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I am trying to configure UART0 on TM4C123G launchpad. All of my configuration registers are perfect. They've been written with desired values. But somehow, my busy flag is always 1 after first write in data register. I'm using UART0 module on PA0 and PA1 pins. Initialization code is given below.

void UART_Init(void){
  SYSCTL_RCGCUART_R |= 0x01;            // activate UART0
  SYSCTL_RCGCGPIO_R |= 0x01;            // activate port A
  while((SYSCTL_PRGPIO_R&0x01) == 0){};
  UART0_CTL_R &= ~UART_CTL_UARTEN;      // disable UART
  UART0_LCRH_R = (UART_LCRH_WLEN_8|UART_LCRH_FEN); // 8 bit word length (no parity bits, one stop bit, FIFOs)
  UART0_IBRD_R = 43;                    // IBRD = int(80,000,000 / (16 * 115,200)) = int(43.40277)
  UART0_FBRD_R = 26;                     // FBRD = int(0.40277 * 64 + 0.5) = 26                          
  UART0_CTL_R |= UART_CTL_UARTEN;       // enable UART
  GPIO_PORTA_AFSEL_R |= 0x03;           // enable alt funct on PA1-0
  GPIO_PORTA_DEN_R |= 0x03;             // enable digital I/O on PA1-0
                                        // configure PA1-0 as UART
  GPIO_PORTA_PCTL_R = (GPIO_PORTA_PCTL_R&0xFFFFFF00)+0x00000011;
  GPIO_PORTA_AMSEL_R &= ~0x03;          // disable analog functionality on PA
}

Configuration registers have following values after running above function:

UART0_IBRD = 0x2B        //43
UART0_FBRD = 0x1A        //26
UART0_LCRH = 0x70    
UART0_CTL  = 0x301

Only problematic register is UART0_FR. It should have a value of 0x90 at all instances except while transmitting data. But even after transmission, its busy bit remains 1 which hinders any transmission.

I'm writing UART0_DATA register as follows:

  while((UART0_FR_R&UART_FR_TXFF) != 0);       //While transmission FIFO is not full, stay here.
  UART0_DR_R = data;                          // Transmit data to serial.
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It's been Solved. There was a very intricate problem. All registers had perfect values. I re-read the datasheet and came to know that LCRH register should be written after baud rate settings. It doesn't make any difference to the final values of the registers, but somehow it keeps UART in busy state. It wasn't mentioned anywhere in datasheet that what are possible reasons of BUSY flag to be 1. I just re-arranged following lines in my code.

UART0_IBRD_R = 43;                                // IBRD = int(80,000,000 / (16 * 115,200)) = int(43.40277)
UART0_FBRD_R = 26;                                // FBRD = int(0.40277 * 64 + 0.5) = 26                          
UART0_LCRH_R = (UART_LCRH_WLEN_8|UART_LCRH_FEN);  // 8 bit word length (no parity bits, one stop bit, FIFOs)
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  • \$\begingroup\$ Acknowledgment: This riddle was solved by Jonathan Valvano- Teacher at university of Texas. He's an expert in embedded systems. users.ece.utexas.edu/~valvano \$\endgroup\$ – Saqib Ahmed Aug 5 '15 at 11:24

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