#include <stdint.h>

void delay(volatile int a) {
    while(a--);
}

int main(void) {
    volatile uint32_t *RCC_APB2ENR = (uint32_t*)0x40021018;
    volatile uint32_t *GPIOA_CRL   = (uint32_t*)0x40010800;
    volatile uint32_t *GPIOA_ODR   = (uint32_t*)0x4001080C;

    *RCC_APB2ENR |= (1 << 2);   // enable GPIOA clock

    
   // *GPIOA_CRL &= ~(0xF << 0);
    //*GPIOA_CRL |=  (0x2 << 0);

    //*GPIOA_CRL &= ~(0xF << 4);
    //*GPIOA_CRL |=  (0x2 << 4);
 
    //*GPIOA_CRL &= ~(0xF << 8);
    //*GPIOA_CRL |=  (0x2 << 8);
    
    //*GPIOA_CRL &= ~(0xF << 12);
    //*GPIOA_CRL |=  (0x2 << 12);
   
    //*GPIOA_CRL &= ~(0xF << 16);
    //*GPIOA_CRL |=  (0x2 << 16);
  
    //*GPIOA_CRL &= ~(0xF << 20);
    //*GPIOA_CRL |=  (0x2 << 20);
   
    //*GPIOA_CRL &= ~(0xF << 24);
    //*GPIOA_CRL |=  (0x2 << 24);
   
    //*GPIOA_CRL &= ~(0xF << 28);
    //*GPIOA_CRL |=  (0x2 << 28);


    *GPIOA_CRL &=~(0XFFFFFFFF<<0);
    *GPIOA_CRL |= (0X22222222<<0);
    
    while(1) {
        
        *GPIOA_ODR |= 0x55;
        delay(500000);
        *GPIOA_ODR &= ~0x55;
        delay(500000);

       *GPIOA_ODR |= 0xAA;
       delay(500000);
       *GPIOA_ODR &= ~0xAA;
       delay(500000);
       
       *GPIOA_ODR |= 0x0F;  
       delay(500000);
       *GPIOA_ODR &= ~0x0F; 
       delay(500000);

      *GPIOA_ODR |= 0xF0;  
      delay(500000);
      *GPIOA_ODR &= ~0xF0; 
      delay(500000);

       for(int i = 0; i < 8; i++) {
      *GPIOA_ODR |= (1 << i);
      delay(50000);
      *GPIOA_ODR &= ~(1 << i);
      delay(50000);
      }

    }
}