#define CLOCK_MHZ       16UL
#define DELAY_LENGTH_MS 1000UL
#define DELAY_VALUE     (uint32_t)((CLOCK_MHZ * 1000UL * DELAY_LENGTH_MS) / 5UL)

void setup() {
  asm volatile (
    "sbi %0, %1 \n" //pinMode(13, OUTPUT);
    : : "I" (_SFR_IO_ADDR(DDRB)), "I" (DDB5)
  );
}

void loop() {
  asm volatile (
    "mov r18, %D2 \n" //delay
    "mov r20, %C2 \n"
    "mov r21, %B2 \n"

    "sbi %0, %1   \n" //liga led

  "1:             \n" //delay 1 segundo
    "subi %A2, 1  \n"
    "sbci %B2, 0  \n"
    "sbci %C2, 0  \n"
    "brcc 1b      \n"

    "cbi %0, %1   \n" //desliga led

  "2:             \n" //delay 1 segundo
    "subi r18, 1  \n"
    "sbci r19, 0  \n"
    "sbci r20, 0  \n"
    "brcc 2b      \n"

    : : "I" (_SFR_IO_ADDR(PORTB)), "I" (PORTB5), "r" (DELAY_VALUE) : "r18", "r19", "r20"
  );
}