Experiment No. 4 : Reading Temperature Values from DS1820 using 1-Wire Protocol

In this experiment, we are going to build a digital temperature meter using DS1820 connected to our PIC16F628A development board. The temperature value will be displayed on the LCD display. I have modified the sample program that comes with the compiler according to our PIC board requirements. Also I have elaborated comments in the program so that every step will be more clear to the readers.

Experimental Setup:
The experimental setup is very straight-forward. Place DS1820 device on the three-pin female header that we recently added to our board. And also connect the data pin of DS1820 to RB.0 pin of PIC16F628A using a jumper wire.

 

 

 Circuit Diagram

 

Software:
Here is the program written in microC that reads temperature values from DS1820 device using OneWire Library.
/* Project name:
     One Wire Communication Test between PIC16F628A and DS1820
 * Copyright:
     (c) Rajendra Bhatt, 2009.
 * Description:
     This code demonstrates how to use One Wire Communication Protocol
     between PIC16F628A and a 1-wire peripheral device. The peripheral
     device used here is DS1820, digital temperature sensor.
     MCU:             PIC16F628A
     Oscillator:      XT, 4.0 MHz
*/
// LCD connections definitions
sbit LCD_RS at RA0_bit;
sbit LCD_EN at RA1_bit;
sbit LCD_D4 at RB4_bit;
sbit LCD_D5 at RB5_bit;
sbit LCD_D6 at RB6_bit;
sbit LCD_D7 at RB7_bit;
sbit LCD_RS_Direction at TRISA0_bit;
sbit LCD_EN_Direction at TRISA1_bit;
sbit LCD_D4_Direction at TRISB4_bit;
sbit LCD_D5_Direction at TRISB5_bit;
sbit LCD_D6_Direction at TRISB6_bit;
sbit LCD_D7_Direction at TRISB7_bit;
// End LCD connections definitions

// String array to store temperature value to display
char *temp = "000.00";

// Temperature Resolution : No. of bits in temp value = 9
const unsigned short TEMP_RES = 9;

// Variable to store temperature register value
unsigned temp_value;

void Display_Temperature(unsigned int temp2write) {
  const unsigned short RES_SHIFT = TEMP_RES - 8;

  // Variable to store Integer value of temperature
  char temp_whole;

  // Variable to store Fraction value of temperature
  unsigned int temp_fraction;
  unsigned short isNegative = 0x00; 

  // check if temperature is negative
  if (temp2write & 0x8000) {
     temp[0] = '-';
     // Negative temp values are stored in 2's complement form
     temp2write = ~temp2write + 1;
     isNegative = 1;   // Temp is -ive
     }

  // Get temp_whole by dividing by 2 (DS1820 9-bit resolution with 
  // 0.5 Centigrade step )
  temp_whole = temp2write >> RES_SHIFT ;


  // convert temp_whole to characters
 if (!isNegative) {
     if (temp_whole/100)
   // 48 is the decimal character code value for displaying 0 on LCD
     temp[0] = temp_whole/100  + 48;
     else
     temp[0] = '0';
  }

  temp[1] = (temp_whole/10)%10 + 48;             // Extract tens digit
  temp[2] =  temp_whole%10     + 48;             // Extract ones digit

  // extract temp_fraction and convert it to unsigned int
  temp_fraction  = temp2write << (4-RES_SHIFT);
  temp_fraction &= 0x000F;
  temp_fraction *= 625;

  // convert temp_fraction to characters
  temp[4] =  temp_fraction/1000    + 48;         // Extract tens digit
  temp[5] = (temp_fraction/100)%10 + 48;         // Extract ones digit

  // print temperature on LCD
  Lcd_Out(2, 5, temp);
}

void main() {
  CMCON  |= 7;                       // Disable Comparators
  Lcd_Init();                                    // Initialize LCD
  Lcd_Cmd(_LCD_CLEAR);                           // Clear LCD
  Lcd_Cmd(_LCD_CURSOR_OFF);                      // Turn cursor off
  Lcd_Out(1, 3, "Temperature:   ");
  // Print degree character, 'C' for Centigrades
  Lcd_Chr(2,11,223);  
 // different LCD displays have different char code for degree
 // if you see greek alpha letter try typing 178 instead of 223

  Lcd_Chr(2,12,'C');

  //--- main loop
  do {
    //--- perform temperature reading
    Ow_Reset(&PORTB, 0);      // Onewire reset signal
    Ow_Write(&PORTB, 0, 0xCC);   // Issue command SKIP_ROM
    Ow_Write(&PORTB, 0, 0x44);   // Issue command CONVERT_T
    Delay_ms(600);
    // If this delay is less than 500ms, you will see the first reading on LCD 
    //85C which is (if you remember from my article on DS1820) 
    //a power-on-reset value. 
    
    Ow_Reset(&PORTB, 0);
    Ow_Write(&PORTB, 0, 0xCC);    // Issue command SKIP_ROM
    Ow_Write(&PORTB, 0, 0xBE);    // Issue command READ_SCRATCHPAD

    // Read Byte 0 from Scratchpad
    temp_value =  Ow_Read(&PORTB, 0);
    // Then read Byte 1 from Scratchpad and shift 8 bit left and add the Byte 0
    temp_value = (Ow_Read(&PORTB, 0) << 8) + temp_value;

    //--- Format and display result on Lcd
    Display_Temperature(temp_value);

    } while (1);
}

Experimental Output:
The temperature reading will be displayed on the LCD screen and will be updated every 600ms. Look at some snapshots below showing output.