CCS/MSP430FR2311: I2C issue

Tool/software: Code Composer Studio

Hi 

I'm using msp430fr2311 and I want to use i2c.

My problem is UCTXSTT bit  never goes to low.

I know,  to transmit data to slave I have to set UCTXSTT bit. next UCTXSTT goes to low (clear) and then TX flag is set.

but after UCTXSTT bit set, It never goes to low and TX flag never sets to high.

I refer to example source and this is part of my code. 

//////////////////////////////////////////////////////////////////////
// I2C PORT set
P2SEL0 = 0x30; // P2.4 SDA , P2.5 SCL
/*
* Disable the GPIO power-on default high-impedance mode to activate
* previously configured port settings
*/
PMM_unlockLPM5();

/////////////////////////////////////////////////////////////////////
// I2C SET

// Configure USCI_B0 for I2C mode
UCB0CTLW0 |= UCSWRST; // put eUSCI_B in reset state
UCB0CTLW0 |= UCMODE_3 | UCMST; // I2C master mode, SMCLK
UCB0BRW = 0x8; // baudrate = SMCLK / 8
UCB0I2CSA = 0xD6;
UCB0CTLW0 &=~ UCSWRST; // clear reset register
UCB0IE |= UCTXIE0 | UCNACKIE; // transmit and NACK interrupt enable


TXByteCtr=4;

while(1)
{
while (UCB0CTLW0 & UCTXSTP); // Ensure stop condition got sent
UCB0CTLW0 |= UCTR | UCTXSTT;

__bis_SR_register(LPM0_bits | GIE);
}

#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector = USCI_B0_VECTOR
__interrupt void USCIB0_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(USCI_B0_VECTOR))) USCIB0_ISR (void)
#else
#error Compiler not supported!
#endif
{
switch(__even_in_range(UCB0IV,USCI_I2C_UCBIT9IFG))
{
case USCI_NONE: break; // Vector 0: No interrupts break;
case USCI_I2C_UCALIFG: break;
case USCI_I2C_UCNACKIFG:
UCB0CTL1 |= UCTXSTT; //resend start if NACK
break; // Vector 4: NACKIFG break;
case USCI_I2C_UCSTTIFG: break; // Vector 6: STTIFG break;
case USCI_I2C_UCSTPIFG: break; // Vector 8: STPIFG break;
case USCI_I2C_UCRXIFG3: break; // Vector 10: RXIFG3 break;
case USCI_I2C_UCTXIFG3: break; // Vector 14: TXIFG3 break;
case USCI_I2C_UCRXIFG2: break; // Vector 16: RXIFG2 break;
case USCI_I2C_UCTXIFG2: break; // Vector 18: TXIFG2 break;
case USCI_I2C_UCRXIFG1: break; // Vector 20: RXIFG1 break;
case USCI_I2C_UCTXIFG1: break; // Vector 22: TXIFG1 break;
case USCI_I2C_UCRXIFG0: break; // Vector 24: RXIFG0 break;
case USCI_I2C_UCTXIFG0:
     if (TXByteCtr) // Check TX byte counter 
     {
          UCB0TXBUF = CHARGE_START_BUF[Buf_Cnt]; // Load TX buffer
          TXByteCtr--; // Decrement TX byte counter
      }
     else
      {
           UCB0CTLW0 |= UCTXSTP; // I2C stop condition
           UCB0IFG &= ~UCTXIFG; // Clear USCI_B0 TX int flag
           __bic_SR_register_on_exit(LPM0_bits); // Exit LPM0
      }
break; // Vector 26: TXIFG0 break;
case USCI_I2C_UCBCNTIFG: break; // Vector 28: BCNTIFG
case USCI_I2C_UCCLTOIFG: break; // Vector 30: clock low timeout
case USCI_I2C_UCBIT9IFG: break; // Vector 32: 9th bit
default: break;
 }

}

I already checked SDA, SCL pull up 3.44V. and after clear reset register, UCBBUSY bit is high in state reg.

But, it is only connected msp430fr2311 <->bq25703a. 

what is my problem? and please give me details. 

Thanks Bye.