Electronic – Why ARM Cortex M0 prints incorrectly Hello World on UART

armcdebugginguart

I am new to ARM CORTEX M0. I am trying to use the Tx pin of UART as debug port.

If I use the XDS200 debugger to step through the code, I am able to see the characters and I can see "Hello World" printed neatly on the Tera-Term through COM port.

But if I let the microcontroller run by itself I don't see any output and the probe connected on the Tx pin shows constant high.

I think only after using the debugger and stepping through the code the signals appear but not exactly representing "Hello World". Following is the part of output I logged from Tera-Term and this is repetitive:

/***********************************************************/

    W

    oldo

    W

    oldo

     Hrl  

    Well WHWelo

    WHWelo

    WHrld HoHrl WHoell Weold 

    olrl WHrllo

    Welllo

    oell  Hrldo

    Welo o

    oelo o

    oelo  Hrl  

    Welo

/***********************************************************/

Following is the main() function code.

/***********************************************************/

    void main(void)

    {

       Configure_peripheral(); /*This function call initializes all the required interrupts and global variables etc... */

       char message[] = "Hello World \n";

       char *ptr;

       for(;;)

       {

           ptr = message;

           while(*ptr)

           {

               UART_TX_BUF = (volatile unsigned char)(*ptr);

               ptr++;

           }

       }

    }

/***********************************************************/

Following is the UART Interrupt Service Routine code

/***********************************************************/

    interrupt void uart_isr(void)

    {

       if(UART_INTERRUPT_STATUS & UART_TXCOMPLETE_I)

       {

              /* Clear UART_TXCOMPLETE_I bit */

              UART_INTERRUPT_STATUS |= 0x02;

              /*datasheet : http://www.ti.com/lit/ds/symlink/pga900.pdf*/

              /*As per PGA900 datasheet : TX_COMPLETE is cleared when the UART_TX_BUF is written*/

              /* Read UART_LINE_STATUS for errors */

              UART_Line_Status = UART_LINE_STATUS;

              /* Transmit flag set here */

              UART_Flag_Tx = 1;

       }

     }

Could someone please help me understand what I am missing to get the expected string "Hello World" printed instead of jumbled up text. I am using a USB-Serial transceiver with operating voltage of 0-3.3v. Thanks

Best Answer

You are putting characters into the transmit buffer as fast as you can, without checking whether the UART is ready to accept a next character. Hence a lot of characters are lost, and what the PC receives is a more or less random subset of the characters you attempt to transmit.

Check for the 'tx buffer empty' or 'tx fifo not full' bits. Before assigning a next character to the Tx buffer, first wait for one of those bits.

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