PIC 16F887 Based Digital Thermometer

Introduction

The hardware configuration when using multiple 1-Wire temperature sensors like the DS1820 is very simple, as illustrated in the block diagram below. A single-wire bus is used for communication between the microcontroller and the temperature sensor. It is also possible to power the devices direclty via this 1-Wire bus. An almost unlimited number of 1-WireTM devices can be connected to the bus because each device has a unique 64-bit ROM code identifier which is used to address each sensor.

 

Temperature measurement using DS1820 sensor. Use of ‘1-wire’ protocol...

Temperature measurement is one of the most common tasks performed by the microcontroller. A DS1820 sensor is used for measurement here. It is capable of measuring temperature in the range of -55 °C to 125 °C with 0.5 °C accuracy. For the purpose of transferring data to the microcontroller, a special type of serial communication called 1-wire is used.

Due to a simple and wide use of these sensors, commands used to run and control them are in the form of functions stored in the One_Wire library. There are three functions in total:

• Ow_Reset is used for reseting sensor;


• Ow_Read is used for receiving data from sensor; and


• Ow_Write is used for sending commands to sensor.

Concretely, you don’t have to study documentation provided by the manufacturer in order to use this sensor. It is sufficient to copy some of these functions in the program.

Program 

/* Embedded Lab

 8-3-2012 */

// LCD module connections

sbit LCD_RS at RB4_bit;

sbit LCD_EN at RB5_bit;

sbit LCD_D4 at RB0_bit;

sbit LCD_D5 at RB1_bit;

sbit LCD_D6 at RB2_bit;

sbit LCD_D7 at RB3_bit;

sbit LCD_RS_Direction at TRISB4_bit;

sbit LCD_EN_Direction at TRISB5_bit;

sbit LCD_D4_Direction at TRISB0_bit;

sbit LCD_D5_Direction at TRISB1_bit;

sbit LCD_D6_Direction at TRISB2_bit;

sbit LCD_D7_Direction at TRISB3_bit;

// End LCD module connections

//  Set TEMP_RESOLUTION to the corresponding resolution of used DS18x20 sensor:

const unsigned short TEMP_RESOLUTION = 9;

char *text = "000.0000";

unsigned temp;

void Display_Temperature(unsigned int temp2write) {

  const unsigned short RES_SHIFT = TEMP_RESOLUTION - 8;

  char temp_whole;

  unsigned int temp_fraction;

  // Check if temperature is negative

  if (temp2write & 0x8000) {

     text[0] = '-';

     temp2write = ~temp2write + 1;

     }

  // Extract temp_whole

  temp_whole = temp2write >> RES_SHIFT ;

  // Convert temp_whole to characters

  if (temp_whole/100)

     text[0] = temp_whole/100  + 48;

  else

     text[0] = '0';

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

  text[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

  text[4] =  temp_fraction/1000    + 48;         // Extract thousands digit

  text[5] = (temp_fraction/100)%10 + 48;         // Extract hundreds digit

  text[6] = (temp_fraction/10)%10  + 48;         // Extract tens digit

  text[7] =  temp_fraction%10      + 48;         // Extract ones digit

  // Print temperature on LCD

  Lcd_Out(2, 5, text);

}

void main() {

  ANSEL  = 0;                                    // Configure AN pins as digital I/O

  ANSELH = 0;

  C1ON_bit = 0;                                  // Disable comparators

  C2ON_bit = 0;

 

  Lcd_Init();                                    // Initialize LCD

  Lcd_Cmd(_LCD_CLEAR);                           // Clear LCD

  Lcd_Cmd(_LCD_CURSOR_OFF);                      // Turn cursor off

  Lcd_Out(1, 1, " Temperature:   ");

  // Print degree character, 'C' for Centigrades

  Lcd_Chr(2,13,223);                             // Different LCD displays have different char code for degree

                                                 // If you see greek alpha letter try typing 178 instead of 223

  Lcd_Chr(2,14,'C');

  //--- Main loop

  do {

    //--- Perform temperature reading

    Ow_Reset(&PORTE, 2);                         // Onewire reset signal

    Ow_Write(&PORTE, 2, 0xCC);                   // Issue command SKIP_ROM

    Ow_Write(&PORTE, 2, 0x44);                   // Issue command CONVERT_T

    Delay_us(120);

    Ow_Reset(&PORTE, 2);

    Ow_Write(&PORTE, 2, 0xCC);                   // Issue command SKIP_ROM

    Ow_Write(&PORTE, 2, 0xBE);                   // Issue command READ_SCRATCHPAD

    temp =  Ow_Read(&PORTE, 2);

    temp = (Ow_Read(&PORTE, 2) << 8) + temp;

    //--- Format and display result on Lcd

    Display_Temperature(temp);

    Delay_ms(500);

  } while (1);

}

Circuit 

This project describes how to read temperature from a DS1820 sensor with a PIC16F887 microcontroller and display the temperature value in an LCD display. The temperature will be displayed in Centigrade unit . The temperature resolution is 1 degree.
PIC16F887 DS1820 data will be read at RE2 port.