关于定时器0溢出中断的疑问

2014-03-271.5万

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#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256))
#define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)
#define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) >> 3)
#define FRACT_MAX (1000 >> 3)
volatile unsigned long timer0_overflow_count = 0;
volatile unsigned long timer0_millis = 0;
static unsigned char timer0_fract = 0;
ISR(TIMER0_OVF_vect)
{
      // copy these to local variables so they can be stored in registers
      // (volatile variables must be read from memory on every access)
      unsigned long m = timer0_millis;
      unsigned char f = timer0_fract;

      m += MILLIS_INC;
      f += FRACT_INC;
      if (f >= FRACT_MAX) {
            f -= FRACT_MAX;
            m += 1;
      }

      timer0_fract = f;
      timer0_millis = m;
      timer0_overflow_count++;
}
unsigned long micros() {
      unsigned long m;
      uint8_t oldSREG = SREG, t;

      cli();
      m = timer0_overflow_count;
#if defined(TCNT0)
      t = TCNT0;
#elif defined(TCNT0L)
      t = TCNT0L;
#else
      #error TIMER 0 not defined
#endif


#ifdef TIFR0
      if ((TIFR0 & _BV(TOV0)) && (t < 255))
            m++;
#else
      if ((TIFR & _BV(TOV0)) && (t < 255))
            m++;
#endif

      SREG = oldSREG;

      return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}
在Arduino底层库中有这样一些代码，看到micros（）函数，我就想到了一个问题：单片机开了定时器0的溢出中断，定义了中断函数，以上代码就是的。如果在快要计数溢出时，比如250，由于需要读取micros，会执行cli()函数，把总中断关闭。过了一会读取完毕后，可能这时定时器已经溢出后重新开始了，比如计数到20。那么退出micros（）会执行sei()函数，把中断打开，此时刚才的中断函数仍会执行吗？还是会忽略上一次未执行的中断直接等待下一次的中断？求高手指点江山啊