INTERFACING MULTIPLE TEMPERATURE SENSORS TO AVR

Here is a simple example to interface multiple temperature sensors to AVR (Atmega8) microcontroller. Atmega8 is a High-performance, Low-power Atmel®AVR® 8-bit Microcontroller Advanced RISC Architecture with 8KB ISP flash, 1KB RAM, 512B EEPROM, Two 8-bit Timer/Counters with Separate Prescaler, one Compare Mode, One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture Mode, Real Time Counter with Separate Oscillator, Three PWM Channels, 6 multiplexed ADC channels with 10-bit resolution. We have used 2 channels to connect 2 LM35 linear temperature sensors to it.

About LM35:

The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature. The LM35 thus has an advantage over linear temperature sensors calibrated in °Kelvin.

· Calibrated directly in ° Celsius (Centigrade)

· Linear + 10.0 mV/°C scale factor

· 0.5°C accuracy guaranteeable (at +25°C)

· Rated for full −55° to +150°C range

· Suitable for remote applications

· Low cost due to wafer-level trimming

· Operates from 4 to 30 volts

· Less than 60 μA current drain

· Low self-heating, 0.08°C in still air

· Nonlinearity only ±1⁄4°C typical

· Low impedance output, 0.1 W for 1 mA load

AVR AMTEGA8 ADC Registers:

BIT	7	6	5	4	3	2	1	0
ADMUX	*REFS1*	*REFS0*	*ADLAR*	–	*MUX3*	*MUX2*	*MUX1*	*MUX0*

BIT	7	6	5	4	3	2	1	0
ADCSRA	*ADEN*	*ADSC*	*ADFR*	*ADIF*	*ADIE*	*ADPS2*	*ADPS1*	*ADPS0*

Functions used:

ADC_init();

To initialize the ADC with reference voltage, adc clock prescalar, interrupts,,,

ADC_readch(ch_no);

To start conversion in the specified channel & return the values.

int ADC_readch(unsigned char chn)

{

ADMUX= (ADMUX & 0xF0) | (chn & 0x0F);

ADC_CLEAR_ADIF(); //Clear ADC Interrupt Flag

ADC_START_CONV(); //Start ADC Conversion.

while(!(CHECKBIT(ADCSRA,ADIF)));

//wait for conversion complete

return(ADC_read());//----read values & return

}

ADC_calcvolt(long int ADCval);

To convert the ADCvalue into equ. voltage value (float).

updatedisp();

To display the temperature in LCD in required format.

To get the Celsius value, the ADC_Value is divided by 2 to convert 5mV/'C to 10mV/'C

Main Program:

unsigned char d1[]="T1= . 'C",d2[]="T2= . 'C";unsigned int t1,t2;
void updatedisp1(){ unsigned int i=t1/2; //convert 5mV/'C to 10mV/'C (directly into Celcius) d1[6]= (i%10) | 0x30; //1s i/=10; d1[5]= (i%10) | 0x30; //10s i/=10; d1[4]= (i%10) | 0x30; //100s d1[8]= (t1%2 ? 5:0) | 0x30;//decimal LCD_putsXY(0,0,d1);}void updatedisp2(){ unsigned int i=t2/2; //convert 5mV/'C to 10mV/'C (directly into Celcius) d2[6]= (i%10) | 0x30; //1s i/=10; d2[5]= (i%10) | 0x30; //10s i/=10; d2[4]= (i%10) | 0x30; //100s d2[8]= (t2%2 ? 5:0) | 0x30;//decimal LCD_putsXY(0,1,d2);}
int main(){
CLEARBIT(DDRC,4)//set as i/p for adc CLEARBIT(DDRC,5) DDRD=0xF8;//enable PD as i/p & en pull ups PORTD=0x07; _delay_ms(10); LCD_init(COFF); _delay_ms(100); LCD_putsXY(0,0,"Welcome"); WaitMs(1000);//wait for some time ADC_init();
LCD_putsXY(0,0,d1); LCD_putsXY(0,1,d2);
while(1) {// t=t2/2;//convert 5mV/'C to 10mV/'C (directly into Celcius) t1=ADC_readch(4); t2=ADC_readch(5); updatedisp1(); updatedisp2(); WaitMs(300); }//end of while return 0;}