Bare Metal RPI 3B UART not working

Question

I have referred many tutorials to get UART working in bare metal environment on RPI 3.

1. https://github.com/dwelch67/raspberrypi/tree/master/uart05

2. https://github.com/mrvn/RaspberryPi-baremetal/tree/master/003-can-you-c-me

3. https://wiki.osdev.org/Raspberry_Pi_Bare_Bones

I tried all these codes nothing worked. Nothing gets sent/received from host computer serial terminal to RPI.

I have following files in my sd card.

1. kernel7.img (uart code)
2. fixup.dat
3. start.elf
4. config.txt
5. cmdline.txt(I think it's not necessary for bare metal tutorials)
6. bootcode.bin

I am not able to find out why it's not working for me. I read about 2 UARTS in RPI3

1. miniUART or UART1 : Which is default linux console etc ttyS0 (weaker has baud rate linked to core_freq)
2. UART0 or PL011: ttyAMA0 (more powerful)

I tried all the following options for uart in config.txt

1. Add dtoverlay=pi3-disable-bt or dtoverlay=pi3-miniuart-bt
2. enable_uart=1
3. core_freq=250

Can someone help me figure out the problem? and if there is problem in config.txt or cmdline.txt then please point me in right direction.

I am really confused between what config.txt parameter turn on which UART hardware.

Thanks in advance

Answer 1

It is a bit late but I may have a solution for you.

I have managed to write a loopback program in C to test if the RaspberryPi4 is receiving any data.

Receiver.c

#include <stdio.h>
#include <fcntl.h>
#include <termios.h>
#include <time.h>
#include <stdlib.h>


//Read Function
void receiveMessage();

//Loop Function
void loopFunctionReceivedMessage(int input);


int main()
{
    loopFunctionReceivedMessage(3);
    return 0;
}

void loopFunctionReceivedMessage(int input)
{
    time_t start_time = time(NULL);
    while(1)
    {
            time_t current_time = time(NULL);
            double elapsed = difftime(current_time, start_time);
            if(elapsed >= input)
            {
                    receivedMessage();
                    start_time = current_time;
            }
    }
 }


 void receivedMessage()
 {
     int file, count;
    if((file=open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY| O_NONBLOCK))<0)
    {
            perror("UART: Failed to open the file.\n");
            return -1;
    }

    fcntl(file, F_SETFL, 0);

    struct termios options;
    tcgetattr(file, &options);
    options.c_cflag= B9600 | CS8 | CREAD | CLOCAL| CSTOPB;
    options.c_iflag =  ECHO | ECHONL | ICANON | IEXTEN | ISIG |IGNPAR | ICRNL;
    tcflush(file, TCIFLUSH);
    tcsetattr(file, TCSANOW, &options);

    char receive[30];
    if((count = read(file,(void*)receive,30))<0)
    {
            perror("UART: Failed to read from the port :(.\n");
            return -1;
    }
    printf("Received: %s", receive);
    close(file);
}

Sender.c

#include <stdio.h>
#include <fcntl.h>
#include <termios.h>
#include <stdlib.h>
#include <time.h>

//Sending Function
void sendMessage(char *input_ptr);

//LoopFunction
void loopFunctionMessage(int input);

int main(int argc, char *argv[])
{

    loopFunctionMessage(3);
    return 0;
}

void loopFunctionMessage(int input)
{
    time_t start_time = time(NULL);
    while(1)
    {
            time_t current_time = time(NULL);
            double elapsed = difftime(current_time, start_time);
            if(elapsed >=input)
            {
                    sendMessage("Michael Jackson\n");
                    start_time= current_time;
            }
    }
}

  void sendMessage(char *input_ptr)
  {
    printf("You have entered: %s\n",input_ptr);
     int file, count;
    if ((file=open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY|  O_NONBLOCK))<0)
    {
            perror("UART: Failed to open the file.\n");
            return -1;
    }

    struct termios options;
    tcgetattr(file, &options);
    options.c_cflag = B9600 | CS8 | CREAD | CLOCAL| CSTOPB;
    options.c_iflag = ECHO | ECHONL | ICANON | IEXTEN | ISIG |IGNPAR | ICRNL;
    tcflush(file, TCIFLUSH);
    tcsetattr(file, TCSANOW, &options);

    char command[100];
    sprintf(command, "echo -e '%s' > /dev/ttyS0",input_ptr);
    system(command);
           if((count = write(file, input_ptr, 70))<0)
    {
            perror("UART: Failed to write to the output.\n");
            return -1;
    }

    close(file);
}