上电复位问题求助

求助, 复位

现在以9S12DG128B单片机构成最小系统，设置锁相环、定时器和PWM模块，使得总线频率为24Hz,定时器模块的2到7口输出低电平，并产生PWM信号。把程序下载到MCU中，由5V的电源通过开关给单片机供电。现在的问题是上电后，所有的I/O口输出都是高电平（包括已经设置的定时器和PWM模块的输出）；当按动复位键使MCU复位后，程序正常，端口按照要求正确输出。
   请高人指点一下，为什么会出现这样的情况？理论上，当给单片机上电的时，MCU就应该完成初始化的过程并且while循环语句中！

程序如下：

#include <stdio.h>
#include <hidef.h>    /* common defines and macros */
#include <mc9s12dg128.h>    /* derivative information */
#pragma LINK_INFO DERIVATIVE "mc9s12dg128b"
#pragma CODE_SEG __NEAR_SEG NON_BANKED
void MCUInit(void)       {
DisableInterrupts;
CLKSEL &= 0x7f;
PLLCTL &= 0xbf;
SYNR = 0x05;
REFDV = 0x02;
PLLCTL |= (1<<6);
while ((CRGFLG&0x08) == 0x00);
CLKSEL |= (1<<7);
INTCR &= 0xbf;
COPCTL = 0x00;
EnableInterrupts;
}
void PWMInit(void)
{
  DisableInterrupts;

  PWMPOL |= (1<<1);
  PWMCLK &= ~(1<<1);
  PWMPRCLK = 0X00;
  PWMCAE &= ~(1<<1);
  PWMCTL = (1<<4);
  PWMCNT01 = 0X0000;
  PWMPER01 = 8000;
  PWMDTY01 = 3000;
  PWME |= (1<<1);
  PWME |= (1<<0);

  EnableInterrupts;
}

void TimerInit(void) //15
{
  DisableInterrupts;
  TSCR2 = 0X83;
  TIOS_IOS0 = 0;
  TIOS_IOS1 = 0;
  DDRT = 0XFC;
  PTT = 0X00;
  TCTL4 = 0x0F;
// TIE_C0I = 1;
// TIE_C1I = 1;
// TSCR1_TEN = 1;
  EnableInterrupts;
}

void main(void)
{
  DisableInterrupts;
  MCUInit();
  TimerInit();
  PWMInit();
  EnableInterrupts;
  while(1)
  {

  } /* wait forever */
  /* please make sure that you never leave this function */
}

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