RS-232 between computer and virtex5 FPGA

Hi guys,

I'm quite new to FPGA's and now I'm trying to make a serial rs232 interfase from my computer to my virtex 5 ml507 FPGA. I'm trying to light some LEDs depending on the character I type in HYPERTERMINAL on the computer. I think there is nothing wrong with the serial connection, because when I switch the non-configured FPGA on, the words of the display appear in hyperterminal.

So the problem should be in the code (see below), I think. The problem could be in the baud speed, I calculated this as 3437 because I have 9600 bits/s (done by the configuration of the hyperterminal) and use a 33 MHz clock signal. What would be my mistake? I'm totally stuck...

Thanks in advance :grin:,

Simon

LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_ARITH.ALL;
USE IEEE.STD_LOGIC_UNSIGNED.ALL;

--SIMONS RS232 project
ENTITY RS232 IS
PORT(
CLK : IN STD_LOGIC;
RS232_IN : IN STD_LOGIC;
LED_DISPLAY : OUT STD_LOGIC_VECTOR(6 DOWNTO 0));
end RS232;


ARCHITECTURE behavior of RS232 IS

--Numbers
CONSTANT zero : STD_LOGIC_VECTOR := "0000001";
CONSTANT one : STD_LOGIC_VECTOR := "0111101";
CONSTANT two : STD_LOGIC_VECTOR := "0010010";
CONSTANT three : STD_LOGIC_VECTOR := "0011000";
CONSTANT four : STD_LOGIC_VECTOR := "0101100";
CONSTANT five : STD_LOGIC_VECTOR := "1001000";
CONSTANT six : STD_LOGIC_VECTOR := "1000000";
CONSTANT seven : STD_LOGIC_VECTOR := "0011101";
CONSTANT eight : STD_LOGIC_VECTOR := "0000000";
CONSTANT nine : STD_LOGIC_VECTOR := "0001000";

--Letters
CONSTANT letter_a : STD_LOGIC_VECTOR := "0000100";
CONSTANT letter_b : STD_LOGIC_VECTOR := "1100000";
CONSTANT letter_c : STD_LOGIC_VECTOR := "1000011";
CONSTANT letter_d : STD_LOGIC_VECTOR := "0110000";
CONSTANT letter_e : STD_LOGIC_VECTOR := "1000010";
CONSTANT letter_f : STD_LOGIC_VECTOR := "1000110";
CONSTANT letter_g : STD_LOGIC_VECTOR := "1000000";
CONSTANT letter_h : STD_LOGIC_VECTOR := "0100100";
CONSTANT letter_i : STD_LOGIC_VECTOR := "0111101";
CONSTANT letter_j : STD_LOGIC_VECTOR := "0110001";
CONSTANT letter_l : STD_LOGIC_VECTOR := "1100011";
CONSTANT letter_n : STD_LOGIC_VECTOR := "0000101";
CONSTANT letter_o : STD_LOGIC_VECTOR := "0000001";
CONSTANT letter_p : STD_LOGIC_VECTOR := "0000110";
CONSTANT letter_s : STD_LOGIC_VECTOR := "1001000";
CONSTANT letter_u : STD_LOGIC_VECTOR := "0100001";
CONSTANT letter_y : STD_LOGIC_VECTOR := "0001000";
CONSTANT letter_z : STD_LOGIC_VECTOR := "0010010";


CONSTANT BAUD : INTEGER := 3157;


SIGNAL SHIFT_REG : STD_LOGIC_VECTOR(7 DOWNTO 0);

SIGNAL START_STOP : STD_LOGIC := '0';

BEGIN

PROCESS
variable timer_cnt: integer range 0 to 100000;
variable char_cnt: integer range 0 to 10;
BEGIN

WAIT UNTIL(clk'EVENT) AND (clk = '1');

--IDLE State
IF(RS232_IN = '0' AND START_STOP = '0')THEN
--Reset Counters
char_cnt := 0;
timer_cnt := 0;
--Start RS232 Capture Process
START_STOP <= '1';
ELSIF(RS232_IN = '1' AND START_STOP = '0')THEN

CASE SHIFT_REG is
--ASCII -> Number, Decoding
WHEN x"30" => LED_DISPLAY <= zero;
WHEN x"31" => LED_DISPLAY <= one;
WHEN x"32" => LED_DISPLAY <= two;
WHEN x"33" => LED_DISPLAY <= three;
WHEN x"34" => LED_DISPLAY <= four;
WHEN x"35" => LED_DISPLAY <= five;
WHEN x"36" => LED_DISPLAY <= six;
WHEN x"37" => LED_DISPLAY <= seven;
WHEN x"38" => LED_DISPLAY <= eight;
WHEN x"39" => LED_DISPLAY <= nine;
--ASCII -> Letter, Decoding
WHEN x"61" => LED_DISPLAY <= letter_a;
WHEN x"62" => LED_DISPLAY <= letter_b;
WHEN x"63" => LED_DISPLAY <= letter_c;
WHEN x"64" => LED_DISPLAY <= letter_d;
WHEN x"65" => LED_DISPLAY <= letter_e;
WHEN x"66" => LED_DISPLAY <= letter_f;
WHEN x"67" => LED_DISPLAY <= letter_g;
WHEN x"68" => LED_DISPLAY <= letter_h;
WHEN x"69" => LED_DISPLAY <= letter_i;
WHEN x"6A" => LED_DISPLAY <= letter_j;
WHEN x"6C" => LED_DISPLAY <= letter_l;
WHEN x"6E" => LED_DISPLAY <= letter_n;
WHEN x"6F" => LED_DISPLAY <= letter_o;
WHEN x"70" => LED_DISPLAY <= letter_p;
WHEN x"73" => LED_DISPLAY <= letter_s;
WHEN x"75" => LED_DISPLAY <= letter_u;
WHEN x"79" => LED_DISPLAY <= letter_y;
WHEN x"7A" => LED_DISPLAY <= letter_z;
--Clear 7 Seg When Others Pressed
WHEN OTHERS => LED_DISPLAY <= "1111111";
END CASE;

END IF;

--Start Getting Serial Data
IF(START_STOP = '1')THEN
IF(timer_cnt = BAUD) THEN
--Update Counter
timer_cnt := 0;

--Update Shift Register
SHIFT_REG <= RS232_IN & SHIFT_REG(7 DOWNTO 1);

--Is 8th Bit?
IF(char_cnt = 7)THEN
START_STOP <= '0';
ELSE
char_cnt := char_cnt + 1;
END IF;
ELSE
timer_cnt := timer_cnt + 1;
END IF;
END IF;

END PROCESS;

END behavior;

- - - Updated - - -

In the code above, the value 3437 isn't used yet, but that makes no difference. Nothing appears when i enter a character in hyperterminal

Several comments:

You have a whole bunch of things being assigned inside a single process. Although this might work, it makes things a lot harder to figure out.
Indenting would help to figure out where IF blocks start and end.
You are correct that 3437 would give you a 9600 Hz clock, however you are using a divider of 3157.
Generally, for serial communications, a faster clock is used to sample incoming data (16x, for example). That way you can make sure you are sampling in the middle of a bit, rather than near the transition. With a clock equal to the data rate you can't guarantee setup and hold times because the two signals are asynchronous.

Using a 'wait' command in a process is not typically used, except for test benches. It is more common to define a clocked process, as you define above, using:


A good starting point of serial communication, including test benches, can be found in this post here:
https://www.edaboard.com/threads/156656/

A direct link to the code mentioned:

I expect you are not using a test bench to check your code. Be advised that a test bench/simulation is, more or less, a necessity. Without a test bench/simulation, finding out how/if your code operates correctly is very hard.

Thanks for the reply's guys, in the meantime I made it work already ;-)
I'm sending data from hyperterminal to the FPGA with the rs-232 to light some LEDs, very basic, I know
If anyone would have the same problem I had, do contact me on simonvandenberge@hotmail.be
I made the same code for a PS/2 keyboard aswell.

please share your solution with all of us. Post your files here.

Thanks