#define LED 1

#define MEM32(address) (*(volatile uint32_t*)(address))
#define GPIO_OUT_CLR MEM32(0xd0000000+0x018)
#define GPIO_OUT_XOR MEM32(0xd0000000+0x01c)

#define TIMER_REG(offset) MEM32(0x40054000+offset)

#define TIMELR TIMER_REG(0x0c)
#define ALARM1 TIMER_REG(0x14)
#define ARMED TIMER_REG(0x20)
#define INTR TIMER_REG(0x34)
#define INTE TIMER_REG(0x38)

#define NVIC_ISER MEM32(0xe0000000 + 0xe100)
#define VTOR MEM32(0xe0000000 + 0xed08)

#define TIMER_IRQ_1 1

void alarm1_irq() {
  ALARM1 = TIMELR + 250000;
  GPIO_OUT_XOR = bit(LED);
  // clear the interrupt in INTR:
  INTR = bit(1);
}

void setup() {
  Serial1.begin(115200);
  Serial1.println(VTOR, HEX);
  pinMode(LED, OUTPUT);
  for (int i = 2; i < 8; i++) {
    pinMode(i, OUTPUT);
  }

  // 1. Define the exception/interrupt handler
  MEM32(VTOR + (16 + TIMER_IRQ_1) * 4) = (uint32_t)alarm1_irq;

  // 2. Enable the interrupt in the Timer (INTE) register
  INTE |= bit(1);

  // 3. Enable the NVIC interrupt (NVIC_ISER)
  NVIC_ISER = bit(TIMER_IRQ_1);

  // 4. Set the ALARM1 register to the target time
  ALARM1 = TIMELR + 250000;
}

void loop() {
  for (int i = 2; i < 8; i++) {
    digitalWrite(i, !digitalRead(i));
    delay(50);
  }
}