Hello all,

first of all: I hope that this is the correct forum, if not, please give 
me a hint.
2nd: I searched a couple of days, but did not find anything, so now, I 
need someone who likes to help me.

I run an Atmel ATMEGA 644 on a Pollin Net-I/O board and try "from 
scratch" to setup a simple timer 0 interrupt.

The code looks like this:

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#include <stdint.h>

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#include <avr/io.h>

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#include <avr/interrupt.h> 

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#include <util/delay.h>

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ISR(BADISR_vect)

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{

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// catch up all interrupts for which no ISR is defined

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}

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ISR (TIMER0_OVF_vect) 

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{

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  PORTC |= (1<< PC1);           // LED on

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  _delay_us(200);

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  PORTC &= ~(1<< PC1);          // LED off

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}

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// ********************** main function **************************

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int main(void) {        // main function begins here

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  DDRC |= (1<<PC0) | (1<<PC1); // set C.0 and C.1 as output  

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  TCCR0B = 1<<CS02;

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  TIMSK0 |= (1<<TOIE0);        // unmask timer0 interrupt

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  sei();                       // enable interrupts 

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  for (;;) {

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    PORTC |= (1<<PC0);         // LED on

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    _delay_ms(200);

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    PORTC &= ~(1<<PC0);        // LED off

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    _delay_ms(200);

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  }

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  return (0);    // return from function main (never reached)

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}

The strange thing is that the code after sei() is never reached, means 
the LED 0 is never blinking. If I comment out the sei() OR the TCCR0B = 
line, the LED 0 blinks.
(LED 1 stays dark in both cases)

If I try to use "external clock source on T0 Pin" (CS02 | CS01) the code 
"LED 0" is also reached, but obviously the ISR is not executed.

I also tried to configure Timer 1 with the same effect.

IMHO, the problem must be very trivial, but I don't have any clue 
anymore.

I really appreciate some ideas / hints from you, TIA!