32-Bit Timer Unchained Mode下，想产生一个10秒的中断，发现每次中断产生的时间都不一样，代码如下：
void int_init()
{ CSR = 0x100; //disable all interrupts
IER = 1; //disable all interrupts except NMI
ICR = 0xffff; //clear all pending interrupts
ISTP = 0x10800000; //set the ISTP equals the INT address in the .cmd
INTC_EVTCLR0 = 0xFFFFFFFF;
INTC_EVTCLR1 = 0xFFFFFFFF;
INTC_EVTCLR2 = 0xFFFFFFFF;
INTC_EVTCLR3 = 0xFFFFFFFF;
INTC_EVTMASK0 = 0xFFFFFFEF; //Timer0-TINT12 is combined
INTC_EVTMASK1 = 0xFFFFFFFF;
INTC_EVTMASK2 = 0xFFFFFFFF;
INTC_EVTMASK3 = 0xFFFFFFFF;
INTC_INTMUX3 = 0x00040000; //map Timer0-TINT12 to INT14
TIMER0_TIM12 = 0x00000000;
TIMER0_PRD12 = 0x1017DF80; //27M时钟，10S计时
TIMER0_TGCR = 0x00000005;
TIMER0_TCR = 0x00000080;//The timer is enabled one time. The timer stops after the counter reaches the period.
IER |= 0x00004002; //enable int14-timer0
CSR=0x01 ; // enable all interrupts
}
以上代码没有产生期望的十秒中断，在中断产生时查看输出的TIMER0_TIM12的值每次都不一样。TIMER0_PRD12这个值是怎么计算的？
