msp430i2021 串口波特率不稳

这两天在开发msp430i2021的串口驱动，串口一直发送都是乱码。然后按照ti的例程测试了串口的波特率，发现很不稳定。下面是测试代码

#include "msp430.h"
void main(void) {
WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer

P1SEL0 |= BIT2 | BIT3; // P1.2/3 eUSCI_A Function
P1SEL1 &= ~(BIT2 | BIT3);

UCA0CTL1 |= UCSWRST; // Hold eUSCI in reset
UCA0CTL1 |= UCSSEL_2; // SMCLK
UCA0BR0 = 142; // 115200 baud
UCA0BR1 = 0;
UCA0MCTLW = 0x2200; // 16.384MHz/115200 = 142.22 (See UG)
UCA0CTL1 &= ~UCSWRST; // Release from reset
// UCA0IE |= UCRXIE; // Enable RX interrupt

// __bis_SR_register(LPM0_bits | GIE); // Enter LPM0
__no_operation(); // For debugger

while(1)
{
while (!(UCA0IFG&UCTXIFG)); // USCI_A0 TX buffer ready?
UCA0TXBUF = 0xaa; // TX -> RXed character
//__delay_cycles(90000000);
}
}

正常的115200测试的脉冲宽度应该是8.6us，现在测到是6.8~13us，这个差太多了，望大家不吝赐教啊