uart配置完成后 会自动发送0x00 找不到问题

配置uart后会自动发送0x00 ，每次上电都会发送，请帮忙看下有什么问题，谢谢~

P3SEL0 |= BIT4 | BIT5; // USCI_A1 UART operation
P3SEL1 &= ~(BIT4 | BIT5);

UCA1CTL1 |= UCSWRST;
UCA1CTL1 = UCSSEL__ACLK;
UCA1BR0 = 3;
UCA1MCTLW |= 0x9200;
UCA1BR1 = 0;
UCA1CTLW0 |= UCPEN | UCPAR;
UCA1CTL1 &= ~UCSWRST; // release from reset
UCA1IE |= UCRXIE; // Enable USCI_A0 RX interrupt