stm32 PWM输入问题？？？？？

程序为求出输入波形的占空比而设计，所用资源为time3的1通道和2通道
当我在管脚输入1个周期为2.2ms，占空为0.5ms左右的波形时，捕获到的数据却为198和78
原理来算应该为   时钟分频为       ：71
                      计数频率为        ：72M/(72+1) = 1us
                      每计数一次时间为：1us
                      2.2ms计数应该为：2200
                      0.5ms计数应该为：500
不知道我推理的对吗？？？？我写的代码有错误吗????请求帮助？？？？？？
//初始化捕获
void Study_Init(void)
{
  NVIC_InitTypeDef NVIC_InitStructure;
  TIM_ICInitTypeDef  TIM_ICInitStructure;
  GPIO_InitTypeDef GPIO_InitStructure;
  RCC_ClocksTypeDef RCC_Clocks;
  RCC_APB1PeriphClockCmd(RF_TIM_RCC ,ENABLE); //////////时钟选始能
  NVIC_InitStructure.NVIC_IRQChannel = RF_TIM_IRQChannel;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);           ////中断配置

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_AFIO|RF_GPIO_RCC ,ENABLE);
  GPIO_PinRemapConfig(GPIO_PartialRemap_TIM3,ENABLE);
  
  GPIO_InitStructure.GPIO_Pin = RF_GPIO_PIN;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;//GPIO_Mode_Out_PP;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_Init(RF_GPIO, &GPIO_InitStructure); //////输入管脚配置

  /* RF_TIM configuration: PWM Input mode ------------------------
     The external signal is connected to RF_TIM CH2 pin (PA.01),
     The Rising edge is used as active edge,
     The RF_TIM CCR2 is used to compute the frequency value
     The RF_TIM CCR1 is used to compute the duty cycle value
  ------------------------------------------------------------ */
  
  RCC_GetClocksFreq(&RCC_Clocks);/////////////////////系统时钟为72M
  
  TIM_TimeBaseInitTypeDef  tim_base;
  tim_base.TIM_Prescaler  = (RCC_Clocks.PCLK2_Frequency/1000000 - 1);////分频系数   71
  tim_base.TIM_CounterMode= TIM_CounterMode_Up;
  tim_base.TIM_Period     = 0xffff;              ////ARR 自动装载寄存器
  tim_base.TIM_ClockDivision = 0;              ////CR1中CDK[1:0]时钟分频因子为0
  tim_base.TIM_RepetitionCounter = 0;
  TIM_TimeBaseInit(RF_TIM, &tim_base);

  TIM_ICInitStructure.TIM_Channel = TIM_Channel_1;
  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 = 0x3;
  TIM_PWMIConfig(RF_TIM, &TIM_ICInitStructure);

  /* Select the RF_TIM Input Trigger: TI1FP1 */
  TIM_SelectInputTrigger(RF_TIM, TIM_TS_TI1FP1);
  /* Select the slave Mode: Reset Mode */
  TIM_SelectSlaveMode(RF_TIM, TIM_SlaveMode_Reset);
  /* Enable the Master/Slave Mode */
  TIM_SelectMasterSlaveMode(RF_TIM, TIM_MasterSlaveMode_Enable);
  /* TIM enable counter */
  TIM_Cmd(RF_TIM, ENABLE);
  /* Enable the CC2 Interrupt Request */
  TIM_ITConfig(RF_TIM, TIM_IT_CC1, ENABLE);


}