Digital Room Thermometer

The firmware was developed on mikroC compiler. The in-built libraries for DS1820 makes the firmware development easier. The code is provided with adequate comments so that the reader won’t have much difficulty in understanding programming logic. The PIC microcontroller uses internal oscillator at 4.0 MHz. In the Edit Project window of mikroC, select internal clock, MCLR enabled, WDT Disabled, and Power On Timer Enabled.

/*
  Digital Room Thermometer using PIC16F688
  Copyright@Rajendra Bhatt
  July 13, 2010
*/
 // LCD module connections
sbit LCD_RS at RC4_bit;
sbit LCD_EN at RC5_bit;
sbit LCD_D4 at RC0_bit;
sbit LCD_D5 at RC1_bit;
sbit LCD_D6 at RC2_bit;
sbit LCD_D7 at RC3_bit;
sbit LCD_RS_Direction at TRISC4_bit;
sbit LCD_EN_Direction at TRISC5_bit;
sbit LCD_D4_Direction at TRISC0_bit;
sbit LCD_D5_Direction at TRISC1_bit;
sbit LCD_D6_Direction at TRISC2_bit;
sbit LCD_D7_Direction at TRISC3_bit;
// End LCD module connections
 
// Back Light Switch connected to RA1
sbit BackLight at RA1_bit;
// Define Messages
char message0[] = "LCD Initialized";
char message1[] = "Room Temperature";
 
// String array to store temperature value to display
char *tempC = "000.0";
char *tempF = "000.0";
 
// Variables to store temperature register values
unsigned int temp_whole, temp_fraction, temp_value;
signed int tempinF, tempinC;
unsigned short C_Neg=0, F_Neg=0, TempH, TempL;
 
void Display_Temperature() {
  // convert Temp to characters
 if (!C_Neg) {
     if (tempinC/1000)
   // 48 is the decimal character code value for displaying 0 on LCD
     tempC[0] = tempinC/1000  + 48;
     else tempC[0] = ' ';
  }
  tempC[1] = (tempinC/100)%10 + 48;             // Extract tens digit
  tempC[2] =  (tempinC/10)%10 + 48;             // Extract ones digit
 
  // convert temp_fraction to characters
  tempC[4] =  tempinC%10  + 48;         // Extract tens digit
 
  // print temperature on LCD
  Lcd_Out(2, 1, tempC);
 
  if (!F_Neg) {
     if (tempinF/1000)
      tempF[0] = tempinF/1000  + 48;
     else tempF[0] = ' ';
  }
 
  tempF[1] = (tempinF/100)%10 + 48;             // Extract tens digit
  tempF[2] =  (tempinF/10)%10 + 48;
  tempF[4] =  tempinF%10  + 48;
  // print temperature on LCD
  Lcd_Out(2, 10, tempF);
}
 
// ISR for LCD Backlight
void interrupt(void){
  if (INTCON.INTF == 1)          // Check if INTF flag is set
     {
     BackLight =~BackLight;  // Toggle Backlight
     Delay_ms(300) ;
     INTCON.INTF = 0;       // Clear interrupt flag before exiting ISR
     }
}
 
void main() {
  TRISC = 0x00 ;
  TRISA = 0b00001100; //  RA2, RA3 Inputs, Rest O/P's
  ANSEL =   0b00000000;
  PORTA =   0b00000000;            //  Start with Everything Low
  PORTC =   0b00000000;            //  Start with Everything Low
  CMCON0 =  0b00000111;
  Lcd_Init();                        // Initialize LCD
  Lcd_Cmd(_LCD_CLEAR);               // CLEAR display
  Lcd_Cmd(_LCD_CURSOR_OFF);          // Cursor off
  BackLight = 1;
  Lcd_Out(1,1,message0);
  Delay_ms(1000);
  Lcd_Out(1,1,message1);             // Write message1 in 1st row
  // Print degree character
  Lcd_Chr(2,6,223);
  Lcd_Chr(2,15,223);
 // different LCD displays have different char code for degree
 // if you see greek alpha letter try typing 178 instead of 223
 
  Lcd_Chr(2,7,'C');
  Lcd_Chr(2,16,'F');
 
  // Interrupt Setup
    OPTION_REG = 0x00;   // Clear INTEDG, External Interrupt on falling edge
    INTCON.INTF = 0;     // Clear interrupt flag prior to enable
    INTCON.INTE = 1;     // enable INT interrupt
    INTCON.GIE  = 1;     // enable Global interrupts
 
  do {
  //--- perform temperature reading
    Ow_Reset(&PORTA, 5);      // Onewire reset signal
    Ow_Write(&PORTA, 5, 0xCC);   // Issue command SKIP_ROM
    Ow_Write(&PORTA, 5, 0x44);   // Issue command CONVERT_T
    INTCON.GIE  = 1;     // 1-wire library disables interrpts
    Delay_ms(600);
    Ow_Reset(&PORTA, 5);
    Ow_Write(&PORTA, 5, 0xCC);    // Issue command SKIP_ROM
    Ow_Write(&PORTA, 5, 0xBE);    // Issue command READ_SCRATCHPAD
 
    // Read Byte 0 from Scratchpad
    TempL =  Ow_Read(&PORTA, 5);
    // Then read Byte 1 from Scratchpad
    TempH = Ow_Read(&PORTA, 5);
    temp_value = (TempH << 8)+ TempL ;
    // check if temperature is negative
    if (temp_value & 0x8000) {
      C_Neg = 1;
      tempC[0] = '-';
      // Negative temp values are stored in 2's complement form
      temp_value = ~temp_value + 1;
      }
    else C_Neg = 0;
    // Get temp_whole by dividing by 2
    temp_whole = temp_value >> 1 ;
    if (temp_value & 0x0001){  // LSB is 0.5C
       temp_fraction = 5;
       }
    else temp_fraction = 0;
    tempinC = temp_whole*10+temp_fraction;
 
    if(C_Neg)  {
     tempinF = 320-9*tempinC/5;
     if (tempinF < 0) {
      F_Neg = 1;
      tempF[0] = '-';
      tempinF = abs(tempinF);
      }
     else F_Neg = 0;
     }
    else tempinF = 9*tempinC/5 + 320;
    //--- Format and display result on Lcd
    Display_Temperature();
 
  } while(1);
}

Snapshots of temperature measurements