/* p5_1.c Toggle Green LED (LD2) on STM32F030R8 Nucleo64 using SysTick

 * This program configures SysTick to be a 24-bit free-running
 * down-counter. Bit 22 of SysTick current value is written to 
 * the LED (PA5) continuously.
 * SysTick is based on system clock running at 8 MHz. 
 * So bit 22 of SysTick current value toggles about 1 Hz
 * (8,000,000 Hz / 2^22 = 1.907 Hz).
 *
 * This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
#include "stm32f0xx.h"

void delayMs(int n);

int main(void) {
    RCC->AHBENR |=  0x00020000;     /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;    /* clear pin mode */
    GPIOA->MODER |=  0x00000400;    /* set pin to output mode */

    /* Configure SysTick */
    SysTick->LOAD = 0xFFFFFF;       /* reload with max value */
    SysTick->CTRL = 5;     /* enable it, no interrupt, use system clock */

    while (1) {
        /* take bit 23 of SysTick current value and shift it to bit 5 
           then write it to PortA */
        GPIOA->ODR = (SysTick->VAL >> (22 - 5)) & 0x00000020;
    }
}