Lập trình System tick STM32

#include "stm32f4xx.h"
#include "stm32f4xx_rcc.h"
#include "stm32f4xx_gpio.h"

__IO uint32_t msTick;

// This function will be called every 1ms
void SysTick_Handler(void)
{
    msTick--;
}

int main(void)
{
    // Enable GPIOD
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);

    // Initialization for GPIO pin 13 (orange LED) as output
    GPIO_InitTypeDef GPIO_InitStruct;
    GPIO_InitStruct.GPIO_Pin = GPIO_Pin_13;
    GPIO_InitStruct.GPIO_Mode = GPIO_Mode_OUT;
    GPIO_InitStruct.GPIO_Speed = GPIO_Speed_2MHz;
    GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_Init(GPIOD, &GPIO_InitStruct);

    // Update SystemCoreClock variable
    SystemCoreClockUpdate();
    // Make SysTick overflow every 1ms
    SysTick_Config(SystemCoreClock / 1000);

    while (1)
    {
        // Toggle LED
        GPIO_ToggleBits(GPIOD, GPIO_Pin_13);

        // Set delay 1s (1000ms)
        msTick = 1000;
        // Do nothing until msTick is zero
        while (msTick);
    }
}