Prescaler auf 64 einstellen

#define TIMER0_PRESC_OFF     0
#define TIMER0_PRESC_1       1
#define TIMER0_PRESC_8       2
#define TIMER0_PRESC_64     3
#define TIMER0_PRESC_256     4
#define TIMER0_PRESC_1024     5
#define TIMER0_PRESC_EXT_FALL   6
#define TIMER0_PRESC_EXT_RISE   7

#define TIMER0_CLOCK_SELECT(Clock)  {TCCR0B &= 0xF8; TCCR0B |= (Clock & 
0x07);}

TIMER0_CLOCK_SELECT(TIMER0_PRESC_64);