Error Real Time Clock implementation AVR microcontroller

Question

I want to create a real-time clock using ATmega328p microcontroller. The microcontroller's frequency is 1 MHz. So, I think I should use timer interrupts in CTC mode to generate interrupt every second. I have calculated the value for timer CTC mode:

`n = t * f - 1

• n - number of cycles for CTC mode.
• t - target time.
• f - frequency.
• - 1 because it takes one cycle to reset the counter.

I set the frequency divider by 64 for the timer.

n = 1 s * (10^6 Hz / 64) - 1
n = 1 s * 15625 Hz - 1
n = 15625 - 1
n = 15624

So, I have code:

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

static unsigned long t = 0;

ISR(TIMER1_COMPA_vect) {
    t++;
}

static void InitializeTimer() {
    cli();

    OCR1A = 15624;
    TCCR1B = (1 << WGM12) | (1 << CS10) | (1 << CS11);
    TIMSK1 = (1 << OCIE1A);

    sei();
}

[[noreturn]] int main() {

    InitializeTimer();

    LCD_SetUp(PC_5, PC_4, PC_3, PD_0, PD_1, PD_2, PD_3, PD_4, PD_5, PD_6, PD_7);
    LCD_Init(2, 16);

    while(true) {
        tm* time = gmtime((time_t*)&t);

        LCD_Clear();
        LCD_Printf("Time: %02d:%02d:%02d", time->tm_hour, time->tm_min, time->tm_sec);

        _delay_ms(100);
    }
}

This code prints number of seconds since the start of the microcontroller to an LCD display. It really does, but since even 3 minutes time error is 2 seconds, since 10 minutes time error is 5 seconds, since 45 minutes time error is 30 seconds. I checked the time error using my phone's stopwatch. Why there is a time error? Error during calculation? What are the reasons for this? Thanks in advance!

Answer 1

The AVR's internal oscillator is unsuitable for your purpose. This is from the datasheet:

You should switch to an external crystal clock source at least. Better yet would be an RTC chip with a precision crystal oscillator if you need accurate timekeeping.