51单片机

精确定时是用单片机内的定时器好还是时钟芯片好

2020-02-05 08:53发布

站内问答 / 51单片机
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刚看了一个帖子http://www.ourdev.cn/bbs/bbs_content.jsp?bbs_sn=4056474&bbs_page_no=1&search_mode=1&search_text=定时&bbs_id=9999
好像用单片机内的定时器定时会有误差，是这样吗？误差来源是什么呢？定时器不是1微妙加一计时的吗？ 怎么会有误差呢？我用过方式2，就是那种自动重装载的方式，用protues仿真运行一个小时，好像也看不到误差啊？
    要是用时钟芯片，会不会降低系统的稳定性呢？毕竟多了一部分电路，且在单片机外部，在工控场合会不会容易受到干扰而计时不准呢？
帖子内容如下：
/********定时器T0中断函数**********/
void t0(void) interrupt 1  
{
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？
aa++;
if(aa==18)//这里的意思是中断18次，需要1秒的时间（晶振12MHz），我怎么感觉应该是中断20次用时1秒？
  {
    aa=0;
    miao++;
    if(miao==60)
    {
      miao=0;
      fen++;
       if(fen==60)
        {
           fen=0;
           shi++;
          if(shi==24)
             shi=0;
        }
    }
  }

----------------问题附在注释里面，请指点一下，看看这个18是怎么算的吧！
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77条回答
millwood0
2020-02-11 14:44
the following hopefully will help you all understand the issue here.

this is a shot of single stepping through an AVR timer interrupt service routine. I stop'd the code right after it entered the isr, before executing the first statement.

on the right, I was showing the value of the timer / counter, TCNT0. The timer here works similar to the C51 timer: once TCNT0 hits a pre-set value (100 here and 0x100 in the case of a C51 8-bit timer), it rolls over and fires an interrupt.

so TCNT0 was just reset and that triggered the interrupt.

because of latency and context saving, by the time we are to execute the very 1st user code in the isr, the timer has advanced from 0x00 to 0x19.

if you ignore the value of the timer / counter at this point, by writing an explicit value to it, you would have created timing errors.

instead, you should simply increment or decrement the timer, as suggested earlier.



(原文件名:avr tmr isr.PNG)

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