CCS/MSP430G2553: SPI Communication with LMP91200: UCB0RXBUF Always 0

Part Number:MSP430G2553

Tool/software: Code Composer Studio

Hello! I am new to working with SPI communication/the MSP430G2xx3 and am simply trying to read pH sensor values from the LMP91200 evaluation board. However, while I am trying to receive the three bytes in the interrupt section, UCB0RXBUF always has value 0. Would appreciate guidance on how to resolve this issue!

//******************************************************************************
//                       MSP430G2xx3
//                    -----------------
//                /|\|              XIN|-       MSP430 3.3V  (J1.13 +3.3V DUT)
//                 | |                 |        MSP430 GND   (J1.10)
//                   |             XOUT|-
//                   |                 |
//                   |                 |
//                   |                 |
//                   |             P1.6|<- Data In (UCB0SOMI) (J1.27 SMSO B)
//                   |                 |
//  (J1.23)ADC SEL <-|P1.3         P1.5|-> Serial Clock Out (UCB0CLK) (J1.25 SCK_B)
//
//******************************************************************************
#include <msp430.h>
#define DUMMY   0xFF

unsigned int RxByteCtr = 2;
unsigned int ReceiveIndex = 0;
unsigned char RxWord;
unsigned char ReceiveBuffer[2] = {0};   //[0] MSB, [1] LSB
unsigned char MST_Data;

int main(void)
{
  volatile unsigned int i;

  WDTCTL = WDTPW + WDTHOLD;                     // Stop watchdog timer
  P1OUT = 0x00;                                 // P1 setup for LED & reset output
  P1DIR |= BIT0 + BIT3;                         //

  P1SEL = BIT5 + BIT6 + BIT7;
  P1SEL2 = BIT5 + BIT6 + BIT7;
  UCB0CTL0 |= UCCKPL + UCMSB + UCMST + UCSYNC;  // 3-pin, 8-bit SPI master
  UCB0CTL1 |= UCSSEL_2;                         // SMCLK
  UCB0BR0 |= 0x01;                              // /1 = 1MHz, 4.5 max, 0.9 min
  UCB0BR1 = 0;                                  //
  UCB0CTL1 &= ~UCSWRST;                     // **Initialize USCI state machine**
  IE2 |= UCB0RXIE;                          // Enable USCI0 RX interrupt

  P1OUT |= BIT3;                                // deselect ADC
  __delay_cycles(50);
  P1OUT &= ~BIT3;                           // select/enable ADC
  __delay_cycles(50);


    UCB0TXBUF = DUMMY;      // transmit first byte

  __bis_SR_register(LPM0_bits + GIE);           // CPU off, enable interrupts
}

// Test for valid RX and TX character
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=USCIAB0RX_VECTOR
__interrupt void USCIB0RX_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(USCIAB0RX_VECTOR))) USCIB0RX_ISR (void)
#else
#error Compiler not supported!
#endif
{
    volatile unsigned int i;
    while (!(IFG2 & UCB0RXIFG));              // USCI_B0 RX buffer ready?
    RxWord = UCB0RXBUF;

    if (RxByteCtr > 0)
    {
        ReceiveBuffer[ReceiveIndex] = RxWord;
        ReceiveIndex++;
        RxByteCtr--;
        while (!(IFG2 & UCB0TXIFG));              // USCI_B0 TX buffer ready?
        UCB0TXBUF = DUMMY;      // transmit the second & third bytes
    }
    if (RxByteCtr == 0)
    {
        RxByteCtr = 2;
        ReceiveIndex = 0;
        P1OUT |= BIT3;  // deselect ADC
        __delay_cycles(50);
        P1OUT &= ~BIT3; // select ADC
    }
}