Creating a precise one millisecond delay using timer 0 of ATMega32

Using a built-in delay function could consume the CPU executing time. It will keep the Program counter at the delay sub-routine. Other task is queue until the delay is finish.

Using a timer, could schedule any task at a specific time without making other task awaiting. In this example we use timer to create a one millisecond tick every time the TOV0 triggered. 

I use this timer tick to schedule the time for an LED turn on and off.

Schematic Diagram. CPU clocks at 16 MHz. PB0 toggle an LED.

Let do some calculation:

• The CPU clock period is T (CPU) = 1/(16000000 MHz) = 62.5 nano seconds.
• I set the prescaler to 1:256 (TCCR0=0x04). So the timer 0 input period T  = 256 x 62.5 nano seconds = 16 micro seconds.
• To get one millisecond: (0.000001 second)/(0.000016 second) = 62.5 counts
• So, when TOV0 is set we assign TCNT0 = (256-62) = 194 counts or -62 counts.

Source code:

#include <avr/io.h>
//A variable to hold one milli second
unsigned int milliSecond=0;
unsigned int oneMilliDelay(void){
   //Check if TIFR is set
  if(TIFR&0x01){
   //Preload TCNT0 to make 1 mS
   TCNT0=-62;
   //Clear flag
   TIFR=0x01;
   milliSecond+=1;
  }   return milliSecond;
}
int main(void)
{
   
 //Set PortC to Output
 DDRB=0xFF;
 //Clear PortC
 PORTB=0x00;
 //Set 1:256 prescaler
 TCCR0=0x04;
 //Clear overflow flag
 TIFR=0x01;
    while (1) 
    {
  if(oneMilliDelay()<1000) PORTB=0x01;
  else PORTB=0x00;
  if(oneMilliDelay()>2000) milliSecond=0;
  /*Do other tasks*/
    }
}

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