MCU：PIC16F690

系统频率：8MHZ

目的：实现对18b20 1-wire协议的实现，通过18b20测得环境温度；

18b20通过1-wire协议与主机实现通讯，1-wire协仪非常简单，关键问题是时序问题，只要时序没问题，就可以通过1-wire协议与18b20实现通讯；

   测试过的程序如下：

/******************************************************************
** Function name  :Reset_18b20(void)
** Description    :reset the 18b20
** Input parameter  :
******************************************************************/
void Reset_18b20(void)
{
  DQ_OUT_HIGH();
  Delay(10);           //do nothing
  DQ_OUT_LOW();
  Delay(180);         //output 0 delay about 639us(480~960us)
  DQ_OUT_HIGH();
  Delay(10);           //output 1 delay about   45us(15~60us)
  DQ_IN();
  NOP();
  while(DQ);           //wait for the response
  Delay(80);           //the response delay   290us(60-240us)
}
/******************************************************************
** Function name  :Write_18b20(unsigned char cmd)
** Description    :write to the 18b20
** Input parameter  :unsigned char cmd
******************************************************************/
void Write_18b20(unsigned char cmd)
{
  unsigned char i;
  for(i=0;i<8;++i)
  {
    if(cmd&(1<<i))
    {
      DQ_OUT_LOW();
           Delay(1);         //13us (1us-15us)
      
      DQ_OUT_HIGH();
      Delay(20);         //delay 80us 写时隙至少60us
    }
    else
    {
      DQ_OUT_LOW();
      Delay(20);       //delay 80us 至少保持60us的时间
      DQ_OUT_HIGH();
      Delay(1);           //delay 13us   时隙恢复至少1us的时间
    }
      
  }
}
/******************************************************************
** Function name  :Read_18b20(void)
** Description    :read from the 18b20
** Input parameter  :
**Output parameter   :unsigned char
******************************************************************/
unsigned char Read_18b20(void)
{
  unsigned char i;
  unsigned char result=0;
  for(i=0;i<8;++i)
  {
    DQ_OUT_LOW();
    Delay(0);   //delay us 读时隙开始，至少1us的时间
    DQ_IN();     //15us内读取
    NOP();
    NOP();
    NOP();
    NOP();
    NOP();
    NOP();
    if(DQ)
    {
      result |= 1<<i;
    }
    Delay(1);       //delay 13us 时隙恢复时间
  }
  return result;
}
/******************************************************************
** Function name  :Read_temp(void)
** Description    :read temperature
** Input parameter  :
**Output parameter   :unsigned int
******************************************************************/
unsigned int Read_temp(void)
{
  unsigned char templ;
  unsigned char temph;
  unsigned int result=0;
  
  Reset_18b20();
  Write_18b20(0xCC);         //跳过rom
  Write_18b20(0x44);         //温度转化命令
  Delay(200);                                   //delay us (200~300us)
  Delay(200);
  Delay(200);
  
  Reset_18b20();
  Write_18b20(0xCC);         //跳过rom
  Write_18b20(0xBE);         //读取温度命令
  
  templ=Read_18b20();
  temph=Read_18b20();
  signal_flag = 0;
  if(temph & 0xF0)                             //读取温度为零下负值，此时将温度高8位取反，低8位取反后加1，即可获得正确温度值
  {
    temph = ~temph;
    templ = ~templ+1;
    signal_flag = 1;
  }
  result=temph;
  result=result<<8;
  result|=templ;
  result =(float)result * 0.625;   //精确到1位小数点
  return result;
}