MSP432 UART Receive Problem

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 *                       MSP432 CODE EXAMPLE DISCLAIMER
 *
 * MSP432 code examples are self-contained low-level programs that typically
 * demonstrate a single peripheral function or device feature in a highly
 * concise manner. For this the code may rely on the device's power-on default
 * register values and settings such as the clock configuration and care must
 * be taken when combining code from several examples to avoid potential side
 * effects. Also see www.ti.com/.../mspdriverlib for an API functional
 * library & https://dev.ti.com/pinmux/ for a GUI approach to peripheral configuration.
 *
 * --/COPYRIGHT--*/
//******************************************************************************
//   MSP432P401 Demo - eUSCI_A0 UART echo at 9600 baud using BRCLK = 12MHz
//
//  Description: This demo echoes back characters received via a PC serial port.
//  SMCLK/ DCO is used as a clock source and the device is put in LPM3
//  The auto-clock enable feature is used by the eUSCI and SMCLK is turned off
//  when the UART is idle and turned on when a receive edge is detected.
//  Note that level shifter hardware is needed to shift between RS232 and MSP
//  voltage levels.
//
//  The example code shows proper initialization of registers
//  and interrupts to receive and transmit data.
//  To test code in LPM3, disconnect the debugger.
//
//  
//
//                MSP432P401
//             -----------------
//         /|\|                 |
//          | |                 |
//          --|RST              |
//            |                 |
//            |                 |
//            |     P1.3/UCA0TXD|----> PC (echo)
//            |     P1.2/UCA0RXD|<---- PC
//            |                 |
//
//   Wei Zhao
//   Texas Instruments Inc.
//   June 2014
//   Built with Code Composer Studio V6.0
//******************************************************************************
#include "msp.h"
#include <stdint.h>

uint8_t rx_buffer[1024];
uint16_t bufferIndex = 0;

int main(void)
{
  WDTCTL = WDTPW | WDTHOLD;               // Stop watchdog timer

  CSKEY = 0x695A;                        // Unlock CS module for register access
  CSCTL0 = 0;                            // Reset tuning parameters
  CSCTL0 = DCORSEL_3;                   // Set DCO to 12MHz (nominal, center of 8-16MHz range)
                                         // Select ACLK = REFO, SMCLK = MCLK = DCO
  CSCTL1 = SELA_2 | SELS_3 | SELM_3;
  CSKEY = 0;                             // Lock CS module from unintended accesses

  // Configure UART pins
  P1SEL0 |= BIT2 | BIT3;                  // set 2-UART pin as second function

  __enable_interrupt();
  NVIC_ISER0 = 1 << ((INT_EUSCIA0 - 16) & 31); // Enable eUSCIA0 interrupt in NVIC module

  // Configure UART
  UCA0CTLW0 |= UCSWRST;
  UCA0CTLW0 |= UCSSEL__SMCLK;             // Put eUSCI in reset
  // Baud Rate calculation
  // 12000000/(16*9600) = 78.125
  // Fractional portion = 0.125
  // User's Guide Table 21-4: UCBRSx = 0x10
  // UCBRFx = int ( (78.125-78)*16) = 2
  UCA0BR0 = 78;                           // 12000000/16/9600
  UCA0BR1 = 0x00;
  UCA0MCTLW = 0x1000 | UCOS16 | 0x0020;

  UCA0CTLW0 &= ~UCSWRST;                  // Initialize eUSCI
  UCA0IE |= UCRXIE;                       // Enable USCI_A0 RX interrupt

  while(1)
  {
	  __no_operation();                       // For debugger
  }
}

// UART interrupt service routine
void eUSCIA0IsrHandler(void)
{
    if (UCA0IFG & UCRXIFG)
    {
    	rx_buffer[bufferIndex] = UCA0RXBUF;
    	bufferIndex++;
      __no_operation();
    }
}