IAR For AVR Timer Overflow Interrupt (Usage Summary)

       Regarding overflow interrupt, no matter which microcontroller it is, it will continue to accumulate, causing its register to overflow and trigger an interrupt, and then jump to the interrupt function to execute the interrupt service program. Setting the initial value of the timer can deepen the understanding of the working principle of the timer.

       ATMega16 has two types of timers, 8-bit and 16-bit. When they overflow is fixed, that is, an interrupt will be generated when the maximum value of their counting range is reached. The maximum counting range of the 8-bit timer is 0~256 (2 to the 8th power), which means it will generate an interrupt when the count reaches 256. The maximum counting range of the 16-bit timer is 0~65536 (2 to the 16th power), which means it will generate an interrupt when the count reaches 65536.

     What we call the initial counting value is to set where the timer starts counting. Take an 8-bit timer as an example: the initial value is 100, so the timer starts to accumulate from 100, accumulates 156 times, and generates an interrupt after adding to 256. This is the time consumed in the middle and the instruction cycle is the time we want to set; for example: the initial value is 200, so the timer starts to accumulate from 200, accumulates 56 times, and generates an interrupt after adding to 256. It can be seen that the first timing will only be interrupted after accumulating 156 times, while the second time will be interrupted as long as it accumulates 56 times. Obviously, the time set for the first time is longer than the second time.

      Timers can not only set time, but we often use timers when we need precise timing. We can calculate how long it will take for the initial value we set to enter the interrupt.

The following are the registers needed when setting the 8-bit timer:

      

 

 

 

Experimental platform: ATMega16    

Crystal: 11.0592 MHz

Calculation of initial value:

                     1, 11059200 / 1024 = 10800 Set the frequency to 1024 times, and get how many accumulations are needed per second

                     2. 10800 / 100 = 108. How many accumulations are needed to get a timing of 10ms?

                     3, 256 - 108 = 148. Subtract the time to be accumulated from the maximum value of the calculation range to get the initial value, that is, where to start accumulating so that the time is 10ms when overflow occurs.        

                     4, 148 <==> 0x94 Get the hexadecimal value and assign it to TCNT0

Experimental code: Timing 10ms

#include
unsigned char flag = 0;
void timer_init(void)
{
TCCR0 = 0x05; //1024 division

TCNT0 = 0x94; //Assign initial value to count
  
TIMSK_TOIE0 = 1; //Enable
SREG_I = 1; //Open total interrupt
}
#pragma vector = TIMER0_OVF_vect
__interrupt void time0_normal(void)
{
    TCNT0 = 0x94; //Re-assign initial value
    flag++;
}
void main(void)
{
    timer_init();
    DDRB_Bit0 = 1;
    while(1)
    {
      if(flag == 100) //10ms repeated 100 times, that is 1 second
      {
        PORTB_Bit0 = ~PORTB_Bit0; //Make LED flash
        flag = 0;
      }
    }
}
      

//++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++//

Experimental platform: ATMega16

Crystal: 11.0592

16-bit timer initial value setting:

         1, 11059200 / 256 = 43200 Set the frequency division to 256 times to get the number of accumulations required per second

         2. 65536 - 43200 = 22336. Subtract the time to be accumulated from the maximum value of the calculation range to get the initial value, that is, where to start accumulating so that the time is 1s when overflow occurs.

         3, 22336 <==> 0x57 0x40 Get the hexadecimal value and assign it to TCNT1H and TCNT1L

Experimental code: Timing 1s

#include
unsigned char flag = 0;
void timer_init(void)
{
TCCR1B = 0x04;

TCNT1H = 0x57; 
TCNT1L = 0x40;

TIMSK_TOIE1 = 1;
SREG_I = 1;
}
#pragma vector = TIMER1_OVF_vect
__interrupt void time1_normal(void )
{
TCNT1H = 0x57; 
TCNT1L = 0x40;

    flag++;
}
void main(void)
{
    timer_init();
    DDRB_Bit0 = 1;
    while(1)
    {
      if(flag == 1)
      {
        PORTB_Bit0 = ~PORTB_Bit0;
        flag = 0;
      }
    }
}