AVR Tutorial - 5. Timers and Counters

1.                  Timers and Counters

·         Timers and Counters are the one of the most commonly used complex peripheral in a microcontroller.

·         They are used for getting time periods, generating pulse signals and also for measuring time period of pulses.

·         There are two main TIMER/COUNTER in ATMEGA32 : TIMER0(8-Bit) and TIMER1(16-Bit)

·         Timer0 will roll over after 255 and Timer1 will rollover after 65,535. So be careful about this roll over when using Timers in a µC.

·         TCNT0 is the counter for TIMER0 ie. It is like a variable which counts with the oscillation of system clock and rolls over back to 0.

·         Similarly TCNT1 is the counter for TIMER1.

Timer0:

Using timer 1 includes setting the appropriate values in the Timer Counter Control Register mainly for setting the speed / frequency at which TCNT0 is counting.

Program:

#include <avr/io.h>

#include <util/delay.h>

#include<avr/interrupt.h>

void main()

{

                DDRB = 0b00000011;

                PORTB = 0b00000000;

               

                TCCR0 = (1<<CS02) | (0<<CS01) | (1<<CS00); //   8MHz / 1024 as prescaler

                TCNT0 = 0;           

                int ctr1 = 0;

                int sec_ctr = 0;

                                               

                while(1)

                {

                                if(TCNT0 == 200)

                                {

                                                PORTB = 0b00000000;

                                                TCNT0 = 0;

                                                ctr1++;

                                                if(ctr1 == 39)

                                                {

                                                                ctr1 = 0 ;

                                                                PORTB = 0b00000001;

                                                                sec_ctr++;

                                                                if(sec_ctr >= 60) //checking for elapsing of one minute

                                                                {

                                                                                PORTB |= 0b00000010;

                                                                                sec_ctr = 0;

                                                                                _delay_ms(10);

                                                                }

                                                }

                                }

                }}

·         An interrupt is also being generated when the overflow of Timer occurs.

·         We need to unmask the TIMER0 overflow interrupt in the TIMSK registers and should use suitable ISRs.

·         The previous example could be done with the help of interrupts as given below:

#include <avr/io.h>

#include <util/delay.h>

#include<avr/interrupt.h>

int ctr1 = 0;

int sec_ctr = 0;

void main()

{

     DDRB = 0b00000011;

     PORTB = 0b00000001;

     TCCR0 = (1<<CS02) | (0<<CS01) | (1<<CS00);

     TIMSK = 0x01; //Enabling TIM0 ovf interrupt

     TCNT0 = 55;

     sei();

     int ctr1 = 0;

     int sec_ctr = 0;

     OCR0 = 0;

     while1)

     {

           //Do nothing…

     }

}

ISR(TIMER0_OVF_vect)

{

     TCNT0 = 55;

     PORTB = 0b00000000;

     ctr1++;

     if(ctr1 == 39)

     {

           ctr1 = 0 ;

           PORTB = 0b00000001;

           sec_ctr++;

           if(sec_ctr >= 60) //checking for elapsing of one minute

           {

                PORTB |= 0b00000010;

                sec_ctr = 0;

                _delay_ms(10);

           }

     }

}

TIMER1:

·         Timer1 is a 16 bit counter and rolls over occur after 65535.

·         Here the Timer Counter Control Registers are TCCR1A and TCCR1B

·         We will be using here just TCCR1B for setting the prescaler bits.

Program:

Source: www.newbiehack.com

#include <avr/io.h>
int main(void)
{

DDRB = 0b01111111;
PORTB = 0b00000000;
DDRD = 0b01111111;
PORTD = 0b00000000;
TCCR1B |= 1<<CS10 | 1<<CS11;
int LEDNumber[2];
while(1)
{

if (TCNT1 > 2232)
{

TCNT1 = 0;
PORTB = 1<<LEDNumber[0];
LEDNumber[0] ++;
if (LEDNumber[0] > 6)
{

LEDNumber[0] = 0;
PORTD = 1<<LEDNumber[1];
LEDNumber[1] ++;
if (LEDNumber[1] > 6)

LEDNumber[1] = 0;

}

}

}

}

·         We could also do this thing using interrupts:

Source: www.newbiehack.com

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

int main(void)
{

sei();

DDRB |= 1<<PINB0;

TCCR1B |= 1<<CS10 | 1<<CS11 | 1<<WGM12;
TIMSK |= 1<<OCIE1A;
OCR1A = 15624;

while(1)
{
}

}

ISR(TIMER1_COMPA_vect)
{

PORTB ^= 1<<PINB0;

}