STM32F4 Rechteck-Signal

GPIO_InitTypeDef  GPIO_InitStructure;

TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;

void TIMER_Config(void);

void GPIO_Config(void);

int i=0;

int main(void) {

  TIMER_Config();          // timer configuration

  GPIO_Config();           // GPIO configuration

  while (1) {

    for(i=0 ; i<600; i++) {

    TIM3->ARR = 999-i;

      if (TIM_GetFlagStatus(TIM3, TIM_FLAG_Update) == SET) {  // EGR UG-Bit

        TIM_ClearFlag(TIM3, TIM_IT_Update);                   // TIM_DIER UIE-Bit

        GPIO_ToggleBits(GPIOD, GPIO_Pin_12);                        

        }

      }

    }

}

void TIMER_Config(void) {

  RCC_APB1PeriphClockCmd (RCC_APB1Periph_TIM3, ENABLE);       // TIM3 clock enable

  TIM_TimeBaseStructInit (&TIM_TimeBaseStructure);

  TIM_TimeBaseStructure.TIM_Prescaler = 4200-1;               // 84Mhz --> 20KHz

  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;

  //TIM_TimeBaseStructure.TIM_Period = 400-1;                // 20KH --> 50Hz

  TIM_TimeBaseStructure.TIM_ClockDivision = 0;

  TIM_TimeBaseInit (TIM3, &TIM_TimeBaseStructure);

  TIM_Cmd(TIM3, ENABLE);                                     // TIM3 enable counter

}

void GPIO_Config(void) {

  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);     // GPIO "D" clock enable

  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12;                // LED green

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;             // output

  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;            // pushpull mode

  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;

  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;          

  GPIO_Init(GPIOD, &GPIO_InitStructure);

}