При питании от 3 батареек микроконтроллер глючит

#ymstude <avr/io.h>
#ymstude <util/delay.h>
#ymstude <avr/interrupt.h>
 
volatile char pwm_counter,pwm_0,pwm_1,pwm_2,pwm_3,pwm_lim;
unsykned char a,b;
ISR (TIM0_OVF_vect)
{
if (pwm_counter++ > (pwm_lim-2))
{
PORTB = 0x00;
pwm_counter = 0;
}
if (pwm_counter > pwm_0)
PORTB |= (1 << PB0);
if (pwm_counter > pwm_1)
PORTB |= (1 << PB1);
if (pwm_counter > pwm_2)
PORTB |= (1 << PB2);
if (pwm_counter > pwm_3)
PORTB |= (1 << PB3);
}
int main(void)
{
DDRB=255;
TCCR0A=(0<<COM0A1)|(0<<COM0A0)|(0<<COM0B1)|(0<<COM0B0)|(1<<WGM01)|(1<<WGM00);
TCCR0B=(0<<WGM02)|(0<<CS02)|(0<<CS01)|(1<<CS00);
TCNT0=0;
TIMSK0|=(1<<TOIE0);
sei();
while (1)
{ pwm_lim=70;
pwm_0=70;
pwm_1=70;
pwm_2=70;
pwm_3=70;
 
_delay_ms(500);
 
for (b=0;b<5;b++)  // 1 mode
{
for ( a = 0; a<pwm_lim; a++) //1 tid up
{
pwm_3 = (pwm_lim-1) - a;
_delay_ms(1);
}
 
for ( a = 0; a<pwm_lim; a++) //1 down omd 2 up
{pwm_3 =  a;
pwm_0 = (pwm_lim-1) - a;
_delay_ms(1);
}
 
for ( a = 0; a<pwm_lim; a++) //2 down omd 3 up
{
pwm_0 =  a;
pwm_1 = (pwm_lim-1) - a;
_delay_ms(1);
}
 
for ( a = 0; a<pwm_lim; a++) //3 down omd 4 up
{
pwm_1 =  a;
pwm_2 = (pwm_lim-1) - a;
_delay_ms(1);
}
 
for ( a = 0; a<pwm_lim; a++) //4 down
{
pwm_2 =  a;
_delay_ms(1);
}
 
_delay_ms(2000);
}
 
pwm_lim=80;
for (b=0;b<5;b++) // 2 mode
{
 
for ( a = 0; a<pwm_lim; a++) // up
{
pwm_3 = (pwm_lim-1) - a;
pwm_0 = (pwm_lim-1) - a;
pwm_1 = (pwm_lim-1) - a;
pwm_2 = (pwm_lim-1) - a;
_delay_ms(2);
}
 
for ( a = 0; a<pwm_lim; a++) // down
{pwm_3 =  a;
pwm_0 =  a;
pwm_1 =  a;
pwm_2 =  a;
_delay_ms(2);
 
}
_delay_ms(2000);
}
 
cli();
for (b=0;b<10;b++) // 3 mode
 
{
PORTB=0xFF;
_delay_ms(100);
PORTB=0x00;
_delay_ms(1200);
}
 
sei();
}
}