【新手】MSP430F5529 用I2C通讯时，没有SCL和SDA 波形 ，求帮助

I2C通信是用MSP430F5529作为主机，向外设AD5274（数字电位器）发送两个字节的数据。用的MSP430F55XX系列的例程，只有从机地址改成外设的固定地址。但是开始条件产生后，SDA和SCL都只是被拉低，没有波形的变化，一直到结束条件STP产生后，SDA和SCL空闲拉高。时钟是SMCLK，时钟源用4M的外部晶振，用IO口显示SMCLK，时钟源信号正常。想请教大家有没有碰到这种情况，问题有可能会出在哪里呢？

#include <msp430.h>

unsigned char *PTxData; // Pointer to TX data
unsigned char TXByteCtr;

const unsigned char TxData[] = // Table of data to transmit
{  0x1C,0x02  };

int main(void)
{
unsigned int i;

WDTCTL = WDTPW + WDTHOLD; // Stop WDT
P3SEL |= 0x03; // Assign I2C pins to USCI_B0
UCB0CTL1 |= UCSWRST; // Enable SW reset
UCB0CTL0 = UCMST + UCMODE_3 + UCSYNC; // I2C Master, synchronous mode
UCB0CTL1 = UCSSEL_2 + UCSWRST; // Use SMCLK, keep SW reset
UCB0BR0 = 40; // fSCL = SMCLK/40 = ~100kHz
UCB0BR1 = 0;
UCB0I2CSA = 0x2E; // Slave Address is 048h
UCB0CTL1 &= ~UCSWRST; // Clear SW reset, resume operation
UCB0IE |= UCTXIE; // Enable TX interrupt

while (1)
{
for(i=0;i<10;i++); // Delay required between transaction
PTxData = (unsigned char *)TxData; // TX array start address
// Place breakpoint here to see each
// transmit operation.
TXByteCtr = sizeof TxData; // Load TX byte counter

UCB0CTL1 |= UCTR + UCTXSTT; // I2C TX, start condition

__bis_SR_register(LPM0_bits + GIE); // Enter LPM0, enable interrupts
__no_operation(); // Remain in LPM0 until all data
// is TX'd
while (UCB0CTL1 & UCTXSTP); // Ensure stop condition got sent
}
}

//------------------------------------------------------------------------------
// The USCIAB0TX_ISR is structured such that it can be used to transmit any
// number of bytes by pre-loading TXByteCtr with the byte count. Also, TXData
// points to the next byte to transmit.
//------------------------------------------------------------------------------
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector = USCI_B0_VECTOR
__interrupt void USCI_B0_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(USCI_B0_VECTOR))) USCI_B0_ISR (void)
#else
#error Compiler not supported!
#endif
{
switch(__even_in_range(UCB0IV,12))
{
case 0: break; // Vector 0: No interrupts
case 2: break; // Vector 2: ALIFG
case 4: break; // Vector 4: NACKIFG
case 6: break; // Vector 6: STTIFG
case 8: break; // Vector 8: STPIFG
case 10: break; // Vector 10: RXIFG
case 12: // Vector 12: TXIFG
if (TXByteCtr) // Check TX byte counter
{
UCB0TXBUF = *PTxData++; // Load TX buffer
TXByteCtr--; // Decrement TX byte counter
}
else
{
UCB0CTL1 |= UCTXSTP; // I2C stop condition
UCB0IFG &= ~UCTXIFG; // Clear USCI_B0 TX int flag
__bic_SR_register_on_exit(LPM0_bits); // Exit LPM0
}
default: break;
}
}