stm32f4discovery PWM InputMode (DutyCycle und Frequenz)

Hallo zusammen,

ich versuche gerade PWM Signal(Frequenzbereich 100-1000 HZ) auszuwerten. 
Um zu testen, schließe ich ein Signalgenerator an. Ich benutze das 
offizielle Beispiel(PWM-Input),im Debugmode versuchte ich Schritt für 
Schritt, aber habe leider nicht die richtigen Werte von DutyCycle und 
Frequenz ermittelt. Jeder Versuch bekam ich ein verschiedenes Ergebnis. 
Ich habe schon viel Zeit damit verbracht, um den Fehler zu finden. Aber 
zur Zeit noch keine Ahnung, wo der Fehler liegt. Ich weiss nicht, ob ich 
irgendwo nicht richtig programmierte. Kann mir jemand helfen? Ich 
bedanke mich im Voraus.

Mfg
Kevin

int main(void)
{
  /* TIM Configuration */
  TIM_Config();
/* TIM4 configuration: PWM Input mode ------------------------
     The external signal is connected to TIM4 CH2 pin (PB.07),
     The Rising edge is used as active edge,
     The TIM4 CCR2 is used to compute the frequency value
     The TIM4 CCR1 is used to compute the duty cycle value
  ------------------------------------------------------------ */
  TIM_ICInitStructure.TIM_Channel = TIM_Channel_2;
  TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising;
  TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
  TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;
  TIM_ICInitStructure.TIM_ICFilter = 0x0;

  TIM_PWMIConfig(TIM4, &TIM_ICInitStructure);

  /* Select the TIM4 Input Trigger: TI2FP2 */
  TIM_SelectInputTrigger(TIM4, TIM_TS_TI2FP2);

  /* Select the slave Mode: Reset Mode */
  TIM_SelectSlaveMode(TIM4, TIM_SlaveMode_Reset);
  TIM_SelectMasterSlaveMode(TIM4,TIM_MasterSlaveMode_Enable);

  /* TIM enable counter */
  TIM_Cmd(TIM4, ENABLE);

  /* Enable the CC2 Interrupt Request */
  TIM_ITConfig(TIM4, TIM_IT_CC2, ENABLE);

  while (1);
}

void TIM_Config(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;
  NVIC_InitTypeDef NVIC_InitStructure;

  /* TIM4 clock enable */
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM4, ENABLE);

  /* GPIOB clock enable */
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOB, ENABLE);

  /* TIM4 chennel2 configuration : PB.07 */
  GPIO_InitStructure.GPIO_Pin   = GPIO_Pin_7;
  GPIO_InitStructure.GPIO_Mode  = GPIO_Mode_AF;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStructure.GPIO_PuPd  = GPIO_PuPd_UP ;
  GPIO_Init(GPIOB, &GPIO_InitStructure);

  /* Connect TIM pin to AF2 */
  GPIO_PinAFConfig(GPIOB, GPIO_PinSource7, GPIO_AF_TIM4);

  /* Enable the TIM4 global Interrupt */
  NVIC_InitStructure.NVIC_IRQChannel = TIM4_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);

}


void TIM4_IRQHandler(void)
{
  RCC_ClocksTypeDef RCC_Clocks;
  RCC_GetClocksFreq(&RCC_Clocks);

  /* Clear TIM4 Capture compare interrupt pending bit */
  TIM_ClearITPendingBit(TIM4, TIM_IT_CC2);

  /* Get the Input Capture value */
  IC2Value = TIM_GetCapture2(TIM4);

  if (IC2Value != 0)
  {
    /* Duty cycle computation */
    DutyCycle = (TIM_GetCapture1(TIM4) * 100) / IC2Value;

    /* Frequency computation
       TIM4 counter clock = (RCC_Clocks.HCLK_Frequency)/2 */

    Frequency = (RCC_Clocks.HCLK_Frequency)/2 / IC2Value;
  }
  else
  {
    DutyCycle = 0;
    Frequency = 0;
  }
}