART_RX is
signal tick_UART : STD_LOGIC; -- Signal "top" passage d'un état à l'autre selon vitesse connexion série
signal double_tick_UART : STD_LOGIC; -- Signal précédent, fréquence * 2
signal compteur_tick_UART : integer range 0 to 10420; -- Compteur pour tick_UART
signal double_compteur_tick_UART : integer range 0 to 5210; -- Compteur pour demi-périodes
type state_type is (idle, start, demiStart, b0, b1, b2, b3, b4, b5, b6, b7); -- Etats de la FSM
signal state :state_type := idle; -- Etat par défaut
signal RAZ_tick_UART : STD_LOGIC; -- RAZ du signal tick_UART;
signal RxD_temp : STD_LOGIC; -- RxD provisoire entre deux FF
signal RxD_sync : STD_LOGIC; -- RxD synchronisé sur l'horloge
begin
D_flip_flop_1:process(clk) -- Clock crossing
begin
if clk = '1' and clk'event then
RxD_temp <= RxD_in;
end if;
end process;
D_flip_flop_2:process(clk) -- Clock crossing
begin
if clk = '1' and clk'event then
RxD_sync <= RxD_temp;
end if;
end process;
tickUART:process(clk, RAZ, state, RAZ_tick_UART) -- Compteur classique (tick_UART)
begin
if clk = '1' and clk'event then
if (RAZ='1') or (state = idle) or (RAZ_tick_UART = '1') then
compteur_tick_UART <= 0;
tick_UART <= '0';
elsif compteur_tick_UART = 10417 then
tick_UART <= '1';
compteur_tick_UART <= 0;
else
compteur_tick_UART <= compteur_tick_UART + 1;
tick_UART <= '0';
end if;
end if;
end process;
doubleTickUART:process(clk, RAZ, state) -- Compteur demi-périodes (double_tick_UART car fréquence double)
begin
if clk = '1' and clk'event then
if (RAZ='1') or (state = idle) then
double_compteur_tick_UART <= 0;
double_tick_UART <= '0';
elsif double_compteur_tick_UART = 5209 then
double_tick_UART <= '1';
double_compteur_tick_UART <= 0;
else
double_compteur_tick_UART <= double_compteur_tick_UART + 1;
double_tick_UART <= '0';
end if;
end if;
end process;
fsm:process(clk, RAZ) -- Machine à état
begin
if (RAZ = '1') then
state <= idle;
data_out <= "00000000";
RAZ_tick_UART <= '1';
elsif clk = '1' and clk'event then
case state is
when idle => if RxD_sync = '0' then -- Si front descendant de RxD (= bit de start) et en idle
state <= start;
RAZ_tick_UART <= '1';
end if;
when start =>if double_tick_UART = '1' then -- Demi période écoulée (pour échantillonage)
state <= demiStart;
RAZ_tick_UART <= '0'; -- Le compteur tick_UART commence à compter
end if;
data_out <= "00000000"; -- Reset des anciennes données
when demiStart => if tick_UART = '1' then
state <= b0;
RAZ_tick_UART <= '0';
end if;
data_out(0) <= RxD_sync; -- Acquisition bit 0
when b0 => if tick_UART = '1' then
state <= b1;
end if;
data_out(1) <= RxD_sync; -- Acquisition bit 1
when b1 => if tick_UART = '1' then
state <= b2;
end if;
data_out(2) <= RxD_sync; -- Acquisition bit 2
when b2 => if tick_UART = '1' then
state <= b3;
end if;
data_out(3) <= RxD_sync; -- Acquisition bit 3
when b3 => if tick_UART = '1' then
state <= b4;
end if;
data_out(4) <= RxD_sync; -- Acquisition bit 4
when b4 => if tick_UART = '1' then
state <= b5;
end if;
data_out(5) <= RxD_sync; -- Acquisition bit 5
when b5 => if tick_UART = '1' then
state <= b6;
end if;
data_out(6) <= RxD_sync; -- Acquisition bit 6
when b6 => if tick_UART = '1' then
state <= b7;
end if;
data_out(7) <= RxD_sync; -- Acquisition bit 7
when b7 => if tick_UART = '1' then
state <= idle; -- state <= stop;
end if;
end case;
end if;
end process;
end Behavioral;
Do you have any idea about the origin of my problem ?
Thank you very much !

Answer 1

First if (raz='1') or (state = idle) or (RAZ_tick_UART = '1') then don't put so many things in an asynchronous reset input. Actually: don't use asynchronous resets at all. They will introduce logic into the clock path.
Second thing: it's probably a good idea to put some clock domain synchronizations on your UART RxD. Just a two stage synchronizer. Else when idle => if RxD = '0' then will be affected by glitches.