MSP430FR5739 SPI的问题

UCTXIFG is automatically
reset if a character is written to UCxTXBUF.

就是说向发送寄存器送字节后,UCXIFG会自动清零.

为什么我第一次写入后,没有自动清零呢?还是1.

用的LAUNCHPAD仿真看到的...

UCA0CTLW0 |= UCSWRST;                     // **Put state machine in reset**
  UCA0CTLW0 |= UCMST+UCSYNC+UCCKPL+UCMSB+UCSSEL_3;   // 3-pin, 8-bit SPI master Clock polarity high, MSB , SMCLK=DCO
  UCA0BR0 = 0x02;                           // /2  baud
  UCA0BR1 = 0;                              //
  UCA0MCTLW = 0;                            // No modulation
  UCA0STATW|=UCLISTEN;                         //Receive the transmitted;
  UCA0CTLW0 &= ~UCSWRST;                    // **Initialize USCI state machine**
  //UCA0IE |= UCTXIE;                         // Enable USCI_A0 TX interrupt   We will use  no interruption.

 P1SEL1 |= BIT5;
  P2SEL1 |= BIT0 + BIT1;

void spiw(unsigned char word)
{
  while((UCA0IFG&UCTXIFG)==0);
  UCA0TXBUF=word;
};

执行到这,写入第一个...但UCTIFG没有自动请0..