Msp430f5529XT2晶振无法起振

P1DIR |= BIT0;
P1SEL |= BIT0; //P1.0输出ACLK

P1.0已有4M的输出

#include <msp430.h>

int main(void)
{
volatile unsigned int i; //循环变量
WDTCTL = WDTPW | WDTHOLD; // stop watchdog timer
P4DIR |= (BIT1+BIT7); //P4.7观察现象，P4.1用于示波器观测
P4OUT |= (BIT1+BIT7); //输出高电平，点亮LED
P1DIR |= BIT0;
P1SEL |= BIT0; //P1.0输出ACLK
P2DIR |= BIT2;
P2SEL |= BIT2; //P2.2输出SMCLK

P5SEL |= BIT2+BIT3; //P5.2和P5.3选择XT2晶振功能
UCSCTL3 |= SELREF_5; //设置FLL参考时钟源为XT2CLK
UCSCTL4 |= SELA_5; //必须要设置这一句配置ACLK = XT2CLK
do
{
UCSCTL7 &= ~(XT2OFFG + XT1LFOFFG + DCOFFG);
//清除XT2,XT1,DCO故障失效标志位
SFRIFG1 &= ~OFIFG; //清除晶振故障失效中断标志位
}while(SFRIFG1&OFIFG); //晶振故障失效中断标志位
UCSCTL6 &= ~XT2DRIVE_0; //XT2 = 4M,减少XT2驱动能力
UCSCTL4 |= SELS_5 + SELM_5; //配置MCLK = SMCLK = XT2CLK

while(1)
{
P4OUT ^= (BIT1+BIT7); //LED状态取反
for(i = 50000;i>0;i--); //delay
}
}
程序如上