MCU：MC9S12DG128。从datasheet上查到对SCF标志位的清除是写1，而我在实际用的时候却发现进入中断后写0才能清除标志位，这是怎么回事呢？

程序代码如下：

/*采用中断方式查询AD结果，每个通道每次采集8次结果，每次只转换一个通道*/
#include <hidef.h> /* common defines and macros */
#include <mc9s12dg128.h> /* derivative information */
#pragma LINK_INFO DERIVATIVE "mc9s12dg128b"
typedef double Voltage;
#define Vref 4.73
volatile word res;
volatile byte ADf=0;
void PLL_Init(void)
{
/*--------锁相环初始化-------*/
CLKSEL=0x00; //disable pll;
CLKSEL_PLLSEL = 0;
PLLCTL_PLLON = 0;
SYNR = 24; //6M晶镇获得K25M总线
REFDV = 5;
PLLCTL = 192;
PLLCTL_PLLON = 1;
while(!CRGFLG_LOCK);
CLKSEL_PLLSEL = 1;
}

/******************AD模块初始化****************/
void AD_Init()
{
ATD0CTL2=0x82; //AFFC=0,ASCIE=0,禁止外部触发；0x84，外部下降沿触发
ATD0CTL3=0x40; //S8C:S1C=1000,即每次系列转换8次,FIFO=0,FRZ1:FRZ0=00
ATD0CTL4=0x69; //10位转换，采样周期16clock,ATDclock=BUSCLOCK/20=1.25M;转换时间约为27.2us
}

/******************ATD采样结果处理*******************/
uint AD_FILTER(void)
{
uchar i;
uint max,min,sum,res[8];
res[0]=ATD0DR0H*4+ATD0DR0L/64;
res[1]=ATD0DR1H*4+ATD0DR1L/64;
res[2]=ATD0DR2H*4+ATD0DR2L/64;
res[3]=ATD0DR3H*4+ATD0DR3L/64;
res[4]=ATD0DR4H*4+ATD0DR4L/64;
res[5]=ATD0DR5H*4+ATD0DR5L/64;
res[6]=ATD0DR6H*4+ATD0DR6L/64;
res[7]=ATD0DR7H*4+ATD0DR7L/64;
//对结果进行处理，剔除最大最小值后，输出平均值
max=res[0];min=res[0];sum=res[0];
for(i=1;i<8;i++)
{
sum=sum+res;
if(max<res) max=res;
if(min>res) min=res;
}
return((sum-max-min)/6);
}
void main(void) {
/* put your own code here */
Voltage vol;
DisableInterrupts;
PLL_Init(); //从6M晶振获得25M总线频率
AD_Init();
EnableInterrupts;
ATD0CTL5=5; //通道5
for(;;)
{
if(ADf)
{
res=AD_FILTER();
vol=Vref*(double)res/1023;
ADf=0;
}
} /* wait forever */
/* please make sure that you never leave this function */
}

#pragma CODE_SEG NON_BANKED
void interrupt ATD_ISR(void)
{
ADf=1;
ATD0STAT0&=0x7F; //清除SCF标志等价于清除ASCIF
}