Here is the guide to teach that how to enable the uart port on cubiboard and how to test the uart port is available .

Cubietruck http://docs.cubieboard.org/a20-cubietruck_gpio_pin

Cubieboard 1 2 http://docs.cubieboard.org/cubieboard1_and_cubieboard2_gpio_pin

Fex Guide http://linux-sunxi.org/Fex_Guide

#mount /dev/nanda /mnt
#cd /mnt
#bin2fex  script.bin script.fex
#vi script.fex

Modify the script.fex to open the uart3 ，uart4 as below ：

[uart_para3]
uart_used = 1
uart_port = 3
uart_type = 4
uart_tx = port:PG06<4><1><default><default>
uart_rx = port:PG07<4><1><default><default>
uart_rts = port:PG08<4><1><default><default>
uart_cts = port:PG09<4><1><default><default>

[uart_para4]
uart_used = 1
uart_port = 4
uart_type = 2
uart_tx = port:PG10<4><1><default><default>
uart_rx = port:PG11<4><1><default><default>

Save the modification.

#fex2bin script.fex  script.bin
#reboot

After the reboot ,the uart3 and uart4 are available.

Write a internal loopback program to test the uart .

#vi uart_test.c

Edit the program

/********************************************************************
 **************************uart_test*********************************
 ********************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <errno.h>
#include <sys/time.h>
#include <string.h>

#define TRUE 1
#define FALSE -1

int speed_arr[] = {B115200, B38400, B19200, B9600, B4800, B2400, B1200, B300,
        B38400, B19200, B9600, B4800, B2400, B1200, B300, };

int name_arr[] = {115200, 38400,  19200,  9600,  4800,  2400,  1200,  300,
        38400,  19200,  9600, 4800, 2400, 1200,  300, };

void set_speed(int fd, int speed)
{
  int i;
  int status;

  struct termios Opt;
  tcgetattr(fd,&Opt);

  for (i= 0;i<sizeof(speed_arr)/sizeof(int);i++)
  {
        if(speed == name_arr[i])
       {
           tcflush(fd, TCIOFLUSH);


        cfsetispeed(&Opt, speed_arr[i]);

        cfsetospeed(&Opt, speed_arr[i]);


        status = tcsetattr(fd, TCSANOW, &Opt);

        if(status != 0)
           perror("tcsetattr fd1");

             return;
         }
        tcflush(fd,TCIOFLUSH);
   }
}

int set_Parity(int fd,int databits,int stopbits,int parity)
{
    struct termios options;
   if( tcgetattr( fd,&options)!= 0)
   {
          perror("SetupSerial 1");

           return(FALSE);
    }

   options.c_cflag &= ~CSIZE;


  switch(databits)
   {
          case 7:
          options.c_cflag |= CS7;
          break;

       case 8:
        options.c_cflag |= CS8;
        break;

       default:
        fprintf(stderr,"Unsupported data size\n");

        return (FALSE);
    }

   switch(parity)
      {
          case 'n':
        case 'N':
        options.c_cflag &= ~PARENB;    /* Clear parity enable */
        options.c_iflag &= ~INPCK;    /* Enable parity checking */
        options.c_iflag &= ~(ICRNL|IGNCR);
        options.c_lflag &= ~(ICANON );
        break;


    case 'o':

    case 'O':
        options.c_cflag |= (PARODD | PARENB);
        options.c_iflag |= INPCK;    /* Disnable parity checking */
        break;


    case 'e':

    case 'E':
        options.c_cflag |= PARENB;    /* Enable parity */
        options.c_cflag &= ~PARODD;
        options.c_iflag |= INPCK;    /* Disnable parity checking */
        break;


    case 'S':

    case 's':  /*as no parity*/
        options.c_cflag &= ~PARENB;
        options.c_cflag &= ~CSTOPB;
        break;

         default:
        fprintf(stderr,"Unsupported parity\n");

        return (FALSE);
    }

   switch(stopbits)
      {

      case 1:

      options.c_cflag &= ~CSTOPB;

    break;


    case 2:

    options.c_cflag |= CSTOPB;

    break;


    default:

    fprintf(stderr,"Unsupported stop bits\n");


    return (FALSE);
    }

  /* Set input parity option */

   if(parity != 'n')
          options.c_iflag |= INPCK;
          options.c_cc[VTIME] = 150; // 15 seconds
       options.c_cc[VMIN] = 0;

         tcflush(fd,TCIFLUSH); /* Update the options and do it NOW */

  if(tcsetattr(fd,TCSANOW,&options) != 0)
  {
          perror("SetupSerial 3");

        return (FALSE);
    }

   return (TRUE);
}

int main(int argc, char **argv)
{
    int fd;
    int nread;
    int nwrite;
    int n=0;
    int i=0;
    char buffer[15];
    char devname_head[10] = "/dev/";
    char dev_name[20];

     if(argc < 2)
    {
      printf("Please input './test_uart ttySx'\n");
      exit(1);
    }
    else
    {
        strcpy(dev_name, devname_head);
        strcat(dev_name, argv[1]);
    }

    fd = open(dev_name, O_RDWR);
    if(fd < 0)
    {
        perror("error to open /dev/ttySx\n");
        exit(1);
    }


    if (fd > 0)
    {
        set_speed(fd,115200);
    }
    else
    {
        printf("Can't Open Serial Port!\n");

        exit(0);
    }

      if (set_Parity(fd,8,1,'N') == FALSE)
      {
        printf("Set Parity Error\n");

        exit(1);
      }

    printf("\nWelcome to uart_test\n\n");

   memset(buffer,0,sizeof(buffer));

   char test[15] = "hello world";

   nwrite = write(fd,test,strlen(test));
    if(nwrite < 0)
    {
        printf("write error\n");
    }

   printf("Send test data------>%s\n",test);

      while(1)
      {
        nread = read(fd,&buffer[n],1);
        if(nread < 0)
        {
            printf("read error\n");
        }

        printf("read char is -> %c \n",buffer[n]);

           if (strlen(buffer) == strlen(test))
           {
               printf("Read Test Data finished,Read Test Data is------->%s\n",buffer);

            memset(buffer,0,sizeof(buffer));
            printf("Send test data again------>%s\n",test);

            write(fd,test,strlen(test));
            n=0;
            sleep(1);

            continue;
        }
       n++;
    }
}

Install software build the compiled enviroment .

#apt-get install gcc build-essential
#gcc uart_test.c -o uart_test

The uart0 3 4 5 6 are available in Cubieboard1 and Cubieboard2 ,while uart0 3 4 7 are available in Cubietruck, but uart0 is debug serial.

To test uart3(or uart4) should make sure the PG06 and PG07 (or PG10 and PG11)internal loopback,use Dupont Line connect them .

If you only open the uart0 4, uart4 is mapping the device ttyS1.

While open the uart0 3 4, uart3 is mapping the device ttyS1. uart4 is mapping the device ttyS2.

#./uart_test ttyS1 // or ttyS2

The termial will prinf as below,it prove the uart3 is working .