period = 256;
on_time = 34; // Or whatever you want
off_time = period-on_time;
count = 0;
while (1);
{
    if (count >= 0) {
        count -= off_time;
        output_high();
    } else {
        count += on_time;
        output_low();
    }
    wait_for_next_cycle();
}