PIC16F887 ADC interfaces to LM35 thermometer

 Introduction LM35

LM35 is thermometer that converter surrounding temperature to analog voltage. Its analog voltage voltage is proportional to temperature that is 10mV per degree Celsius. It's supplied from a DC voltage source ranging from 4 to 30V. Conversion temperature is between -55 to 150 degree Celsius. 

Sample of this example program

 

Dual-In-Line Package of TO-92 is very popular for hobbyist prototyping of this kind of sensor. It's a three-pin transistor-like electronics device. These three pins are VDD, VOUT, and GND.

LM35DZ TO-92 transistor-like package

 From electronics component store this device costs below one USD.

Interfacing and Programming using MikroC

Using ADC module inside PIC16F887, temperature reading could be done easily and precise due to 10-bit resolution inside ADC module. Floating point calculation in program C allow result acquisition calculated within a line of C code.

Circuit Diagram

In this programming example, AN0 of PIC16F887 reads voltage value that represent temperature data. Conversion temperature in only positive since voltage reference of ADC here set to positive rail only.

Temperature data will show on multiplexed display driven by PortB. Device supplied from stabilized 5V DC voltage with 8MHz internal oscillator.

MikroC source code:

#define STEP_SIZE 0.0048
#define ONE_SECOND 30
#define Volt_Per_Celcius 0.01
#define RATE 1
#define DIGIT1 PORTA.RA4
#define DIGIT2 PORTA.RA5
#define DIGIT3 PORTA.RA6
#define DIGIT4 PORTA.RA7
 
unsigned ADC_Value;
float  Voltage,temp;
char int_count,Temp_Data;
char LED[10]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};
bit  ADC_RUN;
 
void interrupt() {
     if(INTCON.T0IF) {
        int_count++;
        if(int_count==ONE_SECOND)
        { int_count=0; ADC_RUN=1; }
        }
     INTCON.T0IF=0;
}
 
void PORT_SETUP() {
     PORTA=0x00;
     PORTB=0x00;
     TRISA=0x01;   // RA0 IS ANALOG IN PUT
     TRISB=0x00;   // PORTB is OUTPUT
     ANSEL=0x01;   // SELECT AN0
     ANSELH=0x00;   // OTHERS ARE DIGITAL IO
     OSCCON|=0x07;  // SELECT 8MHz INT RC
     }
 
void Timer0_Setup(){
     OPTION_REG.T0CS=0; // SELECT FOSC/4
     OPTION_REG.PSA=0;  // SELECT TIMER0 PRESCALER
     OPTION_REG|=0x07;  // SELECT 1:256 PRESCALER
     }
 
void interrupt_setup() {
     INTCON.GIE=1;
     INTCON.T0IE=1;
     INTCON.T0IF=0;
     }
 
void Read_Temp () {
     if(ADC_RUN=1) {
        ADC_Value=ADC_Get_Sample(0);   // READ AN0
        delay_ms(10);
        Voltage=ADC_Value*STEP_SIZE;  // Convert to Decimal [Volt]
      }
       ADC_RUN=0;
}
 
void SSD() {
     Temp_Data=Voltage/Volt_Per_Celcius;  // convert Voltage to integer
 
     PORTB=LED[Temp_Data/10];  //10's
     DIGIT1=1;
     delay_ms(RATE);
     PORTA=0x00;
 
     PORTB=LED[Temp_Data%10];  // 1's
     DIGIT2=1;
     delay_ms(RATE);
     PORTA=0x00;
 
     PORTB=0x63;    // o
     DIGIT3=1;
     delay_ms(RATE);
     PORTA=0x00;
 
     PORTB=0x39;    // C
     DIGIT4=1;
     delay_ms(RATE);
     PORTA=0x00;
}
 
void main() {
     ADC_Value=0;
     int_count=0;
     ADC_RUN=0;
     PORT_SETUP();
     ADC_Init();
     Timer0_Setup();
     Interrupt_Setup();
     while(1) {
     Read_Temp();
     SSD();
     }
}

Timer0 is added to this example program. It's used for creating timer ticks that schedule ADC reading, and activating each digit of multiplexing display.

Click here to download this example.