#include <avr/io.h>
#include <util/delay.h>

int main(void) {
#if defined(DDRC) && defined(DDRD)
#define BITS 8
    DDRC = 0x3f;
    DDRD = DDRB = 0xff;
#else
#define BITS 6
    DDRB = 0x3f;
#endif
    for (;;) {
        for (uint8_t i = 0; i < BITS; ++i) {
#if defined(DDRC) && defined(DDRD)
            PORTC = (PORTD = PORTB ^= _BV(i))&0x3f;
#else
            PORTB ^= _BV(i);
#endif
            _delay_ms(36);
        }
    }
    return 0;
}