AVR programming, displaying wrong value on 7 seg. LED

Question

I am interfacing LM35 with Atmega8. To display digits I use 7 segment LED anode display that I connect to AVR both ends (it handles it without transistors so why not). Strange thing happens: res value after assigning it from adc is 237 (23.7 degrees). I want to print on my display the first digit (2). If I leave last line in the while commented out, the display first shows digit 2 correctly but after the first delay it shows 1 instead of 2. Otherwise I get correctly digit 2. Why is this happening?

#ifndef F_CPU
#define F_CPU 1000000UL
#endif // F_CPU

#include <avr/io.h>
#include <util/delay.h>

#define DELAY_IN_MS 500 /* 0.5 sec */

int numbers[] = {
    0b01000000,
    0b01110011,
    0b00100100,
    0b00100001,
    0b00010011,
    0b00001001,
    0b00001000,
    0b01100011,
    0b00000000,
    0b00000001,
    0b11111111 // off
};

uint8_t digits[3];

void initADC()
{
    ADMUX=(1<<REFS1)|(1<<REFS0);
    ADCSRA=(1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
}

uint16_t ReadADC(uint8_t ch)
{
    //Select ADC Channel ch must be 0-7
    ch=ch&0b00000111;
    ADMUX|=ch;

    //Start Single conversion

    ADCSRA|=(1<<ADSC);

    //Wait for conversion to complete
    while(!(ADCSRA & (1<<ADIF)));

    //Clear ADIF by writing one to it
    ADCSRA|=(1<<ADIF);

    return(ADC);
}

int main()
{
    DDRD = 0xFF;
    PORTD = 0xFF;

    DDRB = 0b00000001;
    PORTB = 1;

    initADC();

    uint16_t adc_value;
    uint16_t res;
    while(1)
    {
        adc_value = 0;
        for (int i = 0; i < 250; i++)
        {
            adc_value += ReadADC(0);
        }

        adc_value=(adc_value/25)/4;
        res = adc_value;

        for(int j = 2; j >= 0; j--) {
            digits[j] = res%10;
            res /= 10;
        }

        uint8_t dig = digits[0];
        PORTD = numbers[dig];
        _delay_ms(DELAY_IN_MS);

        // if following is uncommented there blinks digit two correctly
        // if commented there is unblinking digit 1 
        PORTD = numbers[10]; // display off
    }

    return 0;
}

Answer 1

The problem was induction.

My circuit had many wires in non-soldering-field. When the display was on, there was a lot of induction going on changing resulting voltage on ADC input/LM35 output. There is more than one solution.

1) Software: I moved ADC conversion into the interruption function. It turns of the displays, converts value from lm35 and displays digit on proper display. It happens so fast that the eye cant perceive it. I prefer this one for now, because it makes my circuit simpler.

2) Hardware: adding L/C or R/C filter to adc pin should resolve the issue.

Full code for 1)

#ifndef F_CPU
#define F_CPU 1000000UL
#endif // F_CPU

#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

#define DELAY_IN_MS 5000 /* ms */
#define NUM_OF_MEASUREMENTS 100
#define NUM_DISPLAYS 3

int numbers[] = {
    0b10000001,
    0b10011111,
    0b10100100,
    0b10010100,
    0b10011010,
    0b11010000,
    0b11000000,
    0b10011101,
    0b10000000,
    0b10010000,
    0b11111111 // off
};

int display = 0;
uint8_t digits[NUM_DISPLAYS];
volatile uint16_t adc_values[NUM_OF_MEASUREMENTS];
int adc_read_cycle_index = 0;
uint32_t res;

void initADC()
{
    ADMUX=(1<<REFS1)|(1<<REFS0);
    ADCSRA=(1<<ADEN)|(1<<ADPS2);
}

uint16_t ReadADC(uint8_t ch)
{
    //Select ADC Channel ch must be 0-7
    ch=ch&0b00000111;
    ADMUX|=ch;

    //Start Single conversion
    ADCSRA|=(1<<ADSC);

    //Wait for conversion to complete
    while (ADCSRA & (1<<ADSC));

    return(ADC);
}

void readDegrees()
{
    adc_values[adc_read_cycle_index] = (ReadADC(0)*10)/4;

    if(adc_read_cycle_index + 1 == NUM_OF_MEASUREMENTS) {
        adc_read_cycle_index = 0;
    } else {
        adc_read_cycle_index++;
    }
}

void fetchTemperatureDigits() {
    res = 0;
    for(int i = 0; i < NUM_OF_MEASUREMENTS; i++) {
        res += adc_values[i];
    }
    res /= NUM_OF_MEASUREMENTS;

    for(int j = 2; j >= 0; j--) {
        digits[j] = res%10;
        res = res / 10;
    }
}

void initTimer0()
{
      // Prescaler = FCPU/64
      TCCR0|=(1<<CS01);//|(1<<CS00);

      //Enable Overflow Interrupt Enable
      TIMSK|=(1<<TOIE0);

      //Initialize Counter
      TCNT0=0;
}

ISR(TIMER0_OVF_vect)
{
    // turn off displays
    PORTD = numbers[10];

    // read ADC and convert to degrees
    readDegrees();

    // turn on proper anode
    PORTB &= 0b11111000;
    PORTB |= (1<<display);

    // show digit
    PORTD = numbers[digits[display]];
    // show decimal point for second display (21.5 - second display shows "1.")
    if(display == 1) {
        PORTD &= 0b01111111;
    }
    // next display for next interruption
    display++;
    if(display == NUM_DISPLAYS) {
        display = 0;
    }
}

int main()
{

    initADC();
    for(int i = 0; i < NUM_OF_MEASUREMENTS; i++) {
        readDegrees();
    }

    DDRD = 0xFF;
    PORTD = 0;

    DDRB |= 0b00000111;
    PORTB |= 1;

    initTimer0();

    sei();

    while(1) {
        fetchTemperatureDigits();
        _delay_ms(DELAY_IN_MS);
    }

    return 0;
}