看门狗中遇到的问题

#include "2440addr.h"
#include "2440lib.h"
void watchdog_test(void);
void __irq Watchdog_ISR(void);

//复位次数
int f_ucSencondNo=0;

void xmain(void)
{
ChangeClockDivider(3,1);
ChangeMPllValue(127,2,1); //405MHZ

Isr_Init();
Uart_Init(0, 115200);
Uart_Select(0);
Uart_Printf("watchdog test is beginning\n");
watchdog_test();
   
}

/**********************************
watchdog_init
watchdog interrupt service routine
************************************/  
void watchdog_test(void)
{
  //clear interrupt registers
  ClearPending(BIT_WDT_AC97);
  ClearSubPending(BIT_SUB_WDT);
  ClearSubPending(BIT_SUB_WDT);
   
   
  //建立WatchDog中断
  pISR_WDT_AC97=(unsigned)Watchdog_ISR;
   
  //Prescaler value=100;?lock division factor=128 ;  PCLK=67.5MHz
  //t_watchdog=1/[PCLK/(Prescaler value+1)/Division_factor]=0.0002
  //disable watchdog
  rWTCON=((100<<8)|(3<<3));  
   
  //看门狗时钟中断周期T=WTCNT*t_watchdog=3S
  //看门狗喂狗
  rWTDAT=15000;
  rWTCNT=15000;
  rWTCON &= ~(3); //disable reset
   
  rWTCON |= ((1<<5)|(1<<2));//enable Watchdog timer and watchdog interrupt

  //开中断
  EnableIrq(BIT_WDT_AC97);
  EnableSubIrq(BIT_SUB_WDT);
  while(f_ucSencondNo<11);
}

/**** watchdog_init ****/
void __irq Watchdog_ISR(void)
{
   
  if(rSUBSRCPND & ~BIT_SUB_WDT)
  {//清除中断
  ClearSubPending(BIT_SUB_WDT);
  ClearPending(BIT_WDT_AC97);
  f_ucSencondNo++;
   
  if(f_ucSencondNo<11)
  Uart_Printf("%ds\n",f_ucSencondNo*3);
  else
  {
  //mask watchdog timer interrupt
  DisableSubIrq(BIT_SUB_WDT);
  DisableIrq(BIT_WDT_AC97);
  Uart_Printf("watch dog is ok\n");
  }
  }

  if(rSUBSRCPND & ~BIT_SUB_AC97)
  {
  ClearSubPending(BIT_SUB_AC97);
  ClearPending(BIT_WDT_AC97);
  }


}
请问各位，为什么rSUBSRCPND & ~BIT_SUB_AC97不为零。我单步调试发现rSUBSRCPND的值为0x92，我就是不明白:1,为什么rSUBSRCPND的值是这个。我看了手册，这几个值为1的位是发送中断的标志位：INT_TXD0~2 ，初始化不应该为零吗。2，清除不掉这几个标志位，一直置一。