/* p2_2.c Toggle Green LED (LD2) on STM32F446RE Nucleo64 using SysTick
 *
 * This program uses SysTick to generate 500 ms delay to
 * toggle the LED. System clock is running at 16 MHz. 
 * SysTick is configure to count down from 3199999 to zero 
 * to give a 500 ms delay.
* This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
#include "stm32f4xx.h"
int main(void) {
    RCC->AHB1ENR |=  1;             /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;    /* clear pin mode */
    GPIOA->MODER |=  0x00000400;    /* set pin to output mode */
    /* Configure SysTick */
    SysTick->LOAD = 3200000 - 1; /* reload with number of clocks per second */
    SysTick->VAL = 0;
    SysTick->CTRL = 5;     /* enable it, no interrupt, use system clock */

    while (1) {
        if (SysTick->CTRL & 0x10000) {  /* if COUNT flag is set */
            GPIOA->ODR ^= 0x00000020;   /* toggle green LED */
        }
    }
}