Need help with timer - c

Question

For last 2 weeks am trying to learn timer & interrupt & wrote a program (with my understanding) to blink LEDs on ATMEGA2560 but no matter what I do TCNT0 never increments & ISR() function never gets called. Where am I going wrong and how can I fix it? Here is my code:
#include<avr/io.h>
#include<avr/interrupt.h>
#define READ_ATMEGA(ADDR) *((P_CHAR)(BASE_ADDR + ((ADDR) * ADDR_MULTIPLIER)))
#define WRITE_ATMEGA(ADDR, DATA) *((P_CHAR)(BASE_ADDR + ((ADDR) * ADDR_MULTIPLIER))) = DATA
#define BASE_ADDR 0x20
void init_timer0_ovf_interrupt(void);
void timer0_interrupt_isr(void);
void initialize_ports(void);
void delay(unsigned int no_65_5ms_interrupts);
void __attribute__((ISR)) timer0_interrupt_isr(void);
//#pragma interrupt_handler timer0_interrupt_isr:24
unsigned int delay_timer;
int main(void)
{
initialize_ports();
init_timer0_ovf_interrupt();
delay(46);
return 0;
}
void initialize_ports(void)
{
READ_ATMEGA(4) = 0xff;
WRITE_ATMEGA(5, 0x00);
}
void delay(unsigned int no_65_5ms_interrupts)
{
TCNT0 = 0x00;
delay_timer = 0;
while(delay_timer <= no_65_5ms_interrupts)
{
;
}
}
void init_timer0_ovf_interrupt(void)
{
TCCR0A = 0X00;
TCCR0B = 0x02;
TIMSK0 = 0x01;
TIFR0 = 1<<0;
OCR0A = 25;
sei();
}
void timer0_interrupt_isr(void)
{
delay_timer++;
if(delay_timer >= OCR0A)
{
PORTB = ~(PORTB);
delay_timer = 0;
}
}

Answer 1

The global variable delay_timer is shared between interrupt and non-interrupt code. It should be declared as volatile as the value can change outside of delay().
If you look at the generated code for delay() you'll probably see that the value of delay_timer isn't being re-read while spinning in the while loop.
Also, volatile isn't enough. You've got non-interrupt code and interrupt code both writing to the same variable (delay_timer). You need to protect writes to the variable in non-interrupt code, there's a race-condition there. The easy/lazy way is to disable interrupts & restore them in the non-interrupt code.
(As for setting up your interrupts & starting your timer, that info should be in the chip's datasheet. Usually that's the part that's easier to get right, it's the shared data stuff that bites people.)

Answer 2

3-4 days ago, I wrote the same program a little differently & got LEDs blinking but still not sure whether it is the correct way of using timer & interrupt. Could anyone please see this & tell me whether it's the correct or not? I managed to write this program by reading programs of timers, interrupts.
#include <avr/io.h>
#include <avr/interrupt.h>
volatile uint8_t intrs;
ISR(TIMER0_OVF_vect) {
/* this ISR is called when TIMER0 overflows */
intrs++;
/* strobe PORTB.5 - the LED on arduino boards */
if (intrs >= 61){
PORTB = ~PORTB;
intrs = 0;
}
}
int main(void) {
TCCR0B = 0x02;
/* Enable Timer Overflow Interrupts */
TIMSK0 = 0x01;
/* other set up */
DDRB = 0xff;
TCNT0 = 0;
intrs = 0;
/* Enable Interrupts */
sei();
while (1)
; /* empty loop */
}
If it's the correct way then I can start working on next step.
Thanks

Answer 3

If could be that your while loop in the delay function doesn't do anything and will not increment delay_timer so you are stuck in an endless loop:
void delay(unsigned int no_65_5ms_interrupts)
{
TCNT0 = 0x00;
delay_timer = 0;
while(delay_timer <= no_65_5ms_interrupts)
{
; //Nothing is happening here!!
}
}