Design and Analysis of DS18B20 Digital Thermometer

    1. Basic knowledge of DS18B20
　　The DS18B20 digital thermometer is a 1-Wire device produced by DALLAS, which has the characteristics of simple circuit and small size. Therefore, it is used to form a temperature measurement system, which has simple circuit. Many such digital thermometers can be hung on one communication line, which is very convenient.

    1. Features of DS18B20 products
　　(1) Only one port is required to achieve communication.
　　(2) Each device in DS18B20 has a unique serial number.
　　(3) In actual application, no external components are required to achieve temperature measurement.
　　(4) The measurement temperature range is between -55. C and +125. C.
　　(5) The resolution of the digital thermometer can be selected from 9 bits to 12 bits.
　　(6) There are internal temperature upper and lower limit alarm settings. 2.
Pin introduction of DS18B20 The
　　pin arrangement of DS18B20 in TO-92 package is shown in Figure 1, and its pin function description is shown in Table 1.
(Bottom view) Figure 1
Table 1 DS18B20 detailed pin function description

3. How to use DS18B20
　　Since DS18B20 uses the 1-Wire bus protocol, that is, bidirectional data transmission is achieved on one data line, and for the AT89S51 microcontroller , the hardware does not support the single bus protocol, so we must use software methods to simulate the protocol timing of the single bus to complete the access to the DS18B20 chip.
　　Since DS18B20 reads and writes data on one I/O line, there are strict timing requirements for the data bits to be read and written. DS18B20 has a strict communication protocol to ensure the correctness and integrity of each bit of data transmission. The protocol defines the timing of several signals: initialization timing, read timing, and write timing. All timings use the host as the master device and the single bus device as the slave device. Each transmission of commands and data starts with the host actively starting the write timing. If the single bus device is required to send back data, the host needs to start the read timing to complete the data reception after the write command. The transmission of data and commands is low-order first.
DS18B20 reset timing

DS18B20 read timing The
　　read timing of DS18B20 is divided into two processes: read 0 timing and read 1 timing.
　　For the read time slot of DS18B20, after the host pulls the single bus low, the single bus must be released within 15 seconds to allow DS18B20 to transfer data to the single bus. It takes at least 60us for DS18B20 to complete a read timing process.

DS18B20 write timing The
　　write timing of DS18B20 is still divided into two processes: write 0 timing and write 1 timing.
　　The requirements for DS18B20 write 0 timing and write 1 timing are different. When writing 0 timing, the single bus must be pulled down for at least 60us to ensure that DS18B20 can correctly sample the "0" level on the IO bus between 15us and 45us . When writing 1 timing, the single bus must be released within 15us after being pulled low.

4. Experimental task:
　　Use a DS18B20 chip to form a temperature measurement system. The temperature measurement accuracy reaches 0.1 degrees. The temperature measurement range is between -20 degrees and +100 degrees, and it is displayed with an 8-bit digital tube
. 5. Circuit schematic diagram

6. Hardware connection on the system board
(1). Connect P0.0-P0.7 in the "MCU system" area to the ABCDEFGH terminals in the "Dynamic digital display" area with an 8-core cable.
(2). Connect P2.0-P2.7 in the "MCU system" area to the S1S2S3S4S5S6S7S8 terminals in the "Dynamic digital display" area with an 8-core cable.
(3). Insert the DS18B20 chip into any socket in the "Four-way single bus" area. Be careful not to connect the power supply and ground signals in reverse.
(4). Connect the corresponding DQ terminal in the "Four-way single bus" area to the P3.7/RD terminal in the "MCU system" area.
7. C language source program
#include
#include
unsigned char code displaybit[]={0xfe,0xfd,0xfb,0xf7,
                                0xef,0xdf,0xbf,0x7f};
unsigned char code displaycode[]={0x3f,0x06,0x5b,0x4f,
                                    0x66,0x6d,0x7d,0x07,
                                    0x7f,0x 6f,0x77,0x7c,
                                    0x39,0x5e,0x79,0x71,0x00,0x40};
unsigned char code dotcode[32]={0,3,6,9,12,16,19,22,
                                25,28,31,34,38,41,44,48,
                                50,53,56,59,63,66,69,72,
                                75,78,81,84,88,91,94,97};
unsigned char displaycount;
unsigned char displaybuf[8]={16,16,16,16,16,16,16,16};
unsigned char timecount;
unsigned char readdata[8];
sbit DQ=P 3^7;
bit sflag;
bit resetpulse(void)
{
  unsigned char i;
  DQ=0;
  for(i=255;i>0;i--);
  DQ=1;
  for(i   =60;i>0;i--);   return   ( DQ
  ) ; signed char j;   for(i=0;i<8;i++)     {       if((command & 0x01)==0)         {           DQ=0;           for(j=35;j>0;j--);           DQ=1;         }         else           {
















            DQ=0;
            for(j=2;j>0;j--);
            DQ=1;
            for(j=33;j>0;j--);
          }
      command=_cror_(command,1);     
    }
}
unsigned char readdatafromds18b20(void)
{
  unsigned char i;
  unsigned char j;
  unsigned char temp;
  temp=0;
  for(i=0;i<8;i++)
    {
      temp=_cror_(temp,1);
      DQ=0;
      _nop_ ();
      _nop_();
      DQ=1;
      for(j=10;j>0;j--);
      if(DQ==1)
        {
          temp=temp | 0x80;
        }
        else
          {
            temp=temp | 0x00;
          }
      for(j=200;j>0;j--);
    }
  return(temp);
}
void main(void)
{
  TMOD=0x01;
  TH0=(65536-4000)/256;
  TL0=(65536-4000)%6;
  ET0=1;
  EA=1;
  while(resetpulse());
  writecommandtods18b20(0x CC );
  writecommandtods18b20(0x44);
  TR0=1;
  while(1)
    {
      ;
    }
}
void t0(void) interrupt 1 using 0
{
  unsigned char x;
  unsigned int result;
  TH0=(65536-4000)/256;
  TL0=(65536-4000)%6;
  if(displaycount= =2)
    {
      P0=displaycode[displaybuf[displaycount]] | 0x80;
    }
    else
      {
        P0=displaycode[displaybuf[displaycount]];
      }
  P2=displaybit[displaycount];
  displaycount++;
  if(displaycount==8)
    {
      displaycount=0;
    }
 
  timecount++;
  if(timecount==150)
    {
      timecount=0;
      while(resetpulse());
      writecommandtods18b20(0xcc);
      writecommandtods18b20( 0xbe);
      readdata[0]=readdatafromds18b20();
      readdata[1]=readdatafromds18b20();
      for(x=0;x<8;x++)
        {
          displaybuf[x]=16;
        }
      sflag=0;
      if((readdata [1] & 0xf8)!=0x00)
        {
          sflag=1;
          readdata[1]=~readdata[1];
          readdata[0]=~readdata[0];
          result=readdata[0]+1;
          readdata[0] =result;
          if(result>255)
            {
              readdata[1]++;
            }
        }
      readdata[1]=readdata[1]<<4;
      readdata[1]=readdata[1] & 0x70;
      x=readdata[0];
      x=x>>4;
      x=x & 0x0f;
      readdata[1]= readdata[1] | x;
      x=2; result=         readdata
      [1]; while       (
      result/10)
        {
          displaybuf[x]=result;
          result=       result /10
          ; sflag==1)         {           displaybuf[x+1]=17;         }       x=readdata[0] & 0x0f;       x=x<<1;       displaybuf[0]=(dotcode[x]) ;       displaybuf[1]=( dotcode[x])/10;       while(resetpulse());       writecommandtods18b20(0xcc);       writecommandtods18b20(0x44);     } }