Count Down Timer using ATMega 8
A countdown timer is a down counter that counts from a
specified time to 00:00. It is used in many devices such as washing machine,
televisions, microwave ovens, etc. This countdown timer has three states: the
running state where it counts down, the pause state where it displays the
paused time and the reset state to set the countdown. The countdown is
displayed on a set of four seven segment displays using the ATMega 8
microcontroller. A buzzer beeps when the countdown gets
over.
I/O Port Map: 
Snippets of Code :
1) To enable INT0 and INT1 :
GICR =(1<<INT1)|(1<<INT0); MCUCR =0b00001010; sei();
2) To perform ADC :
ADCSRA = 0x8F|(1<<ADFR); ADMUX = 0xC0|ADC_Channel; sei(); ADCSRA |= 1<<ADSC;
3) To initialize Timer 1:
TCCR1B |= (1 << CS12)|(1<<CS10); TCNT1 =_________;TIMSK|=(1<<TOIE1); sei();
4)
To initialize
Timer 0:
TCCR0 = (1 << CS02)|(1 << CS00); TCNT0 =0; TIMSK |= (1 << TOIE0); sei();
5)
To select the
various digits in 4 - Seven Segment
Display :
Digit 1 : PORTB = 0b00001110; // Common Anode
Digit 2 : PORTB = 0b00001101;
Digit 3 : PORTB = 0b00001011;
Digit 4 : PORTB = 0b00000111;
6)
Interrupt Service
Routine :
INT 0 :
ISR ( INT0_vect )
INT 1 :
ISR ( INT1_vect )
ADC :
ISR ( ADC_vect )
Timer 0 : ISR ( TIMER0_OVF_vect )
Timer 1 : ISR ( TIMER1_OVF_vect )
Psuedo Code :
main()
{
// Initialize the I/O as INPUT or OUTPUT
// Use PULL – UP mode for Inputs
// Initialize ADC Channel
while(1)
{
//
Continuously display values on Seven Segment Display
// If time = 00:00 , blink and beep
}
}
ISR ( TIMER1_OVF_vect )
{ // Decrement seconds value by 1
// Initialize Timer 1 again
}
ISR ( ADC_vect)
{ // Store the
ADC0 channel value which corresponds to Minutes
// Store
the ADC1 channel value which corresponds to Seconds
// Initialize ADC again
}
Prototype:
