UART in MSP430 comunication doesn't work properly

#include  "msp430x23x0.h"

void main(void)
{

  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  if (CALBC1_1MHZ ==0xFF || CALDCO_1MHZ == 0xFF)                                     
  {  
    while(1);                               // If calibration constants erased
                                            // do not load, trap CPU!!
  }   
 // BCSCTL1   = CALBC1_1MHZ;                  // Set DCO
 // DCOCTL    = CALDCO_1MHZ;
  
  P3SEL     = 0x30;                         // P3.4,5 = USCI_A0 TXD/RXD
  UCA0CTL1 |= UCSSEL_2;                     // SMCLK
  UCA0BR0   = 0x84;                         // 3.39 MHz 2400
  UCA0BR1   = 0x05;                         // 3.39 MHz 2400
  UCA0MCTL  = UCBRS2 + UCBRS0;              // Modulation UCBRSx = 5
  //UCA0CTL0   = 0x00;
  UCA0CTL1 &= ~UCSWRST;                     // **Initialize USCI state machine**
  IE2 |= UCA0RXIE;                          // Enable USCI_A0 RX interrupt
 // IE2 |= UCA0TXIE;                        // Enable USCI_A0 TX interrupt

  __bis_SR_register(LPM0_bits + GIE);       // Enter LPM0, interrupts enabled
  while(1)
  {
      
  }
}

// Echo back RXed character, confirm TX buffer is ready first
#pragma vector=USCIAB0RX_VECTOR
__interrupt void USCI0RX_ISR(void)
{
  while (!(IFG2&UCA0TXIFG));                // USCI_A0 TX buffer ready?
  UCA0TXBUF = UCA0RXBUF;                    // TX -> RXed character
 
}