#define LED_1           (1 << 3)
#define LED_2           (1 << 4)
#define LED_MASK   (LED_1 | LED_2)

volatile AT91PS_RTTC pRTT = AT91C_BASE_RTTC;
volatile AT91PS_AIC  pAIC = AT91C_BASE_AIC;
volatile AT91PS_PIO  pPIO = AT91C_BASE_PIOA;

extern void __attribute__((interrupt("IRQ"))) ISR_rtt(void)
{
    unsigned int status;

    status = pRTT -> RTTC_RTSR;

    if ((status & AT91C_RTTC_RTTINC) == AT91C_RTTC_RTTINC) {

        pPIO -> PIO_PER  = LED_MASK;
        pPIO -> PIO_OER  = LED_MASK;
        pPIO -> PIO_CODR = LED_MASK;
        }
}

void ConfigureRtt(void)
{
    unsigned int previousTime;

    // Configure RTT for a 1 second tick interrupt
    pRTT -> RTTC_RTMR = (32768 | AT91C_RTTC_RTTRST);
    previousTime = pRTT -> RTTC_RTVR;
    while (previousTime == (pRTT -> RTTC_RTVR));

    // Enable RTT interrupt
    // Disable the interrupt first
    pAIC -> AIC_IDCR = SOURCE1;

    // Configure mode and handler
    AT91C_BASE_AIC->AIC_SMR[SOURCE1] = 0;
    AT91C_BASE_AIC->AIC_SVR[SOURCE1] = (unsigned int) ISR_rtt;

    // Clear interrupt
    AT91C_BASE_AIC->AIC_ICCR = SOURCE1;

    AT91C_BASE_AIC -> AIC_IECR = SOURCE1;
    pRTT -> RTTC_RTMR |= AT91C_RTTC_RTTINC;
}

int main (void)
{

  ConfigureRtt();

  while (1)
  {

  }
  
  return(0);
}

inline uint32_t __get_CPSR()
{
    uint32_t cpsr;
    asm volatile
    (
    " MRS  %0,CPSR    \r\n"
    :"=r"(cpsr)
    :
    );
    return cpsr;
}

inline void __set_CPSR_c(uint32_t cpsr)
{
    asm volatile
    (
    " MSR  CPSR_c, %0    \r\n"
    :
    :"r"(cpsr)
    );
}

inline void __enable_interrupts() { __set_CPSR_c(__get_CPSR() & ~(1<<7)); }

    MSR     CPSR_c,#(SYS_MODE | F_BIT)
    LDR     sp,=__c_stack_top__                  /* set the C stack pointer */