回复【7楼】majia-2  
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我来学习啊。。晕

try this:

========zero cumulative error timing routine, from Black Roman============
//initialize the timer
void t0_init(unsigned short duration) {
  TH0=duration >> 8;  //th0 has the msb
  TL0=duration & 0xff; //tl0 has the lsb
}

/********定时器T0中断函数**********/  
void t0(void) interrupt 1   
{  
static unsigned long total_ticks=0;  //initialize total_ticks
//clear the flag
//TH0=(65536-50000)/256;  
//TL0=(65536-50000)%256;//这样的定时数据，是不是正好是50000*1微秒=50毫秒？  
  total_ticks += 65536ul;  //advance duration;
  if (total_ticks >= DURATION) { //desired duration has reached;
    total_ticks -= DURATION;  //update total_ticks but retain the error
    t0_flag=1;  //set the flag
  }
}

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DURATION is a macro or global variable that determines how long t0_flag will be set.
t0_flag is set when the desired ticks, as specified by DURATION, have been reached.

so if each tick is 1us, and you want t0_flag is set every 500ms, you set DURATION to 500000ul.

the accuracy here depends entirely on the oscillator, and the approach is highly portable.

我很奇怪，为什么51做不到精确定时？我的小弟也是这么说，我面试过几个，也这么说。但是我自己却从来都是认为51的定时器和时钟芯片的误差是一样的，误差的来源仅仅取决于振荡器。若要定时精确，就选用好的振荡器。
很是不明白为什么总以51的T0说事，52（现在基本上都是标准的52居多吧）是有T2定时器的，自动重载，何来的中断引起的误差？

我面试那些不知道有T2的都被我刷掉了，哈哈。

回复【11楼】takashiki  岚月影
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就我个人来说，优先选择单片机，时钟芯片能不用则不用。使用时钟芯片的考虑仅仅出于耗电方面的考虑，有的51本身带有RTC，比如NXP的，时钟芯片就更没有什么用了。