int SW = 3, CLK = 7, DT = 6, count = 0, last = 0, limit = 15;

void setup() {
    pinMode(SW, INPUT);
    pinMode(CLK, INPUT);
    pinMode(DT, INPUT);
    DDRB = 0xFF;
}

void loop() {
    bool A = digitalRead(CLK), B = digitalRead(DT);
    if (A > last && B) {
        count += 1;
        if (count > limit) count = 0;
    }
    if (A > last && !B) {
        count -= 1;
        if (count < 0) count = limit;
    }
    PORTB = count << 1;
    last = A;
}