/* p5_5.c Toggle Green LED (LD2) on STM32F030RE Nucleo64 using TIM3
 *
 * This program configures TIM3 with prescaler divides by 800
 * and counter wraps around at 10000. So the 8 MHz system clock
 * is divided by 800 then divided by 10000 to become 1 Hz.
 * Every time the counter wraps around, it sets UIF flag
 * (bit 0 of TIM3_SR register). The program waits for the
 * UIF to set then toggles the LED (PA5).
 * 
 * This program was tested with Keil uVision v5.23 with DFP v2.0.0
 */
#include "stm32f0xx.h"
int main(void) {
    // configure PA5 as output to drive the LED
    RCC->AHBENR |=   0X00020000;    /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;    /* clear pin mode */
    GPIOA->MODER |=  0x00000400;    /* set pin to output mode */
    
    // configure TIM3 to wrap around at 1 Hz
    RCC->APB1ENR |= 0x00000002;     /* enable TIM3 clock */
    TIM3->PSC = 800 - 1;            /* divided by 1600 */
    TIM3->ARR = 10000 - 1;          /* divided by 10000 */
    TIM3->CNT = 0;                  /* clear timer counter */
    TIM3->CR1 = 1;                  /* enable TIM3 */

    while (1) {
        while(!(TIM3->SR & 1)) {}   /* wait until UIF set */
        TIM3->SR &= ~1;             /* clear UIF */
        GPIOA->ODR ^= 0x00000020;   /* toggle green LED */
    }
}