BeMicro UART Interface no data on RxD

Question

Hi @ all

I tried the following vhdl process code in order to receive data from pc com interface:

com: process (clk)

variable temp : std_logic_vector(47 downto 0) := "000000000000000000000000000000000000000000000000";

variable zaehler : integer := 100;

variable modus : bit := '0';

begin

-- all led off

if zaehler = 100 then

led0 <= '1';

led1 <= '1';

led2 <= '1';

led3 <= '1';

led4 <= '1';

led5 <= '1';

led6 <= '1';

led7 <= '1';

zaehler := 99;

end if;

if modus = '0' then

------------------------------------------------------------------

if rxd = '1' then

modus := '1';

zaehler := 0;

led1 <= '0';

end if;

------------------------------------------------------------------

-- rxd does not receive data

-- pin planner rxd = PIN_C8

------------------------------------------------------------------

else

end if;

end process com;

the led1 does not turn on as i would expect when sending data via com interface? do you have an clue what i am doin wrong.

I believe that the data that is sent by the pc is not reaching the rxd pin?

best regards thomas

altera_forum · Answer

you should put all the contents of your process inside a  if rising_edge(clk)
    ...
  end ifloop to be sure that everything is clocked.

altera_forum · Answer

In addition, did you notice, that a (logic level) UART has an idle state of '1' and a start bit of '0'?

You could start with a proven UART code, some can be found in Altera forum's previous discussions.

altera_forum · Answer

--- Quote Start ---  you should put all the contents of your process inside a  if rising_edge(clk)
    ...
  end ifloop to be sure that everything is clocked.  --- Quote End ---    Hi I thought The process itself is clk sensitive, every Time The clk Signal changes The Process gets executed. In your Code example it is just rising Edge sensitive or is there something Else that i Miss at that Point?

altera_forum · Answer

Hi I tried the if Statement with rxd ='1' and with rxd='0' but in both Cases i got always nö Response (LED).

altera_forum · Answer

--- Quote Start ---

I thought The process itself is clk sensitive, every Time The clk Signal changes The Process gets executed.

--- Quote End ---

It's common misunderstanding in VHDL design. It possibly works in simulation but never in synthesized logic (FPGA hardware). VHDL is a hardware description language, not a procedural programming language that is executed sequentially.

Examine existing UART designs. They performing everything in an edge sensitive process block.

P.S.: Regarding rxd polarity, I really meant "in addition". Without a reasonable edge sensitive process trigger, it can't work either.

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BeMicro UART Interface no data on RxD

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