/* p5_4.c Toggle Green LED (LD2) on STM32F446RE Nucleo64 using TIM2
 *
 * This program configures TIM2 with prescaler divides by 1600
 * and counter wraps around at 10000. So the 16 MHz system clock
 * is divided by 1600 then divided by 10000 to become 1 Hz.
 * Every time the counter wraps around, it sets UIF flag
 * (bit 0 of TIM2_SR register). The program waits for the
 * UIF to set then toggles the LED (PA5).
 * 
 * This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
#include "stm32f4xx.h"

int main(void) {
    // configure PA5 as output to drive the LED
    RCC->AHB1ENR |=  1;             /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;    /* clear pin mode */
    GPIOA->MODER |=  0x00000400;    /* set pin to output mode */
    
    // configure TIM2 to wrap around at 1 Hz
    RCC->APB1ENR |= 1;              /* enable TIM2 clock */
    TIM2->PSC = 1600 - 1;           /* divided by 1600 */
    TIM2->ARR = 10000 - 1;          /* divided by 10000 */
    TIM2->CNT = 0;                  /* clear timer counter */
    TIM2->CR1 = 1;                  /* enable TIM2 */

    while (1) {
        while(!(TIM2->SR & 1)) {}   /* wait until UIF set */
        TIM2->SR &= ~1;             /* clear UIF */
        GPIOA->ODR ^= 0x00000020;   /* toggle green LED */
    }
}