#include "stm32c031xx.h"

#define GPIOAEN	(1U<<0)
#define PIN5				(1U<<5)
#define LED_PIN		PIN5

int main()
{
	RCC->IOPENR |= GPIOAEN;
	GPIOA->MODER &= ~(3U << (5*2));
	GPIOA->MODER |= (1U << (5*2));
	while(1){
		GPIOA->ODR ^= LED_PIN;
		for (int i=0; i<1000000; i++){}
	}
}