Reading Temperature with AVR ADC

The Atmega32 comes with a 10-bit, 8 channel inbuilt Ananlog to Digital Convertor (ADC). We will set it up and read temperature from LM35 and light with a simple (light dependent Resistor)LDR. So let's get started!

Basics

The ADCSRA register allows us to enable the adc and set the sampling rate. ADC Control and Status Register A (ADCSRA):

7	6	5	4	3	2	1	0
ADEN	ADSC	ADATE	ADIF	ADIE	ADPS2	ADPS1	ADPS0
1	0	0	0	0	0	0	1

♦ Bit 7 - ADEN: ADC Enable

Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off.

♦ Bit 2:0 - ADPS2:0: ADC Prescaler Select Bits

These bits determine the division factor between the XTAL frequency and the input clock to the ADC.

ADC Prescaler Selection

ADPS2	ADPS1	ADPS0	Division Factor
0	0	0	2
0	0	1	2
0	1	0	4
0	1	1	8
1	0	0	16
1	0	1	32
1	1	0	64
1	1	1	128

We will first initialize the ADC with the following code

1. Enable ADC by Setting ADEN bit of ADCSRA and set the sampling frequency to half of oscillator frequency (i.e 8 MHz)
2. Select channel zero by default using the ADMUX register shown below.

ADC Multiplexer Select (ADMUX)

7	6	5	4	3	2	1	0
REFS1	REFS0	ADLAR	MUX4	MUX3	MUX2	MUX1	MUX0
0	0	0	0	0	0	0	0

♦ Bit 7:6 - REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in below table.

REFS1	REFS0	Voltage Reference Selection
0	0	AREF,internal Vref turned off
0	1	AVCC with external Capacitor at AREF pin
1	0	Reserved
1	1	Internal 2.56V Voltage Reference with external Capacitor at AREF pin.

♦ Bits 4:0 - MUX 4:0 : Analog Channel and Gain Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC.

void ADC_Init()
 {
   ADCSRA=(1<<ADEN) | (1<<ADPS0); /* Enable ADC , sampling freq=osc_freq/2 */
   ADMUX=0x00;                    /* Result right justified, select channel zero */
 }

Let us write another function to select the required channel and read the analog data as below :

1. Select the required channel passed as input to the function
2. Start the ADC conversion by setting ADSC bit in ADCSRA
3. Wait till the conversion is over by checking the ADIF bit in ADCSRA
4. Read the ADCW register and return the result of conversion

uint16_t ADC_GetAdcValue(uint8_t v_adcChannel_u8)
 {
 
   ADMUX = v_adcChannel_u8;               /* Select the required channel */
   DELAY_us(10);                          /* Wait for some time for the channel to get selected */
   util_BitSet(ADCSRA,ADSC);              /* Start the ADC conversion by setting ADSC bit */
 
   while(util_IsBitCleared(ADCSRA,ADIF)); /* Wait till the conversion is over */
                                          /* ADIF will be set once ADC conversion is complete */
     return(ADCW);                        /* Return the 10-bit result */
 }

Raw Data

Let's put all of the above together to read raw data from the ADC. We will connect a Potentiometer(Pot) to channel 0, vary it and read the raw voltage.

Hookup Pot

Temperature

Let's connect the LM35 temperature sensor to channel zero of the ADC and read the temperature. Resolution with 5V reference can be calculated as: $$ResolutionOfADC = (5v/1023)=4.887mv = 5mv$$ For every degree celcius the Lm35 provides 10mv voltage change, therefore $$Temperature = ADCValue/2 $$

Hookup LM35

Code to Read Temperature

Video Tutorial

For those of you, who would like to watch instead of read we have made a video with all the gyan.

Downloads

Download the complete project folder from the below link: https://github.com/ExploreEmbedded/ATmega32_ExploreUltraAvrDevKit/archive/master.zip

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