Timer in AVR ATmega16/ATmega32

Introduction

Generally, we use timer/counter to generate time delays, waveforms or to count events. Also, the timer is used for PWM generation, capturing events etc.

In AVR ATmega16 / ATmega32, there are three timers:

Timer0: 8-bit timer
Timer1: 16-bit timer
Timer2: 8-bit timer

Basic registers and flags of the Timers

TCNTn: Timer / Counter Register

Every timer has timer/counter register. It is zero upon reset. We can access value or write a value to this register. It counts up with each clock pulse.

TOVn: Timer Overflow Flag

Each timer has Timer Overflow flag. When timer overflows, this flag will get set.

TCCRn : Timer Counter Control Register

This register is used for setting the modes of timer/counter.

OCRn : Output Compare Register

The value in this register is compared with the content of the TCNTn register. When they are equal, OCFn flag will get set.

Let us see Timer0 to understand the timers in ATmega16 / ATmega32

Timer0

First, we need to understand the basic registers of the Timer0

TCNT0: Timer / Counter Register 0

It is an 8-bit register. It counts up with each pulse.

2. TCCR0: Timer / Counter Control register 0

This is an 8-bit register used for the operation mode and the clock source selection.

Bit 7- FOC0: Force compare match

Write only bit, which can be used while generating a wave. Writing 1 to this bit causes the wave generator to act as if a compare match has occurred.

Bit 6, 3 – WGM00, WGM01: Waveform Generation Mode

Bit 5:4 – COM01:00: Compare Output Mode

These bits control the waveform generator. We will see this in the compare mode of the timer.

Bit 2:0 – CS02:CS00: Clock Source Select

These bits are used to select a clock source. When CS02: CS00 = 000, then timer is stopped. As it gets value between 001 to 101, it gets clock source and starts as the timer.

3. TIFR: Timer Counter Interrupt Flag register

Bit 0 – TOV0: Timer0 Overflow flag

0 = Timer0 did not overflow

1 = Timer0 has overflown (going from 0xFF to 0x00)

Bit 1 – OCF0: Timer0 Output Compare flag

0 = Compare match did not occur

1 = Compare match occurred

Bit 2 – TOV1: Timer1 Overflow flag

Bit 3 – OCF1B: Timer1 Output Compare B match flag

Bit 4 – OCF1A: Timer1 Output Compare A match flag

Bit 5 – ICF1: Input Capture flag

Bit 6 – TOV2: Timer2 Overflow flag

Bit 7 – OCF2: Timer2 Output Compare match flag

Timer0 Overflow

Normal mode: When the counter overflows i.e. goes from 0xFF to 0x00, the TOV0 flag is set.

Creating Delay Using Timer 0

Steps to Program Delay using Timer0

1. Load the TCNT0 register with initial value (let’s take 0x25).

2. For normal mode and pre-scaler option of the clock, set the value in TCCR0 register. As soon as clock prescaler value gets selected, the timer / counter starts to count, and each clock tick causes the value of timer / counter to increment by 1.

3.Timer keeps counting up, so keep monitoring for timer overflow i.e. TOV0 (Timer0 Overflow) flag to see if it is raised.

4. Stop the timer by putting 0 in the TCCR0 i.e. the clock source will get disconnected and the timer / counter will get stopped.

5. Clear the TOV0 flag. Note that we have to write 1 to the TOV0 bit to clear the flag.

6. Return to the main function.

Program for Timer Delay

#include

void T0delay();

int main(void)
{
DDRB = 0xFF; /* PORTB as output*/

while(1) /* Repeat forever*/
{
PORTB=0x55;
T0delay(); /* Give some delay */
PORTB=0xAA;
T0delay();
}
}

void T0delay()
{
TCNT0 = 0x25; /* Load TCNT0*/
TCCR0 = 0x01; /* Timer0, normal mode, no pre-scalar */

while((TIFR&0x01)==0); /* Wait for TOV0 to roll over */
TCCR0 = 0;
TIFR = 0x1; /* Clear TOV0 flag*/
}

The time delay generated by above code

As Fosc = 8 MHz

T = 1 / Fosc = 0.125 μs

Therefore, the count increments by each 0.125 μs.

In above code, the number of cycles required to roll over are:

0xFF – 0x25= 0xDA i.e. decimal 218

Add one more cycle as it takes to roll over and raise TOV0 flag: 219

Total Delay = 219 x 0.125 μs = 27.375 μs

Example

Let us generate a square waveform having 10 ms high and 10 ms low time:

First, we have to create a delay of 10 ms using timer0.

*Fosc = 8 MHz

Use the pre-scalar 1024, so the timer clock source frequency will be,

8 MHz / 1024 = 7812.5 Hz

Time of 1 cycle = 1 / 7812.5 = 128 μs

Therefore, for a delay of 10 ms, number of cycles required will be,

10 ms / 128 μs = 78 (approx)

We need 78 timer cycles to generate a delay of 10 ms. Put the value in TCNT0 accordingly.

Value to load in TCNT0 = 256 – 78 (78 clock ticks to overflow the timer)

= 178 i.e. 0xB2 in hex

Thus, if we load 0xB2 in TCNT0 register, the timer will overflow after 78 cycles i.e. precisely after a delay of 10 ms.

*Note – All calculations are done by considering 8 MHz CPU frequency. If you are using another value of CPU frequency, modify the calculations accordingly; otherwise the delay will mismatch.

Program for 10 ms Delay Using Timer0

#include

void T0delay();

int main(void)
{
DDRB = 0xFF; /* PORTB as output */
PORTB=0;
while(1) /* Repeat forever */
{
PORTB= ~ PORTB;
T0delay();
}
}

void T0delay()
{
TCCR0 = (1<<CS02) | (1<<CS00); /* Timer0, normal mode, /1024 prescalar */
TCNT0 = 0xB2; /* Load TCNT0, count for 10ms */
while((TIFR&0x01)==0); /* Wait for TOV0 to roll over */
TCCR0 = 0;
TIFR = 0x1; /* Clear TOV0 flag */
}

Timer Interrupt

TIMSK: Timer / Counter Interrupt Mask Register

We have to set TOIE0 (Timer0 Overflow Interrupt Enable) bit in TIMSK register to set the timer0 interrupt, so that as soon as the Timer0 overflows, the controller jumps to the Timer0 interrupt routine.

Timer0 Interrupt Program

#include
#include

/* timer0 overflow interrupt */
ISR(TIMER0_OVF_vect)
{
PORTB=~PORTB; /* Toggle PORTB */
TCNT0 = 0xB2;
}

int main( void )
{
DDRB=0xFF; /* Make port B as output */

sei();
TIMSK=(1<<TOIE0); /* Enable Timer0 overflow interrupts */

TCNT0 = 0xB2; /* Load TCNT0, count for 10ms*/
TCCR0 = (1<<CS02) | (1<<CS00); /* Start timer0 with /1024 prescaler*/
while(1);
}