POCKETBEAGLE: UART Help - Garbage data along with requisite data

[Piyush Raj ae19m009]

I am implementing UART using Pocketbeagle wherein i am trying to send simple strings over UART . I have written a code for loopback testing of UART-4 in PB. for a logic Low on P2.1  i am sending String "LOW" and "HIGH" for vice versa. However i am getting garbage data along with the requisite strings. sometimes all the received data is missing or broken. I tried to work at various baud rates (600, 9600,115200) but of no help.

Am i doing any basic mistake. need guidance from community. (I worked with AVR and Arduino UART in the past and never faced such problem)

Further, i also tried to dump the received data in a text file( to rule out possibility of terminal baud rate mismatch), but results did not change.

My code is below:

#include<fstream>

#include<iostream>

#include <string.h>

#include <fcntl.h>

#include <termios.h>

#include<unistd.h> //for usleep

#include"GPIO.h"

using namespace exploringBB;

using namespace std;

int main()

{

    GPIO  outLogicSupply(60); outLogicSupply.setDirection(OUTPUT);  

    GPIO  inLogicSupply(59); inLogicSupply.setDirection(INPUT);

    int file,count;

    char transmit[6];

    char receive[10];

      int bytes_written,read_code;int count1=1;

    //////////////// OPENING UART4 ON POCKETBEAGLE /////////////////////

    printf("UART: Transmission Started.\n");

    if ((file = open("/dev/ttyO4", O_RDWR | O_NOCTTY | O_NDELAY))<0)

    {

      perror("UART: Failed to open the device.\n");

      return -1;

    }

    else

    {

        printf("UART: Opened file successfully\n");

    }

    /////////////////////////////////////////////////////////////////////

   ///////////////    SETTING UP UART OPTIONS  ///////////////////////////

  struct termios options;

   tcgetattr(file, &options);

   options.c_cflag = B9600 | CS8 | CREAD | CLOCAL;

   options.c_iflag = IGNPAR | ICRNL;

   tcflush(file, TCIFLUSH);

   fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK);  // make reads non-blocking

   tcsetattr(file, TCSANOW, &options);

  

   cout << "The value of the input is: "<< inLogicSupply.getValue() << endl;

 

   while(1)

    {

      

        if(inLogicSupply.getValue()==1)

       

        {

        strcpy(transmit,"HIGH\n");    

         outLogicSupply.setValue(HIGH);

            

        }

        else

        

        {

        strcpy(transmit,"LOW\n");

         outLogicSupply.setValue(LOW);

        }

        

        bytes_written = write(file, &transmit,6);

        sleep(1);

        read_code= read(file,receive,10);

        if(bytes_written>0  && read_code>0  ) 

        {

        receiveFile<<count1<<"sec - "<<receive;

        cout<<count1<<"sec - "<<receive;

       

        }

        count1++;

        sleep(0.01);

      }

  

  ///////////////////////////////////////////////////////////////

     receiveFile.close();

   close(file);

  

   return 0;

}

[Graham Stott]

Which Pins are you using for UART4 rcv and have you set the PIN mux correctly for that PIN?

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[Piyush Raj ae19m009]

Hi Graham

Thanks for replying

I am using UART 4 , i.e ttyO4 P2.5 and P2.7. 

Further,  as I understand, ttyO4 is enabled on my device by default since I am able to see it on device tree.

When the string to send is "HIGH", I am receiving strings such as

IGH

@IGH

H 

And so on at each read. I have shorted Rx and Tx so possibility of hardware noise is ruled out. What else can I look at?

Help would be deeply appreciated

Regards

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[Dennis Lee Bieber]

On Mon, 1 Mar 2021 18:30:04 -0800 (PST), in
gmane.comp.hardware.beagleboard.user Piyush Raj ae19m009
<ae19m009-p/GD3RyrBS6E...@public.gmane.org> wrote:


>
> bytes_written = write(file, &transmit,6);

You are writing 6 bytes even though "HIGH\n" is only 5 characters (and
"LOW\n" is only 4!). That means you have 1 or 2 bytes of "garbage"
(whatever was in memory -- and since it appears the buffers are being
allocated on the stack that could mean anything). I'm also not sure of that
&transmit -- I thought char arrays automatically pass as the address of the
array.

> sleep(1);
> read_code= read(file,receive,10);

Here you are asking to read 10 bytes, even though you are only writing
4 or 5.

> if(bytes_written>0 && read_code>0 )

Here you only check that at least 1 byte was written and received...
And a 1 byte receive IS possible (I haven't found any documentation that
indicates how read() handles a serial port that does not have data in it...
It's possible that the outgoing port may still be sending [the
"bytes_written" may only indicate that the kernel buffered that many for
the sending device]).

I'd probably change the write() to specify strlen(transmit) AND confirm
that bytes_written = strlen(transmit). Similar for the read() operation --
check how many bytes were received.


--
Dennis L Bieber