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Forum: FPGA, VHDL & Verilog UART in FPGA for receiver


Author: rushin (Guest)
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hello there i want to implement UART for receiver. anyone have sample 
code?? can you help me??

Author: Lothar Miller (lkmiller) (Moderator)
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rushin wrote:
> UART ... anyone have sample code??
http://www.lothar-miller.de/s9y/categories/42-RS232

> can you help me??
You should start with a simple blinking LED, then do a chasing light. 
Its no good idea to start with a big system from the scratch...

> can you help me??
1. only one clock in the whole design
2. this clock is active on the same edge
3. always synchronize in external signals to that clock
4. keep an eye on the reset: is it really necessary in every process?

Author: Rushin T. (rushin_t)
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hello in this link
http://www.lothar-miller.de/s9y/categories/42-RS232

i can't understand this.
  if (TX_Start='1' and txstart='0') then

because above this statement there is statement like that
 txstart <= TX_Start;

if txstart and TX_Start are equal then how you write condition like
TX_Start='1' and txstart='0'??

Author: Lothar Miller (lkmiller) (Moderator)
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Rushin T. wrote:
> because above this statement there is statement like that
>  txstart <= TX_Start;
I mentioned it already: VH-D-L ist not a Programming language... ;-)

The sequence of this instructions does not matter due to the behaviour 
of signals in a VHDL process: a signal does NOT change its value until 
the end of the process (or the next wait statement). And at that very 
point it takes over the last value assigned to it. So the txstart is 
updated to its "new" value at the end of the process...
   -- Senden
   process begin
      wait until rising_edge(CLK);            -- a signal keeps its value throughout the process!
      txstart <= TX_Start;                    -- So, the assignment here ...
      if (TX_Start='1' and txstart='0') then 
        .....
      end if;
      txstart <= TX_Start;                    -- ... or here. Its all the same.
   end process;  -- finally HERE txstart takes over the last assigned value

BTW: a hint from heart to you as the software programmer:
DO NOT USE VARIABLES INSTEAD!
Just get the idea behind this "strange" behaviour of signals...

: Edited by Moderator
Author: Rushin T. (rushin_t)
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can you explain this process??
i am interested in receiver part. so i think this is useful for me.
   process begin
      wait until rising_edge(CLK);
      rxd_sr <= rxd_sr(rxd_sr'left-1 downto 0) & RXD;
      if (rxbitcnt<9) then    -- Empfang läuft
         if(rxcnt<(Quarz_Taktfrequenz/Baudrate)-1) then
            rxcnt    <= rxcnt+1;
         else
            rxcnt    <= 0;
            rxbitcnt <= rxbitcnt+1;
            rxsr     <= rxd_sr(rxd_sr'left-1) & rxsr(rxsr'left downto 
1); -- rechts schieben, weil LSB first
         end if;
      else -- warten auf Startbit
         if (rxd_sr(3 downto 2) = "10") then                 -- fallende 
Flanke Startbit
            rxcnt    <= ((Quarz_Taktfrequenz/Baudrate)-1)/2; -- erst mal 
nur halbe Bitzeit abwarten
            rxbitcnt <= 0;
         end if;
      end if;
   end process;
   RX_Data <= rxsr;
   RX_Busy <= '1' when (rxbitcnt<9) else '0';

Author: Rushin T. (rushin_t)
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thanks for your reply. in many ways vhdl is confusing me.because i have 
used embedded c lot. but thanks for your support

Author: Rushin T. (rushin_t)
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can you explain me ?? that in receiver part why you used 4 bit shift 
register (rxd_sr).

Author: Lothar Miller (lkmiller) (Moderator)
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Rushin T. wrote:
> that in receiver part why you used 4 bit shift register (rxd_sr).
Its due to what I wrote:
Lothar M. wrote:
>>> 3. always synchronize in external signals to that clock
So the first 3 flipflops are for synching to the FPGA clock (although 2 
would be enough). And the fourth one is for the edge detection of the 
start bit...

Author: Rushin T. (rushin_t)
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hello can you help me in writing test bench??

Author: Lothar Miller (lkmiller) (Moderator)
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See there at the very end for a simple stimuli testbench for a RS232 
receiver:
http://www.lothar-miller.de/s9y/archives/60-RS232-...

Author: Rushin T. (rushin_t)
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thanks for your support. in your code RXD is input and in testbench , 
you are giving C3. dout is output and for sometime i get C3. now i want 
like when i transmit C3, in dout i can read C3. what is 3D,3F,3B

: Edited by User
Author: Lothar Miller (lkmiller) (Moderator)
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So, it seems like its time to start thinking...

With the simulator you can have a look for the internal signals of each 
module. There you can find the timing problems. Be sure: its your 
specific implementation that's faulty, because that RS232 interface from 
my HP runs fine in several designs all over the world.

And to find your problem in your code you should post your 
implementation causing that problem...

Author: Rushin T. (rushin_t)
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ok thanks

Author: Rushin T. (rushin_t)
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have you ever used proasic FPGA??. i am using proasic A3E1500. for that 
i am using libero. in libero when i am doing presynthesis simulation for 
your code, it will give me result. but for post synthesis. i am not 
getting result. can you help me??

Author: Lothar Miller (lkmiller) (Moderator)
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I'm sorry, I don't know that toolchain. But also I didn't need anything 
other than behavioral (= pre synthesis) simulation the last ten years...

Do all you need is a behavioral simulation and constraints.

Rushin T. wrote:
> i am not getting result.
WHAT do you expect? And WHAT do you get instead?

Author: Rushin T. (rushin_t)
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hi in your code dout is 8 bit parallel output. is it required to convert 
it into serial???

Author: Rushin T. (rushin_t)
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this is code for 50 mhz crystal. if I change crystal then what is change 
in code??

Author: Lothar Miller (lkmiller) (Moderator)
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Simply change this generic:
Quarz_Taktfrequenz : integer := 50000000; -- Hertz
That's the crystal frequency...

Author: Rushin T. (rushin_t)
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hello thanks for your great support. can you give me test bench of 
http://www.lothar-miller.de/s9y/categories/42-RS232
this code ??


because in this link
 http://www.lothar-miller.de/s9y/archives/60-RS232-...

in code there is no generic for changing clock frequency.

Author: Rushin T. (rushin_t)
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hey thanks for your support. i don't require testbench now. i have made 
it. i fuse that code in my proasic fpga. but i am not getting result. i 
am transmitting 'J' from hyperterminal. i am trying to get that pattern 
on fpga i/o pin. but my problem is if i am not transmitting any thing 
some i/o pins are high. some are low.

can you tell me?? how you test that code on hardware??

do you think that in code it require power on reset pin??

Author: Lothar Miller (lkmiller) (Moderator)
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Rushin T. wrote:
> in code there is no generic for changing clock frequency.
That code is not good for use in a FPGA. It is designed for a CPLD. Take 
that with only 1 clock!

Rushin T. wrote:
> have made it.
Obviously not... :-/

Rushin T. wrote:
> how you test that code on hardware??
FIRST it had to run in simulation!
THEN I simply implemented it on FPGA and did some tests with a terminal 
software.

Author: Rushin T. (rushin_t)
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can you tell me which code is for cpld and which code is for fpga??

Author: Lothar Miller (lkmiller) (Moderator)
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The code with only 1 clock in the whole design is for synchronous FPGA 
design. That there is it:
http://www.lothar-miller.de/s9y/categories/42-RS232

But to repeat it once more: the simulator is the debugger for the 
behavioral model.

And if the simulation is working and the "one and only crystal 
oscillator clock" is used throughout the whole design and every async 
input is synched to that "one and only crystal oscillator clock" then 
the design will run in reality also.

: Edited by Moderator
Author: Rushin T. (rushin_t)
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thanks for your great support. today i made simple code. i am giving 
input to fpga from my rs232 port. and in my code i assign this input to 
my output when rising edge of clock arrives.i am using 48.86Mhz crystal. 
but at my output i am not getting data. when i gave clock to that pin, i 
am getting clock at output..


so i think problem occurs when i am trying to give input from fpga...!

Author: Lothar Miller (lkmiller) (Moderator)
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Rushin T. wrote:
> today i made simple code.
I don't know that code, so I cannot help you any further...

If you post it as an attachment named *.vhdl I can setup a testbench 
(that's only a few minutes) and tell you what's going wrong. As already 
said several times: You MUST run a simulation! Its very easy here!

Author: Rushin T. (rushin_t)
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hello thanks.your code helps me lot. now it is running on hardware. when 
i am transmitting serial input from pc , i am getting output on fpga i/o 
pins. now my output is parallel. but i want to convert it into serial. i 
am trying it. i have written second process for it can you help me??

Author: Rushin T. (rushin_t)
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hey there my output is RXDATA1, from screenshot, you can see that it is 
2B which is parallel. now i want to convert into serial at 9600 hz. can 
you help me?? i have also uploaded my code and test bench here

Author: Rushin T. (rushin_t)
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waiting for your reply

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