vel.

static int8_t lookup_table[] = {0,-1,1,0,1,0,0,-1,-1,0,0,1,0,1,-1,0};
static uint8_t enc_val = 0;

int A = 2;
int B = 3;
volatile long enc_count = 0;
volatile long last_time = 0;


void encoder()
{
    last_time = millis();
    enc_val = enc_val << 2;
    enc_val = enc_val | ((PIND & 0b1100) >> 2);

    enc_count = enc_count + lookup_table[enc_val & 0b1111];
}

void setup()
{
  attachInterrupt(0,encoder,CHANGE);
  attachInterrupt(1,encoder,CHANGE);
  Serial.begin(9600);
}

void loop()
{
  Serial.println(1/(millis()-last_time));
}